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Analytica Chimica Acta 400 (1999) 333–379

200 years of practical electroanalytical chemistry: past, present andfuture directions illustrated by reference to the on-line, on-stream and

off-line determination of trace metals in zinc plant electrolyte byvoltammetric and potentiometric techniques

Alan M. Bond∗Department of Chemistry, Monash University, Clayton, Vic. 3168, Australia

Accepted 28 June 1999

Abstract

The millennium being celebrated this year coincides with the 200th anniversary of the birth of practical electrochemistrymade possible via Volta’s publication of the battery in the year 1800. The analytical chemists at the beginning of the 19thcentury were very quick to take advantage of this newly reported device and the first qualitative electrochemical determinationof copper rapidly followed this pioneering discovery. In the last 200 years, electrochemical analysis, in its various forms, hasbeen undertaken routinely in countless laboratories all over the world. However, in view of the long and distinguished history ofthe discipline, and some limitations that have been identified at the time of the celebration of the millennium, electrochemicalanalysis is regarded in some quarters as being a mature and conservative discipline whose importance in the future, whenfaced with severe competition from newly emerging alternative analytical techniques, is somewhat unclear. In this paper, anoverview of past and present developments in electroanalytical chemistry and the possible future status of the technique ispresented. In particular, emphasis is given to describing applications relevant to the also very mature field of electrowinningof zinc from plant electrolyte. This overview encompasses the author’s 25 years’ experience in developing polarographic,stripping voltammetric, adsorptive stripping voltammetric and ion-selective electrode (ISE) methods of analysis in on-line,on-stream and off-line modes for the determination of elements such as Cd, Pb, Ge, Sb (oxidation states (III) and (V)), Co,Ni, Zn, Fe, (oxidation states (II) and (III)), Tl, As (total) and Cu in zinc plant electrolyte. Developments that may contribute toan important future for analytical voltammetry are also considered as are limitations that could inhibit the extent of practicaluse of these electroanalytical techniques in the 21st century. ©1999 Elsevier Science B.V. All rights reserved.

Keywords:Voltammetry; Potentiometry; Trace metals; Zinc plant electrolyte

1. Introduction

Electrochemistry is a mature discipline and via anice coincidence, the publication of contributions to

∗ Tel.: +61-3-905-4593; fax: +61-3-905-4597E-mail address:[email protected] (A.M. Bond)

the Measurement for the Next Millennium Meeting(Egmond, The Netherlands, April 14–16, 1999) in An-alytica Chimica Acta in the year 2000 coincides withthe 200th Anniversary of the publication of the in-vention of the battery (pile) by Volta [1], the use ofVolta’s pile to demonstrate electrolysis by Nicholsonand Carlisle [2] and the first example of qualitative

0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved.PII: S0003-2670(99)00527-9

334 A.M. Bond / Analytica Chimica Acta 400 (1999) 333–379

electrochemical analysis (copper) by Cruikshanks [3].Thus, clearly, the early analytical scientists were ableto very rapidly exploit a new technology and this highlevel of innovation remains today in the subject nowcalled electroanalytical chemistry.

The opportunity to present a review article on thesubject of electroanalytical chemistry at a meeting heldto celebrate the rare occurrence of the commence-ment of a millennium caused this author to deliberatefor a considerable time before deciding on a form ofpresentation that could be relevant to such a histori-cal occasion. However, after becoming aware of theabove-mentioned coincidence of the celebration of the200th anniversary of what may be considered to bethe birth of electroanalytical chemistry, and the com-mencement of the millennium, I immediately decidedit was appropriate to continue with a historical themeand reflect on the role of electroanalytical chemistryin the also very mature field of electrowinning of zinc.

Zinc was first smelted in China and India, in about1000 AD [4,5]. This author’s contact with the zincelectrorefining industry represents only a modest 25years, relative to the thousand year history of the pro-duction of zinc. Nevertheless, during this one quarterof a century, the zinc electrolyte research programmeswith which I have been associated have utilised nu-merous forms of voltammetry, developed on-line andon-stream instrumentation for the routine determina-tion of many elements at trace concentration levels,studied the relevant electrode processes at a fundamen-tal level, and employed ion-selective electrodes (ISEs)and pH electrodes, which represent other tools of tradeof the electroanalytical chemist. Thus, by focussing onknowledge gained during the determination of tracemetals in zinc plant electrolyte obtained from the Pas-minco, Risdon, Hobart, Tasmania and Port Pirie, SouthAustralia Zinc Plants in Australia1 , I am hopeful thatan informative tutorial on the past, present and pre-dicted future uses of electroanalytical chemistry willhave emerged.

Obviously, a 25 year project concerning the de-termination of trace elements in zinc electrolyteinvolves very extensive interactions with a largenumber of co-workers. Consequently, before com-

1 For convenience, these zinc smelter/refineries will be referredto as the Pasminco Hobart and Pasminco Port Pirie Plants, respec-tively, in the remainder of this article.

mencing the scientific component of the review, Iwould like to gratefully acknowledge the extensivecollaboration with Dr. Michael Newman, that madethe majority of this work possible. Acknowledgementalso is given to extensive interactions with the staffat the Pasminco Hobart and Port Pirie Zinc Plantsand Technical Support Laboratories, the substantialcontributions of Roger Knight, Robert Mrzljak andSteven Kratsis who gained their doctorates workingon University–Pasminco sponsored research projects,and numerous colleagues in academia, industry andgovernment laboratories who are co-authors of publi-cations from which this review is derived. The namesof these colleagues are contained in the referencelist associated with Table 1, where the problems ad-dressed and results achieved in this long term researchproject of 25 years’ duration are summarised.

2. The zinc industry

2.1. History of zinc production

Although first smelted 1000 years ago in China andIndia (see Section 1), sophisticated technology for zincproduction was developed only in western civilisationin the 18th century when the metal was called In-dian tin or calamine (a term now reserved for naturalzinc carbonate). Two major manufacturing processeshave emerged since then. The first is a thermal pro-cess where, initially, the zinc sulfide ore (most com-mon source of zinc) is roasted in air to produce thecrude oxide (ZnO). ZnO is then reduced with carbonat high temperatures and the resultant metal vapourcondenses to the solid metal. The reduction of zinc inthis process can be summarised as

ZnO(solid) + C(solid) → Zn(metal) + CO(gas) (1)

Unfortunately, due to the production of harmfulgreenhouse gases and a zinc product which is onlyabout 98% pure, this type of zinc production has itsdrawbacks. The second process, the one relevant to thisreview, is the electrolytic one where, initially, the zincore is roasted at high temperatures and then dissolvedinto solution before the zinc ions are electrolysed tometallic zinc at a cathode. The overall reaction for the

A.M. Bond / Analytica Chimica Acta 400 (1999) 333–379 335

Table 1Summary of papers published by the author and colleagues related to the determination of metals in zinc electrolyte

Elements studied Techniques used Reference Comments

Cd, Cu, Pb, Sb,Co, Ni, Tl, As

DPP, DPASV [6] forerunner of instrumentation for an on-streamanalyser described

Pb, Tl DPASV [7] method of resolving overlapping processes developedCd Fourier transform admittance [8] correction methods for matrix interference effects developedCd, Co, Cu, Pb,

Ni, Hgsolvent extraction, liquid

chromatography, spectrophotometricand electrochemical detection.

[9] separation methods developed to avoid problems withvery high zinc concentration

Cu, Cd ISEs [10] on-line system with redundancy principles developedCd, Cu, Sb, Pb DPASV [11] first on-line, on-stream voltammetric system used in

the plant describedCu, Cd, Co, Ni DPASV, DPAdSV, DPV [12] experiences with on-line, on-stream monitoring describedCo DPASV [13] highly sensitive catalytic method described for

CodeterminationFe(II), Fe(III),

total FeDPP, DCP [14] methods for iron in different oxidation

states developedZn, acid DFA, spectrophotometric and ISE

detection[15] methods for determination of high concentrations of

zinc and acid developedCo DPAdSV [16] on-line method for Co describedNi DPAdSV [17] on-line and off-line methods for Ni describedTotal As DPCSV [18] total As determined after separation by the

reductillation processSb(III), Sb(V),

total SbDPASV, DPAdSV [19,20] combination of techniques described for antimony

speciationGe DPAdSV [21] Ge method with electrochemical separation of

Pb, Cd reported

electrolytic preparation of zinc in the electrowinningprocess is given by

Zn2+(solution) + 2e− → Zn(metal) (2)

2.2. General aspects of the electrolytic zinc process

The key features of electrolytic production of zinccan be described by the simple flow diagram in Fig. 1which outlines the major steps involved in producingmetallic zinc.

2.2.1. RoastingMost electrolytic zinc plants, including the Pas-

minco Hobart one, roast a concentrate of the zinc sul-fide ore, in which zinc blende (ZnS) is the predomi-nant component. A zinc concentration of about 55%is typical, with iron often being present at levels upto 10%, together with a number of significant contri-butions from relatively minor compounds of copper,cadmium and lead.

2.2.2. Neutral leach/iron purificationThis process typically has two stages. The first in-

volves the dissolution of zinc oxide with sulfuric acid(spent electrolyte) generated from the cell room. Thisprocess is also known as neutralisation as the acidis neutralised by zinc oxide to form zinc sulfate andwater. The second stage involves purification of theimpure zinc sulfate solution by precipitation of ferricsulfate. The hydrolysed ferric ion acts as a metal ioncollector and partially removes arsenic, antimony andgermanium.

2.2.3. First and second stage purificationFirst and second stage purification are the final

modes of purification prior to the electrolyte solutionentering the cell room. Obviously, for this reason, thisstage is very important for the electrolytic produc-tion of zinc. Purification is carried out by zinc dustprecipitation over two stages and is driven by classi-cal electrochemical metal displacement reactions inwhich metal ions electropositive to zinc are reduced


Fig. 1. Basic flow diagram describing the process for the elec-trolytic production of zinc.

while the zinc metal is oxidised, for example

Zn(metal) + Cd2+(solution)

→ Cd(metal) + Zn2+(solution) (3)

At the conclusion of the zinc dust purification stage,the electrolyte is filtered and then pumped into ‘checktanks’ until a full analysis of the purity of the solutionbecomes available. If the check analysis is within theprescribed limits, the batch is sent forward to neutralsolution storage, otherwise the batch is returned tosecond stage purification for further treatment. Thetypical concentrations of various impurities present inpurified neutral solutions in the Pasminco Port PiriePlant are shown in Table 2.

2.2.4. ElectrolysisThe final stage in the electrolytic production of

zinc is the electrowinning process, where zinc is elec-trodeposited on aluminium cathodes, while oxygen isevolved at the lead anodes. These two half-cell reac-tions are described in Eqs. (4) and (5):

Table 2Typical analysis of trace elements (total concentrations) in purifiedneutral zinc plant electrolyte solutions

Element Concentration (mg l−1)

Nickel 0.01Cobalt 0.02Copper 0.018Cadmium 0.35Arsenic 0.01Antimony 0.064Germanium 0.005

Cathode : ZnSO4(solution) + 2e−

→ Zn(metal) + SO2−4 (solution) (4)

Anode : H2O(liquid)

→ 2H+(solution) + 12O2(gas) + 2e− (5)

whilst the overall reaction is given in Eq. (6):

2ZnSO4(solution) + 2H2O(liquid)

→ Zn(metal) + 2H2SO4(solution) + O2(gas) (6)

Aluminium cathodes have an oxide layer perform-ing the vital function of providing a substrate fromwhich plated zinc can be readily stripped, while at thesame time preventing corrosion of aluminium in thecorrosive acidic electrolyte which contains fluoride.Lead anodes containing 0.5–1 wt.% silver are gener-ally used. They possess good corrosion resistance inthe acidic electrolyte. Manganese (II) ions, which arenaturally present or added during leaching, form aprotective manganese dioxide scale on the surface ofthe lead anode during electrolysis. At the conclusionof the electrolysis stage, the electrodeposited zinc isstripped off the aluminium cathodes by hydraulicallyoperated stripping machines. The stripped zinc metalis then melted in a large furnace and cast into slabsand blocks for sale.

2.3. Control of impurities

Purified neutral zinc sulphate solution supplied tothe cell room contains a number of other components,termed impurities. Their impact can be classified asfollows:(a) co-depositing with zinc and downgrading the pu-

rity of zinc metal;


Fig. 2. The effect of impurity concentration (Ge, Sb, Se, Te andSn) on the current efficiency for 1 h zinc deposits electrowon at430 A m−2 from industrial acidic zinc sulfate electrolytes. (Adaptedfrom J. Appl. Electrochem. 17 (1987) 1129.)

(b) decreasing the current efficiency (% of the currentpassed which produces zinc) and hence increasingpower consumption. State of the art zinc produc-tion involves current efficiencies of around 94%.

For example, impurities like antimony and germa-nium catalyse the competing reaction of hydrogen ionreduction, and in this manner, decrease the current ef-ficiency and hence the process efficiency. Quantita-tive details of the adverse impact of single elementimpurities on current efficiency are shown in Fig. 2.However, combined effects are synergistic. The mainmetals found in the electrolyte solutions and theirimpacts on zinc electrowinning are summarised inTable 3. Best practice places emphasis on Feed solu-tion purity continually pushing process standards tolower concentrations.

Additives such as glue are added during electrolysisto limit the impact of impurities, facilitate the strippingof the zinc deposit and to prevent acid mist emissionfrom the cell caused by gas evolution.

2.4. Monitoring of impurities

In view of the potentially adverse affects of impu-rities present in the electrolyte on zinc production, it

is of paramount importance that the electrolyte solu-tions are monitored, thereby allowing early correctiveaction to be taken. During the last 25 years, a substan-tial amount of effort has been devoted to research anddevelopment of on-line, on-stream and off-line moni-toring methods in both the author’s and the Pasmincolaboratories.

2.4.1. Advantages of determination of trace metalimpurities by voltammetry

In principle, there is a wide variety of analyticalmethods which can be used for direct trace metal deter-mination in zinc plant electrolyte media, e.g. flame andelectrothermal atomic absorption spectrometry (AAS),inductively coupled plasma (ICP) and emission spec-trometry. The use of separation methods such as liq-uid and ion exchange chromatography, coupled withsensitive detection techniques also are available tominimise interferences. However, voltammetric tech-niques commonly hold two distinct advantages whenemployed in zinc plant electrolyte media.

2.4.1.1. Advantage of voltammetry in high ionicstrength media. The high ionic strength and high con-ductivity of zinc plant electrolyte is an ideal mediumfor the application of voltammetric techniques. Incontrast, the very high concentration of salt in zincplant electrolyte can cause problems with blockagesof aspirators in AAS and ICP methods. Dilution ofthe sample does not necessarily provide an answer as,frequently, it decreases the concentration of the tracemetal to below the AAS or ICP detection limit.

2.4.1.2. Advantages of voltammetry in the presence ofa large concentration excess of zinc.The large con-centration excess of zinc ions can be discriminatedagainst in stripping voltammetric methods by selec-tively choosing deposition potentials that are less neg-ative than required for their reduction. In cases wheresevere interference caused by the presence of the largeexcess of zinc remains, regardless of the use of a se-lective deposition potential, simple adjustment of theelectrolyte conditions may still permit accurate de-terminations to be made. For example, the additionof dimethylglyoxime into zinc plant electrolyte medialeads to complexation with cobalt and nickel ions, butnot zinc, and forms the basis of a sensitive adsorptive


Table 3Main metals found in zinc plant electrolyte solutions and their impact on zinc electrowinning

Element Effect Control

Nickel can cause holes in deposits when other impurities are present zinc dust cementationCobalt lowers current efficiency when other impurities are present zinc dust cementationCopper decreases current efficiency iron purification and zinc dust cementationCadmium contaminates the final zinc product removed by zinc dust cementationArsenic can have harmful effects on electrolysis by lowering the

current efficiencylevels can be reduced by iron purification and zinc dust

cementationAntimony while small concentrations may be beneficial, concentrations

greater than 50 ppb may affect current efficiencyIron purification and zinc dust cementation

Manganese deposits as MnO2 on lead anode but mainly has an effect onthe distribution of other impurities

manually removed from cells

Germanium very low concentrations severely affects electrolysisand overall current efficiency

iron purification and zinc dust cementation

stripping method for their determination (see Sections5.2 and 5.3).

Without sample pre-treatment, high concentrationsof zinc sometimes interfere with spectroscopic deter-minations. For example, in ICP methods, the high con-centrations of zinc sulphate may form a flux at the veryhigh temperatures used with this technique which maycause the quartz nebuliser to melt. Again, dilution maynot be a simple answer as the trace metal to be deter-mined may fall to a value below the detection limit.Of course, advances in technology may lead to theelimination of problems of the above kind presentlyassociated with some spectroscopic methods.

3. Principles of voltammetric techniques usedin the determination of metals in zinc plantelectrolyte

3.1. DC polarography and voltammetry

DC polarography, or voltammetry at a droppingmercury electrode (DME), is a classical electrochem-ical technique [22], which, for example, can be usedfor the determination of the high concentrations of to-tal iron present in zinc plant electrolyte, as well as todistinguish between the iron(II) and iron(III) oxidationstates of this element (see Section 6.1). The excitationsignal is a constant DC potential or a sufficiently slowDC potential ramp (Fig. 3a) so that it can be assumedthat the current (I) is measured at a constant appliedpotential (E). The resulting current (usually sampledat the end of the drop life) is displayed as a func-

tion of the applied potential as shown in Fig. 3b. ThecharacteristicI–E (current–voltage) curve or (currentsampled) DC polarogram allows the electroanalyticalchemist to obtain vital pieces of information. For ex-ample, in Fig. 3b, the difference between the poten-tial independent current and the background currentis called the limiting currentIL, which frequently de-pends on the concentration of the electroactive speciesin a linear manner [22]. Another important parameteris the half-wave potential (E1/2) which is the potentialon a polarographic curve at which the current reacheshalf of its limiting value [22]. For most applications,the E1/2-value is independent or almost independentof the concentration of the electroactive species andit is this parameter which enables the identity of theelectroactive species to be determined. Further detailsof this technique are available in [22]. If a hangingmercury drop electrode (HMDE) or a solid electrodeis used, the term voltammetry is used rather than po-larography, which by definition, specifically impliesthe use of a DME as the working electrode. If thewaveform in Fig. 3a is used, the resultant asymmetricpeak shapedI–E curve (Fig. 3c) is called a voltammo-gram and the technique is called linear sweep voltam-metry (LSV) in which the peak potential (Ep) is re-lated toE1/2 and the peak height (Ip) is proportionalto concentration and hence akin toIL.

3.2. Differential pulse and other transient forms ofvoltammetry (polarography)

DC techniques are not inherently sensitive, withconcentration detection limits down to only about


Fig. 3. Schematic diagrams of (a) waveform used in DC polaro-graphy (very slow scan rate) or voltammetry (faster scan rate),(b) a typical current sampled DC polarogram, (c) linear sweepvoltammogram.

10−6–10−5 M usually being available [22]. Transientwaveforms in which pulse, square wave or alternat-ing potentials are superimposed periodically onto theDC potential to give differential pulse (DPP), squarewave (SWP) and alternating current (ACP) polarogra-phy, respectively, are available and give significantlyimproved sensitivity when applied at the DME [22]as are their voltammetric counterparts (differentialpulse voltammetry (DPV), square wave voltamme-try (SWV), alternating current voltammetry (ACV),respectively) when applied at the HMDE or a solid

electrode. Fig. 4a illustrates the principle of DPV.The resultant peak shaped curve is obtained by plot-ting the difference in current measured before andafter the application of a small amplitude pulsed po-tential (amplitude 10–100 mV) as a function of DCpotential. As in DPP and DPV, the peak height (Ip)is proportional to concentration and readily enablesconcentrations down to about 10−7 M to be deter-mined [22], whilst the peak potential (Ep) is related toE1/2. The differential pulse method has been used forthe determination of numerous elements in zinc plantelectrolyte (Table 1). In the square wave or alternat-ing current voltammetric (polarographic) techniques,as the names imply, a square wave or an alternatingpotential is superimposed onto the DC ramp [22] toalso produce a peak shaped response. Clearly, theresolution as well as the sensitivity of these so-calledtransient techniques is superior to DC polarography.

3.3. General features of DC stripping voltammetry

Various voltammetric stripping methods are usedfor trace metal determination of metals such asCd, Cu, Pb, Ni, Co, Ge, Sb in zinc electrolyte(Table 1). Essentially, all stripping techniques possessthree main steps viz. deposition, equilibration andstripping.

3.3.1. Deposition or accumulation stepThe deposition step usually involves the electrolytic

or adsorptive deposition of a chemical species ontoan electrode surface at a constant DC potential. Whenmetal ions are determined by anodic stripping voltam-metry (ASV) at an HMDE, a sufficiently negative po-tential is applied to the working electrode to cause themetal ion of interest to be reduced to the metal, which,in many cases, forms an amalgam with the mercuryelectrode (e.g. Cd, Pb, Tl and Sb) [11]. Similar depo-sition principles apply in cathodic stripping voltam-metry (CSV), although, in this case, the stripping stepis different (see Section 3.3.3). In all forms of adsorp-tive stripping voltammetry (AdSV), a metal complexis accumulated at the electrode surface by adsorption(e.g. Ni [17], Co [16], Sb [20], Ge [21]). In strippinganalysis, the deposition step is usually facilitated byconvective transport of the analyte to the surface of theworking electrode. This can be achieved by rotation


Fig. 4. Schematic diagram of the waveforms used in (a) DPV, (b) DC stripping voltammetry.

of the electrode, by stirring the solution or by flowingthe solution over the electrode [22–25].

3.3.2. Equilibration stepWhen the deposition step occurs under convective

conditions, a quiet time usually follows this step inorder to enable the electrode to return to a quiescentstate. This period is usually in the range of 10–30 sand is called the equilibration step.

3.3.3. Stripping stepIn ASV, the stripping step is achieved by the ap-

plication of a linear or ramped voltage applied in thedirection of positive potential which, therefore, causes

the metal or metal in the amalgam to be oxidised backto the solution soluble metal ion state. In CSV, strip-ping, as in the case of the determination of As [18], isachieved by a negative direction potential scan. Duringthe potential scan, the accumulated metal is strippedfrom the surface, yielding a peak height for each an-alyte, which is proportional to concentration. Ideally,the peak current is linearly proportional to the concen-tration of the analyte in the bulk solution and to thedeposition time. The different stages associated withDC stripping voltammetry in the ASV form are de-scribed in Fig. 4b. In AdSV, the stripping step gener-ally involves reduction of the adsorbed metal complexby applying a negative direction potential scan.


3.4. Differential pulse stripping voltammetry

Differential pulse stripping voltammetry (DPSV) atan HMDE or solid electrode as for the DPV methodand as the name implies, utilises a periodic rather thanDC waveform. As in conventional voltammetry, theuse of a transient waveform minimises the contributionfrom the background current [22,23], and therefore,enables increased analytical sensitivity to be achievedover traditional DC stripping techniques. Hence, (a)the waveform again consists of pulses of constant am-plitude superimposed on a linear or ramped wave-form, (b) the current is sampled twice, once prior tothe pulse and again at the end of the pulse, (c) thedifference in current between the two measurementsis plotted as a function of potential and (d) the peakheight of the resultant peak shaped voltammogram isproportional to the concentration of the electroactivespecies. In DPSV, a deposition time and an equilibra-tion time are added to the waveform used in DPV.By analogy, square wave (SWSV) and AC stripping(ACSV) methods (not discussed in this review) use theanalogous waveforms employed in their voltammetriccounterparts.

AdSV evolved from the need of electroanalyticalchemists to develop very sensitive voltammetric tech-niques capable of the determination of elements thatare inaccessible by classical stripping methods, be-cause of their irreversible electrochemical character-istics. The method also offers greater sensitivity thanconventional stripping methods due to the immediateavailability of accumulated material for reaction at thesurface.

In DPASV, as in ASV (see Section 3.3), the accu-mulation involves reduction of the metal ion of in-terest to its elemental or amalgam form, whilst inDPAdSV or AdSV (see above), the metal ion is com-plexed in solution with an organic ligand to form ametal complex which adsorbs at the surface of theHMDE without the transfer of charge. In DPASV, thedeposited metal is stripped from the working elec-trode by scanning the potential in the direction of pos-itive (anodic) potential, whilst in DPAdSV, the ad-sorbed metal complex is usually reduced on the sur-face by scanning the potential in the direction of neg-ative potential. Fig. 5 provides a comparison of thewaveform and timing sequences used in DPASV andDPAdSV.

Fig. 5. Schematic representation of the current sampling and timingsequences employed in (a) DPASV and (b) DPAdSV. The shortquiet time that follows the accumulation step (see Fig. 4 has beenomitted for simplicity.

4. Protocols, instrumentation and practicalconsiderations for on-line, on-stream monitoringof metals in zinc plant electrolyte by voltammetrictechniques2

The development of on-stream analysis proceduresfor inorganic impurities has been a long term objec-tive of many electrolytic zinc producers. The Elec-trolytic Zinc Company of Australasia (a forerunnerof the present day Pasminco), evaluated this possibil-ity during the 1970’s and targeted polarographic andvoltammetric techniques as prospective technology forthe application. In the early studies by Pilkington,Weeks and Bond [6], computerised forms of DPP andstripping voltammetry were interfaced to a PDP8/Eminicomputer. The computer controlled the nitrogen

2 More detailed information on the instrumentation and operationin a zinc plant is available in [12], from which section 4 is adopted.


degassing step required to remove electroactive oxy-gen from solutions and the timing of all stages in ASV,initiated the potential step, and processed theI–Ecurveto generate the final print-out in units of concentra-tion. However, at that time, a number of technical dif-ficulties precluded the development of an automatedon-stream voltammetric analyser. These difficulties in-cluded the problem of supplying reliably a sample toan on-stream analyser without manual intervention.Since that time, the advent of modern solid state elec-tronics at a realistic cost has resulted in a renaissanceof voltammetry and related techniques and facilitatedthe development of automated sampling and analyti-cal systems. During the 1980’s and 1990’s, Pasmincohas achieved its aspirations of the 1970’s [12].

The successful development of an on-streamvoltammetric analyser involved a collaborativePasminco–University project. Following an initialphase of development in the author’s laboratories[11], the technology was transferred to the PasmincoHobart plant. Subsequently, a prototype voltammet-ric analyser was successfully commissioned in 1987and used on-line for several years at the outlet of theold zinc dust purification stage available at that time.The purification sequence in the old circuit involveda two stage iron purification, a single zinc dust stageremoving copper and cadmium at about 50◦C, andpartial removal of cobalt usinga-nitrosob-naphthol.Significant improvements in plant performance wereachieved after the on-stream analyser was introduced[12].

In the modernised circuit used in the 1990’s, this pu-rification sequence has been replaced by a hot neutralleach and two proprietary high temperature zinc dustpurification stages. The first zinc dust stage removescopper, while cadmium, cobalt, nickel, thallium andother trace metals are removed in the second stage.Voltammetric analysers developed from concepts de-veloped at the prototype stage and applied to the oldplant have been used to monitor and control the per-formance of the new zinc dust purification stages.

4.1. Instrumental procedures

The majority of the on-line microprocessor con-trolled methods are based on a combination of metaldeposition or adsorption of metal complexes ontoa hanging mercury drop electrode from a flowing

solution of zinc plant electrolyte, with detection byDPASV or DPAdSV, often in another electrolyte un-der stopped flow conditions and after matrix exchange(see Section 5 for an explanation of the chemical prin-ciples and concepts associated with matrix exchange).A schematic diagram of the initial instrumentationdeveloped for on-stream analysis in the old plant isshown in Fig. 6a, whilst Fig. 6b is a photograph ofthe secondary purification DPASV analyser used forthe simultaneous determination of Cd, Pb, Cu andSb(III) in the new plant.

During deposition of the metal or adsorption ofmetal complex, the very dense zinc electrolyte sam-ple solution flows past the HMDE, falls to the baseof a specially designed electrochemical cell and is re-moved through a bottom drain facility (Fig. 7). Fig.7a represents an example of an initial design used inthe old plant, whereas Fig. 7b is a more recent de-sign presently in use. The stripping cycle commencesafter the flow has been stopped and the cell has equi-librated, with the mercury drop fully immersed in thereceiving or exchange electrolyte solution. Since de-position or adsorption occurs when the mercury dropis surrounded by the sample electrolyte and strippingoccurs when the receiving solution surrounds the mer-cury drop, an in situ matrix exchange occurs withinthe cell. As will be seen in Section 5, the receivingsolution can be varied to suit the specified analyticaldetermination; for example, to remove interferencesotherwise present if the stripping is carried out directlyin the sample solution.

The on-stream analytical method involves a numberof sequential steps to achieve the desired sensitivityand resolution, which are summarised as follows:1. Operating parameters are entered into the control

unit, and the analytical procedure is started at thecontrolling unit.

2. The cell containing the appropriate exchange so-lution is purged with nitrogen to remove interfer-ing oxygen.

3. A new mercury drop is formed at the HMDE afterthe old drop is dislodged.

4. Zinc electrolyte is sampled automatically from theplant, filtered, diluted (1 : 1 with 0.05% H2SO4),and purged with nitrogen in preparation for deliv-ery to the electrochemical cell.

5. The deposition potential is applied to the freshmercury drop as the sample is introduced into the


Fig. 6. On-stream voltammetric analysers for monitoring of trace metal in zinc plant electrolyte; (a) block diagram of the prototypeautomated on-line voltammetric analyser used in the old plant, key: (1) voltammetric experimental parameters, (2) cycle start signal, (3)sampling unit start signal, (4) voltammetric experimental start signal, (5) multimode electrode control signal, (6) voltammetric waveform, (7)voltammetric cell response, (8) experimental data, (9) sample electrolyte; (b) photograph (provided by courtesy of Pasminco) of on-streamPasminco Secondary Purification Filtrate Voltammetric Analyser used in the new plant for the determination of 0–2.5 mg Cd l−1, 0–3.0 mgPb l−1, 0–1.0 mg Sb(III) l−1 and 0–1.0 mg Cu l−1 by DPASV (matrix exchange method).


Fig. 7. Schematic diagrams of bottom-drain flow-through cells used for the on-line matrix exchange voltammetric method; (a) example ofdesign used with prototype instrumentation, (1) exchange electrolyte reservoir, (2) dense plant electrolyte offtake, (3) dense plant electrolytedrain, (4) air vent (syphon break), (5) mercury reservoir, (6) mercury drain; (b) example of design used in new plant, (1) sample inletline, (2) nitrogen purge line, (3) reference electrode, (4) Metrohm hanging mercury drop electrode, (5) auxiliary electrode, (6) PAR Model310 flow adaptor, (7) sample electrolyte drain to waste, (8) control valve. (Adapted from Anal. Chem. 60 (1988) and Anal. Chim. Acta281 (1993) 281.)

electrochemical cell and it impinges directly overthe drop.

6. The cell is allowed to equilibrate for a pre-determinedtime with all flow stopped after the depositionperiod is over.

7. The stripping waveform is applied to the cell, andthe response in the exchange electrolyte is mea-sured and stored in the control unit.

8. When all data have been collected, graphical rep-resentation and data evaluation are performed. Re-sults are transmitted to the required location.

9. Fresh exchange electrolyte, which is introducedinto the top of the cell at completion of the exper-iment, replaces contaminated electrolyte and mer-cury drops, which are drained from the bottom ofthe cell.

10. At pre-selected time intervals, a standard solutionis introduced for calibration purposes.

11. The analytical cycle, which typically takes be-tween 2 and 15 min, depending on the experimen-tal parameters being used, is recommenced.

In the case of the simultaneous determination ofcadmium, lead, antimony and copper in purified zincplant solution using the analyser shown in Fig. 6b, atypical cycle takes about 10 min.

4.1.1. Prototype instrumentation descriptionThe prototype on-stream voltammetric analyser

tested in the old Risdon, Hobart plant consisted offour major components (Fig. 8a):

1. Controlling unit: an Apple II computer system ef-fected this function, which involved sequencingand timing the operations of other units, receivingand processing experimental data, and producingreports and alarms.

2. Voltammetric controller: a Motorola MEK 6800D2 kit controlled the HMDE operation, generationof electrochemical waveforms and collection ofraw experimental data.

3. Analysis unit: this unit was a modified Applikontitro-analyser ADI 2020 (Applikon DependableInstruments, Schiedam, Netherlands) fitted withan HMDE (Metrohm multimode electrode), ref-erence and auxiliary electrodes, a proprietary po-tentiostat and solenoid driver card. Functions ofthe analysing unit include effecting sample andreagent handling and starting the sampling unit.

4. Sampling unit: this was a proprietary systemwhich sampled, filtered (coarse filter medium),diluted and purged process solution with nitrogenbefore delivery to the analysis unit.

4.1.2. Voltammetric analyser developmentThe use of the ADI 2020 analyser allowed the rapid

development (Fig. 8b and c) from a prototype formof the instrumentation (Fig. 6a and Fig. 8a) to theform now used in the new plant (Fig. 6b). The ADI2020 titroanalyser used in the prototype was retainedin all later versions of the voltammetric analyser. Keyfeatures which contributed to the value of the ADI


Fig. 8. Schematic diagram showing the development of a voltammetric analyser through three generations of instrument configuration. See[12] for further details.

2020 analyser include its burettes for precision reagentand sample handling, a well designed ‘wet parts area’,easily modified construction and easily programmedsequencing software.

The instrumentation finally used in the new plant(Fig. 8c) was consolidated into two units, namely theanalysis and sampling units, by rebuilding the con-trolling and voltammetric controller units into a formsuitable for installation within the ADI 2020 cabinet.

4.1.3. Electronics and software developmentIn the prototype instrumentation (Fig. 8a), the con-

trolling unit activated the functions of the voltammet-

ric controller, the analysis unit and the sampling unitthrough the analysis unit. The first stage of develop-ment to the form of instrumentation used in the newplant involved the replacement of the Apple II com-puter controlling unit by an IBM PC computer, and thefabrication of a voltammetric processor card to replacethe voltammetric controller (Fig. 8b). The voltammet-ric processor card was mounted in the ADI 2020 titro-analyser. Two RS232C serial links were used to com-municate between the IBM PC controlling unit, theADI 2020 analysis unit and the sampling unit.

The second and final stages of instrumental de-velopment (Fig. 8c) involved the building of a


communications processor card for installation in theADI 2020, thus allowing the analysis unit to be pro-grammed directly for operation as a self-containedunit independent of the controlling unit.

4.1.4. Mercury electrodeA PAR Model 303 static mercury drop electrode

Model 310 flow adaptor system was used in the initialstudies (Fig. 7a). However, Metrohm 663VA voltam-metric stands and Metrohm multi-mode mercuryelectrodes replaced the PAR 303 electrode in the sec-ond generation on-line voltammetric analysers (Fig.7b, Fig. 8c). An important feature of these Metrohmstands is the spring loaded support arm, which ismounted rigidly onto the ADI 2020 ‘wet parts’ panelby its hinge plate. This hinged arm provides easyaccess to the analysis vessel and electrodes withoutremoving the analysis vessel. This feature is partic-ularly useful when servicing the mercury electrodecapillary.

4.1.5. Sample handlingIf required, the sample can be diluted and acidified

in the sampling unit and the sample purge vessel fedwith either the diluted sample for analysis or the testsolution for calibration purposes.

4.2. An overview of plant experiences

4.2.1. Installation and commissioning in the old plantThe prototype instrument (Fig. 6a and Fig. 8a) was

assembled and subjected to pre-commissioning trialsoff-site in the Pasminco Hobart Research Laborato-ries before installation and commissioning in the oldplant in 1987. This prototype on-stream analyser waslocated immediately adjacent to the plate and framepresses, which filtered zinc dust precipitate derivedfrom the purification stage to remove copper and cad-mium in the old circuit. The sampling and analysisunits were situated within the actual operating envi-ronment, and hence, were subject to high dust and vi-bration levels. The high vibration levels did not affectthe mercury drop electrode. The units were designedto cope with being occasionally sprayed with waterand process liquor, and therefore, the hardware of bothsampling and hardware units was contained in sealedcabinets. The controlling unit and the voltammetric

controller were located approximately 4 m away ina small laboratory which was also available for rou-tine manual analytical purposes. The purified processstream was sampled from a launder located one floorbelow and 15 m away from the sampling unit, using aprobe located in the launder such that it was alwaysimmersed in solution, irrespective of the flow rate. Anin-line filter (8mm) was installed between the sam-pling and analysing units to reduce line blockages inthe fine sample tubing.

4.2.1.1. Process benefits.In the old circuit, zincdust was used to remove copper and cadmium bycementation at a low temperature (50◦C) stage. Zincdust was added to agitated tank reactors and the puri-fied solution was pumped to filter presses to separatethe zinc dust precipitate. The prototype form of thevoltammetric analyser was used to monitor copperlevels in the launder carrying the combined flow offiltrate from all the presses currently on-line. Sub-sequently, the analysis set was increased to includecadmium, antimony and lead [11]. However, withrespect to process control, only copper and cadmiumwere important. At this stage, the on-stream measure-ments were made in addition to the routine manualmeasurements.

When the on-stream analyser indicated high cop-per levels in the combined filtrate from the presses,an immediate investigation was initiated to find andeliminate the cause. Typical causes of high copperlevels were associated with poor operating practices[12]. For instance, after the filter presses were cleanedmanually at the end of a cycle, the filters and the sur-rounding filter floor were hosed down to remove anyprecipitate spills. High on-stream analysis copper lev-els highlighted a number of instances of plant opera-tors using poor practices which resulted, in part, of thecontaminated cleaning water entering the launder car-rying the purified filtrate. By modifying operating pro-cedures and making some minor modifications to theequipment, these occurrences were eliminated [12].Another frequent operating problem detected throughon-stream analysis was the failure of the press filtermedium, either because of a fold or a hole in the filtercloth. Fortunately, each press plate had an indepen-dent discharge spigot, which could be inspected visu-ally. Consequently, following a high copper indication


Table 4Effect of on-stream analysis (OSA) on copper and cadmium con-centrations in zinc dust purified solutiona

Metal Without OSAb With OSAb

Copper(mg l−1)Average 54± 21 41± 17Maximum 89± 48 65± 27Minimum 27± 11 23± 10

Cadmium(mg l−1)Average 262± 87 219± 56Maximum 430± 194 322± 173Minimum 141± 58 151± 40

a Data obtained from [12].b The values quoted are based on the maximum, minimum and

average values obtained from daily determinations each week overa 1-year period.

by on-stream analysis, each spigot discharge was in-spected and any unsatisfactory discharge was rectifiedby plugging that outlet [12].

Increased base line cadmium levels indicated byon-stream analysis, at times when presses were not be-ing changed, provided an indication of unsatisfactoryconditions in the zinc dust purification reactors (e.g.inadequate zinc dust addition rate or high pH). In or-der to assist in detecting such problems, the on-streamanalyser was programmed to provide regular reportson trends in process performance [12].

The plant operators rapidly gained confidence in thereliability of the prototype on-stream analyser, whichallowed them to improve greatly the surveillance ofthe process, particularly the physical aspects of sepa-rating and handling zinc dust precipitate. The extent towhich this provided process benefits is shown in Table4. For instance, comparing performance 1 year beforeand after the commission of on-stream analysis, theaverage concentration of copper and cadmium in pu-rified solution passing to the cell room decreased by27 and 25%, respectively. This resulted in decreasedpower consumption for zinc production [12].

Encouraged by the successful use of the prototypein the old pant, it was decided that extensive usebe made of voltammetric analysers in the new twostage zinc dust purification which was designed forhighly automated operation and which was commis-sioned in 1989. Furthermore, in the new plant, theon-stream analysers are used for process control ratherthan purely for surveillance as was the case in the oldplant.

4.2.2. Use in the new circuitAn extensive range of analysers were developed for

metal determination in the new plant. Fig. 9 representsan example of configurations of on-stream analysersthat were developed or proposed for deployment inthe new zinc dust purification part of the plant.

4.2.2.1. Use in primary purification. In the formatshown in Fig. 9, Analyser No. 1 could be used tomeasure copper and cadmium concentrations in solu-tion flowing to the primary purification stage in thenew plant. If the concentration changes in this processstream are relatively slow, this analyser also could beused to measure samples taken from within the purifi-cation reactors and press filtrate, at hourly intervals.Under this scenario, these measurements could be usedto control zinc dust additions automatically, using anappropriate algorithm which includes logic checks forthe failure of the zinc dust addition equipment. Onaccount of the relatively high levels of copper andcadmium in these pre-purified samples, a polarogra-phy based procedure can be used as an alternative tostripping and is more rapid than the matrix exchangedifferential pulse anodic stripping voltammetric tech-nique (see later). However, with the scheme presentedin Fig. 9, Analyser No. 2 also is available for provid-ing an accurate DPASV check on the copper, lead andantimony levels in the filtrate, and can also be used toprovide redundancy in the event of the failure of theother analyser.

4.2.2.2. Use in secondary purification.In thescheme shown in Fig. 9, Analyser No. 3 would mea-sure the cobalt concentration in primary purificationfiltrate and in solution sampled from a tank in the sec-ondary purification reactor train, whilst Analyser No.5 would determine the cobalt concentration in the fil-trate from the secondary purification stage. Both theseanalysers also could be used to determine nickel, butthis measurement is not likely to be essential becausenickel removal is invariably satisfactory when targetcobalt concentrations are achieved.

The copper, cadmium, antimony and lead levels inthe third reactor and the combined filtrate from the sec-ondary purification section filter presses can be mea-sured by Analysers No. 4 and 6, respectively, usingthe automated matrix exchange method. Analyser No.


Fig. 9. Simplified flowchart showing an example of a configuration in which voltammetric analysers could be employed in the new zincdust purification circuit at the Pasminco Hobart plant. See [12] for further details.

6, as depicted in the scheme in Fig. 9, is set in aparticularly sensitive configuration and can measureconcentrations of copper, cadmium and lead as lowas 10mg l−1. Finally, Analyser No. 7 would measurecadmium, and it has a very rapid response. Its func-tion would be to detect as to when to cease the recycleof filtrate from individual filters, as well as to detectthe dissolution of cadmium from the filter cake dur-ing the filtration cycle, particularly towards the end ofthe filtration cycle, and any filter cloth failures duringfiltration.

Using appropriate control algorithms, the processparameters determined by the analysers are used to

adjust reagent additions to the secondary purificationstage. For example, the output from Analyser No. 7,the fast response cadmium analyser, can be used tocontrol filter cycles and to determine the extent towhich out of specification filtrate must be recycled tothe purification reactors.

4.2.2.3. Analyser installation in the new plant and anoverview of features of the operational performanceThe voltammetric analysers used in the new plantwere installed in two air-conditioned dedicated rooms,known as the tank analyser room and the filter analyserroom. Data generated by the voltammetric analysers


Fig. 10. Display of data as observed for the on-stream determination of metals in zinc plant electrolyte in the Pasminco Hobart PurificationControl Room. From left to right, VDU displays of Primary Purification Feed Analyser, Secondary Purification Feed Analyser and ZincLeach Analyser (0–2500 mg Cd l−1). Photograph provided by courtesy of Pasminco.

were read by the Hobart plant Taylor Mod 300 dis-tributed control system, which was used to implementprocess control functions. Fig. 10 shows a photographof the data display as viewed in the plant control room.

In summary, the voltammetric method has beenfound to be extremely versatile and can be adaptedto the determination of a large range of impuritiesincluding copper, cadmium, antimony, lead, nickeland cobalt (see relevant chemistry in Section 5).Large ranges in impurity concentrations are readilyaccommodated through software adjustments to theanalyser systems. Moderate reagent consumption andself-calibration checks, via the introduction of a stan-dard solution, make prolonged periods of unattendedanalyser operation possible. Each on-stream anal-yser is periodically calibrated automatically by theintroduction of a standard solution.

5. Chemical aspects of methods used for theon-line, on-stream determination of metals in zincplant electrolyte

In Section 4, details of the instrumentation usedfor on-stream monitoring of trace elements in zincplant electrolyte are provided. The chemistry associ-ated with each determination in these on-stream as

well as off-line applications for use in conventionallaboratories is equally as complex as the instrumenta-tion. In Section 5, details of the chemical ‘tricks’ in-troduced to avoid interference in on-line applicationsare revealed.

5.1. The on-line, on-stream determination of traceelements by stripping voltammetry in highly densezinc plant electrolyte

5.1.1. General considerationsASV and related techniques, as noted in Section

3, are widely acknowledged as important electroana-lytical methods because of their high sensitivity andselectivity and ease of automation, particularly whenused in flow-through electrochemical cell configura-tions [23,26,27]. The methods briefly referred to inSection 4, which are used for ‘on-line’ monitoring oftrace metals in zinc plant electrolyte, therefore, havebeen based on flow-through cells of a range of designs(Fig. 7). However, there are a number of unique prob-lems that had to be overcome with ASV detection inzinc (sulfate) plant electrolyte, before a ‘chemicallyreliable’ on-line, on-stream method could be installedin a plant situation. In particular, the electrolyte is ex-ceptionally dense relative to common electrolytes, and


Fig. 11. Attempted determination by DPASV at an HMDE of (a)10−7 M thallium and 10−7 M lead in 1 M zinc sulfate where theresponses are only partially resolved, and (b) thallium in zinc plantelectrolyte when the concentration excess of lead is approximately40-fold and no visual evidence of thallium is present. Depositiontime, 2 min. Equilibration time, 15 s. (Reproduced by courtesyfrom Anal. Chem. 48 (1976) 1624).

modified flow-through electrochemical cells had to bedesigned (Fig. 7) for handling these very high den-sity solutions (also see Section 5.1.2 below). Further-more, direct determination of many elements in zincelectrolyte is not feasible because of inadequate res-olution [6,7,9,28,29], and interferences from organicimpurities also can be significant [6,8].

5.1.1.1. Problems with resolution.The problemwith resolution is conveniently illustrated by refer-ence to the simultaneous determination of Tl and Pbin zinc electrolyte [7]. Fig. 11a shows the attempteddetermination of a mixture of 10−7 M Tl and 10−7 MPb in 1 M zinc sulphate by DPASV in a stationary cellconfiguration, whilst Fig. 11b shows the attempteddetermination of thallium in zinc plant electrolyteusing the same technique. The peak potentials forthe Pb(Hg)Pb2+ + 2e− and Tl(Hg)Tl+ + e−DPASV processes at an HMDE are−0.37 and−0.43 V versus Ag/AgCl, respectively, and therefore,too close to resolve completely. Thus, in Fig. 11a,where the concentrations of both elements are equal,

the Tl stripping peak is only seen as a shoulder onthe Pb peak. Even worse, in Fig. 11b, Tl is not evendetectable by eye when lead is present in about a40-fold concentration excess, as is the case in plantelectrolyte. Sophisticated subtraction methods weredeveloped to overcome this resolution problem (in[7]), but matrix exchange methods of the kind de-scribed in Section 5.1.1.3 also achieve this objective.

5.1.1.2. Problems with organic impurities.Thedifficulty introduced by the presence of organic im-purities (surfactants) is illustrated in Fig. 12a wherethe response for the reduction of 1× 10−3 M Cd2+(Cd2+ + 2e− Cd(Hg)) in zinc sulphate under condi-tions of AC polarography is shown in the presence ofincreasing concentrations of butanol. The peak height,

Fig. 12. (a) AC polarograms (24 mV peak-to-peak AC potentialat 200 Hz superimposed onto DC ramp) for the reduction at 25◦Cof 1.0× 10−3 M Cd2+ at a DME in 1.0 M ZnSO withn-butanolconcentrations of 0 M (A), 22× 10−3 M (B), 44× 10−3 M (C), and88× 10−3 M (D), (b) DC polarograms of the same system as in(a). (Reproduced by courtesy from Anal. Chem. 49 (1977) 1805).


Fig. 13. Difference in the DPASV response for the determinationof Cd, Pb and Cu in zinc electrolyte when a flow through cell isused and when the matrix exchange electrolyte is changed from(a) 0.1 M NaNO3 to (b) 0.1 M KCl.

which is expected to be linearly related to the concen-tration of cadmium ions when using a highly sensitivetransient polarographic technique, can be altered sub-tly by the presence of a surfactant [8]. In contrast,limiting currents obtained by DC polarography are notinfluenced by this kinetic effect [8] as shown in Fig.12b. The solution exchange method developed foron-stream determinations also avoids a matrix effectof the kind shown in Fig. 12 (see Section 5.1.1.3).

5.1.1.3. Examples of the use of matrix exchange.Fig. 13 shows the significance of changing the ex-change electrolyte from 0.1 M NaNO3 to 0.1 M KCl,when determining Cd, Pb and Cu. The relevant peakpotential data in both electrolyte exchange mediafor these and other elements are contained in Table5. Obviously, matrix exchange can be used easilywith carefully selected exchange electrolytes so thatstripping occurs in a medium that minimises the pos-sibility of having overlapping peaks, and therefore,maximises the prospect of an interference free de-termination. Fig. 14a and b, respectively, show the

Table 5Dependence of the DPASV peak potentials (Ep) for different metalson the nature of electrolytea

Metal ion Ep (V vs. Ag/AgCl)

1 M KCl 1 M NaNO3

Cd −0.68 −0.63Cu −0.20 −0.01Sb −0.20 −0.11Pb −0.47 −0.44Tl −0.52 −0.49

a Data obtained from [11] where experimental details are avail-able.

DPASV long time scale monitoring of Cd and Cu inzinc plant electrolyte when the exchange electrolyte is1 M KCl. Fig. 14c shows the results of similar DPASVfor the simultaneous determination of Cd, Cu, Sb(III)and Cu when 1 M NaNO3 is the exchange electrolyte,whereas Fig. 14d gives an example of the determina-tion of Ni and Co by DPAdSV when the exchangeelectrolyte is the medium described in Section 5.2.

5.1.2. Details of the on-line determination ofcadmium, copper, antimony and lead in zinc plantelectrolyte3

The DPASV method may be used for the simul-taneous determination of cadmium, copper, lead, andantimony in zinc plant electrolyte by using matrix ex-change techniques in a flow-through configuration toachieve ease of automation, adequate resolution andavoidance of interference from organic compounds(Section 4). The sequence of events that occurs dur-ing continual automated determination of the requiredelement is listed in Section 4.1.

All trace elements show similar responses when de-termined directly or after dilution with 0.05% H2SO4.However, acidification of the samples inhibits the pre-cipitation of hydroxides from the zinc electrolyte. Fil-tration to remove particulate matter and also dilutionwith acid prior to voltammetric analysis is used to re-duce the incidence of blockage of the sample linesfrom precipitated gypsum.

No increase in response was observed for all metalsof interest in samples UV-irradiated for 2 h, comparedto that in non-irradiated samples. This result impliesthat organic surfactants present in plant electrolyte do

3 Adapted from Anal. Chem. 60 (1988) 2445.


Fig. 14. Long-term monitoring of (a) 185 g l−1 cadmium, (b)95mg l−1 copper, (c) copper, antimony, lead and cadmium, and (d)cobalt and nickel, in zinc plant electrolyte using a matrix exchangecell configuration. Experimental parameters; (a) and (b) DPASV,the exchange electrolyte is 1 M KCl, (reproduced by courtesy fromAnal. Chem. 60 (1988) 2445) (c) and (d) DPAdSV, A and B arethe first and fiftieth runs, other conditions as per Table 7.

not cause interference because of the use of matrixexchange.

5.1.3. Determination of cadmium by DPASVIn cell feed solution, the determination of cadmium

by DPASV can usually be achieved with no interfer-ences when either 1 M KCl or 1 M NaNO3 is usedas the exchange electrolyte and with typical param-eters of deposition time, 30 s; zinc electrolyte flowrate, 2.4 ml min−1; pulse amplitude, 50 mV; durationbetween pulses, 1 s; scan rate, 5 mV s−1; pulse width,5 ms; temperature, 20± 2◦C. As shown from data inTable 5,Ep values of cadmium are well separated fromthose of other elements in both of these electrolyteexchange media. In principle, in the presence of verylarge concentration excesses of lead and thallium, thecadmium response may have to be measured on theshoulder of the lead and thallium responses, but inpractice, this problem rarely arises in cell feed solu-tions. The electrode process used for the determinationof cadmium is

Cd(II ) + 2e− Cd(Hg) (7)

Pulse times used in DPP, DPV and DPASV havebeen typically of the order of 50 ms to ensure a sat-isfactory Faradaic to charging current ratio [22]. Asignificantly increased cadmium response can be ob-tained by reducing the pulse time from 50 to 5 ms. Atcadmium levels of interest in even a relatively purecell feed electrolyte, the DPASV method gives a re-sponse that is orders of magnitude above the detectionlimit, and consequently, charging or background cur-rent is not an important consideration. The increasein the magnitude of the response available with shortpulse widths allows decreased deposition times andconsiderably faster overall measurement times sincethe scan rate can be increased to 50 mV s−1 or greaterwith the pulse width as short as 5 ms.

Excellent agreement was obtained for all cadmiumdeterminations in all high-purity cell feed solutions ex-amined with the automated method and from off-linedeterminations using DPP in a conventional labo-ratory situation. Typical cell feed levels encompassthe cadmium concentration range of 200–500mg l−1.Other zinc plant streams, which contain higher con-centrations of cadmium, are also amenable to con-tinuous monitoring via the method described in this


Table 6Automated monitoring of cadmium by DPASV in zinc plant electrolytea,b,c

Run no. Peak heightc (nA) Run no. Peak height (nA) Run no. Peak height (nA)

1 1265 31 1132 58 10982 1265 32 1165 59 10653 1148 33 1132 60 10984 1198 34 1265 61 10985 1098 35 1165 62 10986 1098 36 1165 63 11327 1165 37 1165 64 11658 1132 38 1232 65 1132

10 1082 39 1098 66 116511 1032 40 1132 67 106512 1065 41 1231 68 119813 1132 43 1198 69 103214 1132 44 1098 70 116415 1032 45 113216 1115 46 116518 1098 47 126520 1032 48 123123 1265 49 116524 1165 50 126525 1132 52 126526 1165 53 116527 1198 54 116528 1231 55 119829 1032 56 117730 1231 57 1132

a Peak potential is 680(±5) mV vs. Ag/AgCl.b Matrix exchange electrolyte is 1 M KCl. Other experimental parameters are available in [11] from which the data are derived.c Average peak height was 1150(±60) nA.

paper. Table 6 illustrates results obtained via automat-ically monitoring a zinc plant electrolyte of constantcadmium concentration for extended periods of timein an operator-unattended mode. Very occasionally, awidely erratic result is obtained that is attributable tomercury drop malfunction. These data are excludedvia the computer software in the control unit.

5.1.4. Determination of copper by DPASVCopper determinations in zinc plant electrolyte are

successfully achieved when the exchange electrolyteis 1 M NaNO3. In this medium, the stripping processis the result of the overall two-electron oxidation pro-cess:

Cu(Hg) Cu(II ) + 2e−

The copper stripping response in 1 M NaNO3 isgenerally observed without interference from anti-mony responses. However, if required, the antimony

response can be suppressed to enable determinationof small quantities of copper in the presence of largeamounts of antimony by selecting a deposition poten-tial between−0.25 and−0.1 V versus Ag/AgCl. In1 M KCl, the copper and antimony processes are notadequately resolved, as shown from data containedin Table 5. This determination of copper highlightsa great advantage of being able to employ matrixexchange techniques. As for cadmium, the copperresponse is also increased and still well defined atreduced pulse times. Thus, short pulse width, fastscan rate techniques can be used to minimise the timetaken for each copper determination This method canalso be used for the determination of copper in otherzinc electrolyte streams where the levels of copperare much higher. Reproducibility of data for unat-tended long-term monitoring of copper is similar tothat for cadmium. Typical cell feed levels are in the10–100mg l−1 range.


5.1.5. Determination of antimony by DPASVAs is the case with copper, antimony(III) can be

determined reliably by DPASV with 1 M NaNO3 asthe exchange electrolyte solution. In this medium, thestripping response is observed without interferencefrom the neighbouring copper process. However, ac-ceptable resolution of the copper and antimony pro-cesses is dependent on having only a low chloride con-centration in the exchange solution during the strip-ping stage so that contamination from chloride presentin the zinc plant electrolyte must be minimal. This cri-terion is satisfied by using the bottom drain cells de-signed for dense zinc plant electrolyte solutions (Fig.7). The magnitude of the antimony stripping response(Eq. (9))

Sb(Hg) → Sb(III ) + 3e− (9)

is highly dependent on the deposition potential andis greatly reduced if deposition occurs at a potentialmore positive than−0.3 V versus Ag/AgCl. Decreasedpulse times, which increase substantially the responsefor copper and cadmium, offer a smaller increase inthe peak height for antimony, consistent with the ir-reversibility of the stripping process. It is, therefore,recommended that pulse widths of the order of 50 msbe used for determining antimony, unless it is knownthat copper levels are low.

5.1.6. Determination of lead by DPASVThe determination of lead according to the re-

versible process

Pb(Hg) Pb(II ) + 2e− (10)

is often hindered in many commonly used electrolytesdue to similar half-wave potentials of lead, tin, andthallium. In acid chloride medium, Pilkington et al.[6] observed interferences in lead determinations inzinc plant electrolyte. At concentrations typical forcell feed solutions, lead can be determined in 1 M KClas the exchange solution without interference fromneighbouring elements, provided deposition occurs at−0.52 V versus Ag/AgCl. At this potential, the extentof the reduction process for Tl(I) is minimised. Thus,careful choice of deposition potential removes the thal-lium interference. With 1 M NaNO3 as the exchangeelectrolyte, no interference from tin is observed. Tinexists in oxidation state (IV) in the zinc electrolyte and

does not produce voltammetric waves in acidic sulfatemedia at potentials near the lead process, as is the casein acidic chloride or bromide media [21]. Short pulsewidths of 5 ms may also be used advantageously atfast scan rates of 50 mV s−1 with the reversible leadprocess, as is the case with cadmium and copper.

5.1.7. Simultaneous determination of cadmium,copper, lead and antimony(III) in zinc plantelectrolyte by DPASV

Fig. 14c shows that Cd, Cu, Pb and Sb(III) canbe determined simultaneously in zinc plant electrolytefrom a single DPASV experiment when 1 M NaNO3 isused as the matrix exchange electrolyte. A summary ofthe reproducibility associated with this measurementmethod is contained in Table 7.

5.2. Details of two methods for the on-linedetermination of cobalt in zinc plant electrolyte byDPAdSV techniques4

As shown in Section 5.1.7, an on-line DPASVvoltammetric method has been developed for the de-termination of cadmium, lead, antimony and copper.However, DPAdSV rather than DPASV methods arerequired for the development of an on-line method forthe determination of cobalt. The presence of cobaltin the zinc electrolyte leads to decreased current ef-ficiency, and hence, lower power efficiency duringthe deposition process. The maximum concentrationallowable in the zinc electrolyte is in the sub-mg l−1

cobalt range. Consequently, very sensitive methodsare required for the determination of cobalt.

The determination of cobalt by voltammetry atstationary mercury (HMDE) and polarography atmercury drop electrodes have been reported by manyworkers (see for example [30–33]). To overcome theproblems associated with irreversibility and poor sen-sitivity of stripping methods due to the low solubilityof cobalt in mercury [6,34], many workers have em-ployed complexing agents and utilised the reductionwave of the adsorbed cobalt complex. For example,adsorption and then reduction of the cobalt dimethyl-glyoxime (DMG) complex has been used to determinecobalt in solutions containing large excesses of othermetals [35–37].

4 Adapted from Anal. Chim. Acta 281 (1998) 281.


Table 7Long-term reproducibility of automated monitoring of trace metalsin zinc plant electrolyte

Simultaneous monitoring of Cd, Pb, Sb and Cua,b

Parameter Element

Cadmium Lead Antimony Copper

Ip (nA) 338± 6 128± 3 94± 3 329± 6E1/2 (mV) −610± 5 −435± 5 −160± 5 −20± 5

Simultaneous monitoring of Co and Nic,d

Cobalt Nickel

Ip (nA) 208± 10 45± 3E1/2 (mV) −975± 3 −870± 3

a Experimental conditions: technique, DPASV at an HMDEwith in situ matrix exchange; sample electrolyte, first stage pu-rification (old plant) diluted 1 : 1 with 0.05% (v/v) sulphuric acid;receiving electrolyte, 1 M sodium nitrate acidified to pH 3.5 withsulphuric acid; sample electrolyte deposition volume, 6.0 ml; celldrainage volume, 60 ml per run; deposition time, 100 s; depositionpotential, −850 mV final potential, +50 mV vs. Ag/AgCl; pulseamplitude, 50 mV; pulse duration, 50 ms; ramp step, 5 mV; rampduration, 0.5 s; temperature, 20± 2◦C; metal concentrations, Cu,140mg l−1; Sb, 140mg l−1; Pb, 140mg l−1; Cd, 200mg l−1.

b Runs = 50 (4% rejected).c Experimental conditions: technique, DPAdSV at an HMDE

with in situ matrix exchange; sample electrolyte, cell feed (newplant) diluted 1 : 1 with 0.05% (v/v) sulphuric acid and then 1 : 5with 0.05 M tri-sodium citrate, 0.4% (v/v) ammonia, 5× 10−4 MDMG, pH 6.4; receiving electrolyte, 0.1 M tri-sodium citrate, 0.1 Mammonium chloride, 0.04% (v/v) ammonia, 3× 10−3 M DMG, pH8.4; sample electrolyte deposition volume, 0.15 ml; cell drainage,25 ml per run; accumulation time, 5 s; accumulation potential,−600 mV; nitrogen purge time, 300 s; equilibration time, 10 s;pulse amplitude,−50 mV; pulse duration, 5 ms; ramp step,−3 mV;ramp duration, 1.0 s; temperature, 20± 2◦C; drop size, 0.45 mm2;metal concentrations, Co, 230mg l−1; Ni, 320mg l−1.

d Runs = 79 (4% rejected).

A range of off-line voltammetric methods has beendescribed for the determination of cobalt in the pres-ence of a large excess of zinc. Geissler and Da Maia[38] developed a method for determining cobalt in zincplant electrolyte solutions in a supporting electrolytewhich contained 0.1 M sodium citrate, 0.1 M ammo-nium chloride and DMG. It was found that cobaltcould be determined in the presence of zinc up toa Co : Zn ratio of 1 : 104 using this supporting elec-trolyte. However, this falls short of the required Co : Znratio of 1 : 106, which occurs in purified zinc plantelectrolyte. Schmidt et al. [39] improved the sensi-tivity for cobalt in this system by adding the sur-

factant 1-benzylsulfonyl-(N-morpholino)ethane to thesupporting electrolyte. Subsequently, Bobrowski [40]developed a method in which cobalt could be deter-mined in solutions containing the required zinc con-centration of 107 times higher than cobalt.a-Benzildioxime, which also forms a 2 : 1 complex with cobaltin the same manner as DMG, was used as the com-plexing agent. The complex adsorbs onto the mer-cury electrode surface when a suitable potential is ap-plied and the reduction potential is−0.98(±0.01) Vversus Ag/AgCl. The sensitivity and resolution rela-tive to the interfering zinc process may be further in-creased by the addition of the nitrite ion to the support-ing electrolyte [40,41]. This latter technique is calledcatalytic adsorptive stripping voltammetry (cat-AdSVor cat-DPAdSV) and has both the sensitivity and theselectivity necessary for the on-line determination ofcobalt in zinc plant electrolyte, without the need formatrix exchange.

5.2.1. An on-line cobalt DPADSV method withmatrix exchange

In the first on-line method developed for the de-termination of cobalt, a DPAdSV method is coupledwith in situ matrix exchange. Initially, in the matrixexchange method, the zinc plant electrolyte is sam-pled automatically from the process stream, filteredautomatically, diluted 1 : 1 with 0.05% H2SO4 toavoid precipitation in the sample lines that may other-wise occur during the cooling of the plant electrolyteand then transported pneumatically to the analyser.Zinc plant electrolyte is then further diluted in a reac-tion loop in a 1 : 5 ratio with 0.5 M tri-sodium citrate,0.4% (v/v) ammonia and 5× 10−4 M DMG at pH6.4. The cobalt-DMG complex is then adsorbed ontothe HMDE and then reduced using the voltammetricparameters and matrix exchange electrolyte describedbelow. This twice diluted sample is delivered bymeans of a burette system at a rate of 11 ml min−1

directly onto the mercury drop in the measurementcell (Fig. 7b), which contains the receiving matrixexchange electrolyte composed of 0.1 M tri-sodiumcitrate, 0.1 M ammonium chloride, 0.04% (v/v) am-monia and 3× 10−3 M DMG at pH 8.4. The differentDMG concentrations and pH values in both steps arecrucial to the success of the matrix exchange method(see below). The initial dilution steps mean that


adsorption of the Co–DMG complex takes place in amedium which contains a significantly lowered zincconcentration, relative to that in plant electrolyte, andthis feature also is important. The injected sample,after passing the electrode, sinks to the bottom of thecell because of its higher specific gravity than that ofthe receiving electrolyte, so that the only zinc presentduring the adsorptive stripping method is the residualamount left after matrix exchange.

During sample delivery, the potential of the mercurydrop is held at−0.6 V for 5 s and then for a further10 s with all flow stopped after the adsorptive depo-sition period is complete. A scan in the direction ofnegative potential using the differential pulse wave-form gives rise to a cobalt peak at−0.98(±0.01) Vversus Ag/AgCl (Fig. 14d). Results are transmittedto the computer for processing. Prior to the next de-termination, fresh receiving electrolyte is introducedinto the top of the cell to replace contaminated elec-trolyte which is drained from the bottom of the cell.A cell drainage of 25 ml per run was found to besuitable. No interference was encountered from thepresence of nickel in plant electrolyte. Under thesesame conditions, the nickel–DMG complex is reducedat −0.87(±0.01) V versus Ag/AgCl in the matrix ex-change method (Fig. 14d), but clearly, if present at rel-atively high concentrations, could cause interferences.However, in the Pasminco Hobart plant electrolytes,nickel levels have always been found to be too lowto cause interferences with the cobalt determination.Reproducibility and other data for the determinationof cobalt by the on-line matrix exchange method aresummarised in Table 7.

5.2.2. An on-line cobalt catalytic-DPADSV methodwithout matrix exchange

The even more sensitive and specific cat-DPAdSVmethod may be used without matrix exchange, usingthe procedure described below and typically using thefollowing instrumental parameters: initial potential,−0.5 V; final potential,−1.2 V vs. Ag/AgCl; nitrogenpurge time, 300 s; deposition time, 0 s (adsorption oc-curr during scan); equilibration time, 10 s; drop size,0.45 mm2; pulse duration, 50 ms; pulse amplitude,−50 mV; ramp step,−2 mV; scan rate, 10 mV s−1.On this occasion, a stationary cell is used, which forthe determination, contains 20 ml of supporting elec-

trolyte having the composition 1.3× 10−5 M a-benzildioxime, 1.0 M ammonium chloride, 0.5 M sodiumnitrite and 1.3% (v/v) ammonia at pH 9.4. The zincelectrolyte is transported automatically from the plantto the voltammetric analyser in the same way as de-scribed for the matrix exchange method. Prior to eachdetermination of cobalt, the cell is washed twice withwater, which in turn is drained from the cell bottomoutlet to ensure no residual contamination from theprevious run. For the determination of Co, 20 ml ofcatalytic supporting electrolyte in the cell is combinedwith 200ml of acidified plant electrolyte sample froma sample loop and transferred to the analysis cell.The solution is purged with nitrogen to remove inter-fering oxygen. A new mercury drop is then formedat the HMDE and a potential of−0.5 V vs. Ag/AgClwas applied to the drop. After a 10 s equilibrationperiod, a negative potential scan of the voltage us-ing the differential pulse waveform yields a peak at−0.98(±0.01) V (Fig. 15a) which corresponds to thecatalytic reduction of the adsorbed cobalt complex.The calibration curve is shown in Fig. 15b, with thereported analytical data being confined to the linearregion. Results of the experiment are transmitted tothe central Plant computer for processing. The cell isthen drained, flushed and refilled with water and thecycle recommenced for the determination of cobalt inthe next sample delivered from the process stream.

5.2.3. Comparison of DPAdSV with and withoutmatrix exchange

The major difficulty to be overcome in determiningcobalt by DPAdSV is the interference caused by theneighbouring zinc reduction process. The in situ ma-trix exchange method needs to minimise the zinc inter-ference and this has been achieved as clearly shown inFig. 16a, where DPAdSV curves obtained in station-ary (no matrix exchange), and flowing solution (withmatrix exchange) configurations are compared.

A further benefit of using in situ matrix exchangein the flowing solution mode is the decreased time re-quired for the determination of low concentrations ofcobalt because of the enhanced efficiency of the ac-cumulation step. For example, a deposition time ofonly 5 s is required to give adequate sensitivity underflowing solution, matrix exchange conditions. In con-trast, for a stationary solution experiment, the depo-


Fig. 15. Determination of Co in purified zinc plant electrolyte bythe on-line cat-DPAdSV method. (a) Cat-DPAdSV Co responsewith 3× 5mg l−1 additions of a cobalt standard, (b) cobalt cali-bration curve, (c) comparison of cobalt results obtained with theon-line voltammetric analyser using (d) the catalytic method and(j) an off-line spectrophotometric method. (Reproduced by cour-tesy from Anal. Chim. Acta 28 (1993) 281).

Fig. 16. (a) DPAdSV voltammograms obtained for the determina-tion of cobalt and nickel in purified zinc plant electrolyte using(1) a stationary cell and (2) the bottom-drain cell incorporatingthe matrix exchange procedure, (b) calibration curve obtained withthe matrix exchange method for the determination of cobalt. (Re-produced by courtesy from Anal. Chim. Acta 281 (1993) 281).

sition time for achieving the same sensitivity (signalto background) is in the minutes time regime. Fur-thermore, while it was found necessary to perform a300-fold dilution of the zinc plant electrolyte to enablethe determination of cobalt in the conventional station-ary cell, with the on-line method, a dilution ratio ofonly one part zinc plant electrolyte to five parts diluentcontaining 0.05 M tri-sodium citrate, 0.4% ammoniabuffer and 5× 10−4 M DMG can be used to achievethe required lowering of the zinc concentration. Thissmall dilution also retains the necessary difference be-tween the specific gravity of the sample and that of the


receiving electrolytes as required to allow efficient op-eration of the bottom-drain cell.

The matrix exchange, bottom-drain flow-cell com-bination also provides the opportunity for decreasingthe concentration of tri-sodium citrate required to shiftzinc reduction to sufficiently negative potentials. Thisfeature minimises the decrease in the cobalt responsewhich occurs with increasing concentration of thisreagent [38]. Better sensitivity for cobalt can, there-fore, be achieved whilst retaining the necessary dis-crimination from zinc. Another advantage of the ma-trix exchange method is the ability to be able to varythe pH of the diluent and receiving electrolytes. Theoptimum pH for determining cobalt is between 8 and9, but basic precipitates are also formed in this pH re-gion in zinc plant electrolyte. To overcome this prob-lem, the adsorption step is undertaken at pH 6.4 whichsuppresses the formation of precipitates but still al-lows complexation of cobalt with DMG. The reduc-tion of the adsorbed Co–DMG complex is then per-formed at pH 8.4 after the majority of the zinc elec-trolyte has been removed by matrix exchange. Finally,since the matrix exchange method is operated with ashort deposition time, saturation coverage of the mer-cury drop occurs at a higher concentration than with astandard stationary solution experiment. This extendsthe linear analytically useful concentration range (Fig.16b) and hence the upper detection limit for cobaltto 600mg l−1 which allows the range of interest forlow-purity zinc electrolyte to be covered with a sin-gle calibration curve. At concentrations higher than600mg l−1, saturation coverage of the electrode sur-face causes curvature of the calibration graph to beobserved (Fig. 16b).

5.2.4. Features of the cat-DPAdSV methodThe major benefit obtained with this catalytic

method is that it is so much more sensitive and specificfor cobalt that there is no need to use matrix exchangeto eliminate zinc interference. The complexation, ad-sorption and reduction of the cobalt–benzil dioximecomplex accumulated by adsorption gives rise toenhanced sensitivity over the Co–DMG method. Ad-dition of nitrite to the supporting electrolyte resultsin a further 10-fold increase in sensitivity. Nitrite is astrong oxidising agent and is believed to play a rolein the regeneration of Co2+ by oxidising the elec-

trochemically reduced form, hence giving rise to acatalytic effect [41]. However, the mechanism of thisprocess is not completely understood.

The concentration of ammonium chloride is crucialin minimising the interference of the zinc reductionwave. A concentration of 1.0 M ammonium chloridewas chosen because it gave peak potential separationsof 250 mV for the cobalt and zinc reduction processes.Fig. 15a clearly shows that excellent separation ofthe cobalt and zinc reduction peaks is achieved eventhough the ratio of cobalt to zinc is of the order of1 : 106.

The linear range for the determination was found toextend from the detection limit of 0.25 to 30mg l−1.This range is achieved by keeping the adsorption timeto a minimum and using a large HMDE drop size of0.45 mm2, which was the largest stable drop size thatcould be obtained in the supporting electrolyte withthe Metrohm electrode. The adsorption time used is10 s, which was, in fact, the equilibrium period be-fore commencement of the negative potential directionscan. Above a concentration of 30mg l−1, saturationof the electrode occurs which leads to curvature of thecalibration graph (Fig. 15b).

5.2.5. Overview of methods used in the on-line,on-stream determination of cobalt

Both the matrix exchange and the catalytic DPAdSVmethods have been incorporated into on-line monitor-ing programs developed at the Pasminco Hobart plant.In the new plant, a cobalt determination is performedon zinc electrolyte in both purification tanks and puri-fied electrolyte samples three times an hour, 24 h a day.The decision as to whether further purification of theelectrolyte is required is based on the result, and so,high accuracy is essential. The supporting electrolyteis made up in 20 l batches which last for 2 weeks.The electrode set-up is reliable and requires very lit-tle maintenance, with the only manual input being thechanging of the electrode capillary and the filling ofthe reservoir with mercury. A self-calibrating systemfully eliminates operator intervention. Fig. 15c showsa graphical comparison of cobalt cat-DPAdSV resultsobtained from an on-line voltammetric analyser andthe off-line manual spectrophotometric method [42]performed by shift chemists at the plant. Results ofthis kind obtained over several years have shown that


cobalt can be monitored routinely in an on-line modein zinc plant electrolyte using DPAdSV methods withan error of approximately±4%, based on comparisonswith a reference off-line spectrophotometric method.The catalytic method is required to determine cobaltin purified zinc electrolyte where the cobalt levels arein the sub- mg l−1 range. The flow-based DPAdSVmethod with in situ matrix exchange can be used todetermine cobalt in zinc electrolyte from purificationtanks where the cobalt concentrations are higher andin the milligram per litre range. The main objective ofperforming routine on-line cobalt determinations con-tinually, with high precision and with minimal opera-tor intervention, has been achieved.

5.3. Determination of nickel in zinc plant electrolyte5

The routine monitoring of nickel may be requiredin the course of the electrolytic zinc process, althoughin practice, as noted in Section 4.2.2.2 in the newPasminco Hobart Plant, this has not been found to benecessary because the levels are too low.

If required, the voltammetric determination of lowconcentrations of nickel would ideally be undertakensimultaneously with cobalt using the DMG adsorptivestripping method at the HMDE [43–48]. Reliable re-sults in stationary solution configurations (non-matrixexchange) are generally obtained when the levels ofcobalt and nickel are in close proximity. However,when the concentration of cobalt exceeds that of nickelby more than 10-fold, the reduction processes for thetwo metals may overlap [49]. Additionally, competi-tion with cobalt and zinc for the dimethylglyoximeligand could also result in an error in the determi-nation of nickel under conditions prevailing in zincplant electrolyte. Consequently, methods of minimis-ing both zinc and cobalt interference must be ad-dressed in the methodology developed for the on-linedetermination of nickel in zinc plant electrolyte. Fortu-nately, it emerged that the matrix exchange method de-scribed above for the determination of cobalt achievedthis objective, which makes the simultaneous determi-nation of both elements possible.

5 Adapted from Analyst 119 (1994) 1057.

5.3.1. On-line matrix exchange-voltammetricprocedure for the simultaneous determination ofnickel and cobalt

As noted when discussing the determination ofcobalt, the use of the in situ matrix exchange methodcombined with dilution and inclusion of citrate into thesupporting electrolytes minimises interference fromzinc by shifting the zinc reduction wave to a morenegative potential (Fig. 16b). However, unfortunately,the introduction of citrate also can lead to suppressionof the nickel signal [17,38]. Hence, a compromisebetween sensitivity and resolution is required for thesimultaneous determination of both cobalt and nickeland the optimum concentration of citrate for this pur-pose has been found to be (see Section 5.2.1) 0.05 Mfor the diluent and 0.1 M for the receiving electrolyte.

A major problem usually encountered when deter-mining nickel by the DMG method without matrix ex-change procedures are not available arises if the con-centration ratios of nickel and cobalt vary, which iscertainly the case in zinc plant electrolyte. Owing tothe competition between nickel and cobalt for DMG,an increase or decrease in the concentration of eitherelement introduces errors into the determination. Thus,an increase in the nickel concentration leads to a smalldecrease in the cobalt response. In contrast, increas-ing the cobalt concentration to a 10-fold excess overnickel, which can occur commonly in zinc plant elec-trolyte, decreases the nickel signal by approximately50% when using the conventional DPAdSV method ina stationary solution configuration. Use of matrix ex-change, which readily allows both the DMG concen-tration and the pH to be changed between the adsorp-tion and stripping processes, minimises this problem.The problem of cobalt interference is, in fact, min-imised in the matrix exchange method by the increasein the pH of the receiving electrolyte which increasesthe nickel signal, retards the cobalt signal and leadsto an increased separation between the two processes.However, the pH of the initial receiving electrolytecannot be set at too high a value as the formation ofbasic precipitates would occur in alkaline medium onthe addition of zinc electrolyte. Consequently, the dilu-ent pH is 6.4 where complexation and adsorption takeplace, while the pH is 8.4 in the receiving electrolytewhere the stripping of the adsorbed complex occursafter matrix exchange. The on-line method with ma-trix exchange can, therefore, be used routinely on a


24 h per day basis in the plant situation for the deter-mination of nickel, as well as cobalt. Reproducibilityand other data for this determination are summarisedin Table 7.

The linear range for nickel determination, usingthe matrix exchange method, is 60 (detection limit)to 600mg l−1. At higher nickel concentrations, curva-ture of the calibration graph is encountered (Fig. 16b).However, nickel concentrations of interest in mostprocess streams fall within the linear range (up to400mg l−1 of nickel in some solutions). In some highlypure electrolytes, nickel levels are below 60mg l−1 andcannot be determined by the on-line method. Fortu-nately, concentration levels this low are not a problemfor the electrowinning process.

6. Off-line methods for the determination ofdifferent oxidation states of metal ions present inzinc plant electrolyte

AAS and other spectroscopic techniques generallyprovide information on the total metal concentra-tion in solution. In contrast, voltammetric techniquescan be used to distinguish between the different ox-idation states of metals such as Fe(II)/Fe(III) andSb(V)/Sb(III). Access to this information may repre-sent a significant advance in the monitoring of zincplant performance.

6.1. Polarographic determination of total iron,iron(III) and iron(II) in zinc plant electrolyte6

Iron occurs in leach solutions used for zinc produc-tion at concentrations in excess of 20 g l−1, the dom-inant oxidation state being iron(III). At the PasmincoHobart Plant, and at the time of this study, it was re-moved partially from iron rich solutions as jarosite, abasic iron sulphate. However, iron was incompletelyremoved by this process and an additional iron removalstep was required which involved the precipitation ofhydrated iron oxide at pH 5.2, under aerated condi-tions, so that, at this stage, any iron(II) was oxidisedto iron(III). Iron-purified solutions obtained via theirmethod typically contain 1–50 mg l−1 iron partly insolution and partly as suspended particulates; the plant


operation ideally involving the 1–10 mg l−1 range, asis the case with new purification methods presentlyused.

Polarographic methods for the determination of ironhave been developed for many matrices [50,51]. How-ever, application to zinc plant electrolyte presents arange of problems not usually encountered in othermatrices. For example, the widely used oxalic or tar-tric acid electrolytes, in which a reversible well de-fined Fe(III) + e− Fe(II) response is observed [52],are unsatisfactory because the presence of saturatedsolutions of calcium sulphate in the zinc plant elec-trolyte leads to the precipitation of calcium oxalate ortartrate salts. Additionally, the presence of many elec-troactive species other than iron means that achievingadequate resolution of the iron response is difficult.Furthermore, standard complexing agents which mightbe used to mask the interferences in zinc plant elec-trolyte may not work because they complex preferen-tially with the large excess of zinc rather than with ironor interfering elements of interest. In Section 6.1.1,the results of an investigation into the development ofa reagent combination are presented, which enablesthe determination of total iron in zinc plant electrolyteand information on the distribution between iron(III)and iron(II) to be obtained.

6.1.1. Determination of total iron in zinc plantelectrolyte by differential pulse polarography

Ideally, for the sensitive determination of total iron,a technique such as DPP, and a reversible process (Eq.(11)) of the kind found in oxalate, tartrate or citratemedia would be used [22]:

Fe(III ) + e− Fe(II ) (11)

The differential pulse method, as used with mostforms of instrumentation, gives the same response forthe reduction of Fe(III) as for the oxidation of Fe(II) ifthe electrochemical process being used for the deter-mination is reversible [22]. In contrast, methods suchas current sampled DC polarography, which have thecapability of measuring the sign of the current andhence the oxidation state may be used to distinguishand hence determine iron(II) and iron(III) [22].

Whilst, as noted above, neither oxalate or tartrateelectrolytes can be used for the determination of ironin zinc plant electrolyte because of the precipitation


of insoluble calcium salts, citrate electrolytes do notsuffer from this problem. To achieve adequate resolu-tion of the iron and copper responses, EDTA, whichshifts the copper reduction to significantly more neg-ative potentials than the iron process, is added to thezinc plant electrolyte solution, along with a buffer tocontrol the pH value. However, the concentrations ofcitrate and EDTA as well as the pH and buffering ca-pacity must be controlled very carefully as theE1/2values of iron, as well as copper, cadmium and otherpotential interferents, are all very sensitive to varia-tions in these parameters. Additionally, the volume ra-tio of zinc plant electrolyte to analytical reagents usedfor the determination is critical. The high concentra-tions of zinc in the plant electrolyte need to be dilutedprior to the determination of iron, so that complexationwith the vast excess of zinc does not preclude suffi-cient complexation taking place with iron and copper.

After an extensive investigation of the influence ofthe variation of pH and buffer capacity and the con-centration of citrate and EDTA as well as the ra-tio of volume of zinc plant electrolyte to that of thereagent, the following procedure for total iron is rec-ommended: an aliquot of zinc plant electrolyte (acidicor neutral) is added to 20 ml of reagent solution (0.1 Msodium citrate–0.1 M EDTA) and adjusted to pH 6.0with ammonia. After 5 min of degassing of the so-lution with nitrogen to remove oxygen, a differen-tial pulse polarogram (drop time = 0.8 s, pulse ampli-tude =−50 mV) recorded over the potential range of0.10 to−0.30 V versus Ag/AgCl gives an extremelywell defined response with a peak potential,Ep, of–0.09± 0.02 V versus Ag/AgCl. Additionally, copperand cadmium may be determined simultaneously withtotal iron via the use of their well resolved reductionprocesses which have peak potentials at−0.37± 0.02and−0.59± 0.02 V versus Ag/AgCl, respectively, ifthe potential range is extended. DPV at an HMDE maybe used as an alternative to the polarographic methodin order to conserve mercury.

In complexing reagent solution only (no zincpresent), an iron(II) standard gave a linear calibra-tion graph for the oxidation process with a slope of75 nA l mg−1 and a correlation coefficient of 0.9998(n= 6) via standard regression analysis over the range1–20 mg l−1. The corresponding slope and correlationcoefficient values for a calibration graph preparedfrom the reduction of iron(III) standard solution were

Fig. 17. Simultaneous determination of total (a) iron, (b) copperand (c) cadmium by (a,b) DPP (pulse amplitude,−50 mV; droptime, 0.8 s); and (c) DPASV (pulse amplitude, 50 mV; durationbetween pulses, 0.6 s; plating time with stirring, 20 s; equilibra-tion time, 5 s) in high iron ‘acidic’ zinc plant electrolyte. Curves(lowest to highest) refer to addition of 0, 1, 2 and 3× 200ml of1.00 g l−1 iron(II), copper and cadmium standards. Other experi-mental parameters are available in [14]. (Reproduced by courtesyfrom Anal. Chim. Acta 277 (1993) 148).

−72 nA l mg−1 and 0.9993 (n= 6), respectively. Thesedata demonstrate that total iron can be determinedby use of either an iron(III) or an iron(II) standard.However, in practice, the method of standard addi-tions had to be used in preference to direct calibrationto minimise matrix problems which were found tovary considerably in samples obtained from differentstages of zinc production.

Fig. 17 shows responses for the simultaneous de-termination of total iron, copper and cadmium when1 ml of high iron ‘acidic’ zinc plant electrolyte con-taining 10 g l−1 sulphuric acid is added to 20 ml ofthe reagent solution. Extremely well defined and sep-arated peaks were obtained for each metal. The cad-mium concentration in this particular sample was de-termined by DPASV as the concentration was too lowto be determined accurately by polarography. Fig. 18shows voltammetric curves obtained on the low iron‘neutral’ zinc plant electrolyte in which 5 ml of sample


Fig. 18. Simultaneous determination of total (a) iron, (b) copper and(c) cadmium by DPP (pulse amplitude,−50 mV; drop time, 0.8 s)in low iron ‘neutral’ zinc plant electrolyte. Other experimentalparameters are available in [14]. (Reproduced by courtesy fromAnal. Chim. Acta 277 (1993) 148).

was added to 20 ml of reagent solution. In this sam-ple, which involves less dilution of plant electrolyte,the polarographic method could be applied directly tothe determination of cadmium as well as the other el-ements, provided the method of standard addition isused for calibration to remove matrix effects. How-ever, the determination of cadmium and copper can beachieved efficiently by the on-line method describedin Section 5.1.2 where the matrix variation problem isremoved by the matrix exchange procedure which canbe used for metals that can be reduced to the metallic(amalgam) state at an HMDE.

6.1.2. Determination of iron(II) and iron(III) in zincplant electrolyte by DC polarography

The differential pulse method described above mayonly be used for the determination of iron(II) andiron(III) when the relevant oxidation and reductionelectrode processes are irreversible, and therefore,well separated because otherwise no distinction be-tween oxidative and reductive components of theexperiments is possible. However, DC polarogra-

phy may be used with a reversible process for thispurpose because the reduction current representsthe iron(III) concentration as measured from theFe(III) + e− Fe(II) component of the experimentand the oxidation current represents the iron(II) con-centration as measured by the Fe(II)Fe(III) + e−component.

To determine total iron as well as iron(II) andiron(III) in a single experiment using an efficient cali-bration procedure, a 1.5 ml aliquot of zinc electrolytemay be added to 20 ml of 0.1 M sodium citrate–0.1 MEDTA reagent and the pH adjusted to 6.0 with ammo-nia. DC and DP polarograms are then recorded eithersimultaneously [53] or sequentially on the same so-lution as in [14]. Standard addition experiments withiron(II) are then used to determine both total iron andiron(II) and standard addition with iron(III) is usedto determine iron(III). The baseline reference point(or zero Faradaic current) in the DC experiment isascertained from experimental data obtained by using1.5 ml of a synthetic zinc solution made from analyt-ical reagent grade zinc sulphate at the approximatezinc concentration present in the plant electrolyte andat the same pH as the solution to be determined.

Fig. 19 illustrates the responses obtained from themethod as applied to a high iron acidic zinc plantelectrolyte solution. In the first experiment, the DP

Fig. 19. Simultaneous determination of (a) total iron by DPP,and (b) iron(II) and iron(III) by DCP with sequential addition ofiron(II) and iron(III) standard solutions. Experimental parametersare available in [14]. (Reproduced by courtesy from Anal. Chim.Acta 277 (1993) 148).


and DC polarograms are recorded. Two standard addi-tions of iron(II) followed by two standard additions ofiron(III) then enable total iron, iron(II) and iron(III) tobe determined. Linear calibration plots are obtained.In the solution used to obtain the data presented inFig. 19, four determinations gave values (mean± SD)of 1032± 48, 50± 12 and 907± 53 mg l−1 for totaliron, iron(II) and iron(III), respectively. For this so-lution, iron is predominantly in oxidation state (III).Furthermore, when the method was applied to the‘low iron’ neutral zinc plant electrolyte, no iron(II)could be detected within the limit of experimental er-ror. The validity of the total iron measurement methodwas confirmed by comparison of data obtained polaro-graphically with established spectrophotometric meth-ods [54,55].

6.2. DPASV and DPAdSV techniques for thedetermination of total antimony, antimony(III) andantimony(V) in zinc plant electrolyte7

6.2.1. Determination of total antimony andantimony(III) in hydrochloric acid media by DPASV

One of the target impurities during the course of thezinc dust precipitation purification step is antimony[56]. Both antimony(III) and (V) are detrimental to theefficient electrolysis of zinc, although their impact onzinc plant operation is different [57]. Thus, monitor-ing of antimony levels is important in maintaining anefficient plant operation, and ideally, quantification ofboth oxidation states would be included in the plantmonitoring program.

In acidic halide containing electrolytes, it has beenwell established that antimony(III) can be reversiblyreduced to the elemental (amalgam) state at a mercuryelectrode:

Sb(III ) + 3e− Sb(Hg) (12)

The reverse process gives rise to a sensitive strippingmethod capable of detecting antimony at themg l−1 orlower level. In principle, antimony(V) may be reducedin two steps:

Sb(V) + 2e− → Sb(III ) + 3e− Sb(Hg) (13)

7 Adapted from Electroanalysis 9 (1997) 13 and Anal. Chim.Acta 372 (1998) 307.

Thus, if the two-step reduction (Eq. (13)) is avail-able, it also follows that a very sensitive strippingmethod is available for the determination of anti-mony(V). However, since this determination wouldalso utilise the Sb(Hg)Sb(III) + 3e− stripping pro-cess, this kind of stripping experiment does notdistinguish between the two oxidation states, so to-tal antimony would be determined. In contrast, ifan electrolyte can be found where antimony(III) iselectroactive but antimony(V) is electronactive, astripping method which is specific for antimony(III)will become available. Thus, via access to both kindsof electrolytes, total and antimony(III) concentrationscould be determined directly with the antimony(V)concentration being given by the difference.

Fig. 20a shows typical Sb(III) and Sb(V) DPASVstripping curves obtained in concentrated 5 M hy-drochloric acid media. Data contained in [19] showthat HCl concentrations greater than 4 M producemaximum peak current stripping signals irrespectiveof whether antimony(III) and (V) are present in solu-tion. An HCl concentration of 5 M is, therefore, idealfor determining total antimony. In contrast, HCl acidconcentrations less than 5 M lead to a rapid deteri-oration in the antimony(V) signal, while the signalfrom antimony(III) only decreases significantly foran HCl concentration below about 0.2 M. Use ofa 0.1 M HCl concentration, therefore, provides thepossibility of a method for specifically determiningantimony(III).

As may be concluded from the above data, a verysuitable medium for determining total antimony isfound by using a 1 : 1 mixture of zinc plant electrolytesample and concentrated 10 M hydrochloric acid [6].Fig. 20b shows that a well defined response is ob-tained when using a Pasminco Port Pirie secondarypurification filtrate sample diluted 1 : 1 with 10 M HCl.Other zinc plant electrolytes available at either the Pas-minco Hobart or Port Pirie Plants gave equally welldefined DPASV responses. A suitable calibration plotcan be prepared via use of either Sb(III) or Sb(V) stan-dards by using a 1 : 1 mixture of 1 M ZnSO4 or highlypurified plant electrolyte and 10 M concentrated hy-drochloric acid or else the method of standard addi-tions can be used as is the case in Fig. 20b. In theexample given in Fig. 20b, the total antimony concen-tration was determined to be 0.25± 0.01 mg l−1. Thesame sample, when monitored by the Pasminco Port


Fig. 20. (a) DPASV curves obtained in 5 M HCl when (A) 15 mg l−1 antimony(III) and (B) 15 mg l−1 antimony(V) is in solution. (b)Determination of total antimony by DPASV in Pasminco Port Pirie secondary purification filtrate in a 1 : 1 mixture of plant electrolyte and10 M HCl. Curve (A) sample response; (B) addition of 50 mg l−1 Sb(III); (C) further addition of 50 mg l−1 Sb(V); (D) further addition of50 mg l−1 Sb(III). Conditions: (a) deposition time, 10 s; pulse amplitude, 50 mV; pulse time, 200 ms; scan rate, 20 mV s−1 and depositionpotential,−0.40 V; (b) as for (a), but with deposition time, 30 s, deposition potential,−0.33 V. (Reproduced by courtesy from Electroanalysis9 (1997) 13).

Pirie on-stream analyser, gave a total antimony con-centration of 0.28 mg l−1.

Antimony(III) in zinc plant electrolyte may bereadily determined by preparing 1 : 1 mixtures ofplant electrolyte and 0.2 M HCl and using the DPASVmethod. In this case, antimony(III) standards must beused and it needs to be noted that a small system-atic but calculable error is present because Sb(V) isnot 100% electroinactive at this HCl concentration[19]. Antimony(V) is then determined from the dif-ference in total antimony and antimony(III). The useof 1 M NaNO3 as the matrix exchange electrolyte,as described in the on-line method discussed in Sec-tion 5, also provides a determination of the Sb(III)concentration.

6.2.2. Determination of antimony(III) by DPAdSV inthe presence of antimony(V) in zinc plant electrolyte

A DPAdSV method has been developed byHenze and co-workers for the determination of anti-mony(III) and antimony(V) in water and phosphoricacid samples [58]. The adsorptive stripping methodutilises a complexing ligand known as chloranilicacid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone)(Structure 1) and the DPAdSV method is simply un-dertaken by addition of the appropriate amount ofchloranilic acid, which acts as the complexing agent,to acidified sample solutions. However, as zinc plantelectrolyte contains a very large concentration excessof zinc over antimony, it was necessary, as usual, to as-certain if the DPAdSV method for the determination of


antimony(III) and (V) could tolerate the very high zincconcentration encountered in these types of samples.

Structure 1.

At a Zn(II) concentration of 65 g l−1, the anti-mony(III) peak current decreases to 23% of the valueobtained in the absence of zinc. However, adequatesensitivity is still available for the determination ofantimony(III) in zinc plant electrolyte where con-centrations in the range 20 to 2000mg L−1 are ofinterest. The DPAdSV antimony(V) response is notas severely affected by the addition of Zn(II) as is theantimony(III) response, with only a 11% decrease inpeak current being found for a Zn(II) concentration of65 g l−1. However, while a well defined antimony(V)response is found in zinc electrolyte, problems arise.For instance, the fact that the peak potential and peakcurrent obtained for solutions of antimony(V) are sig-nificantly affected by the addition of small amounts ofantimony(III) means that the DPAdSV method for thedetermination of antimony(V) based on chloranilicacid is unsuitable for use in zinc plant electrolytemedia.

On the basis of results obtained in [20], some ofwhich are summarised above, the DPAdSV methodfor the determination of antimony in zinc electrolytewas found to be suitable only for the determination ofantimony(III). Fig. 21 shows the DPAdSV responseof antimony(III) in zinc electrolyte media. Succes-sive additions of antimony(V) over the concentrationrange 1–30mg ml−1 gave no additional response, norwas the antimony(III) response modified, confirmingthat the method is selective for antimony(III) in zincelectrolyte.

6.2.3. Recommended method for the interference-freedetermination of total antimony, antimony(III) andantimony(V) in zinc plant electrolyte

The initially described DPASV method for thedetermination of antimony(III) requires the addi-

Fig. 21. DPAdSV response obtained from a 1 M ZnSO4 (pH 3)solution containing 8mg ml−1 antimony(III) and 5× 10−5 M chlo-ranilic acid. Conditions: deposition potential, 100 mV vs. Ag/AgCl;initial potential, −200 mV; final potential,−700 mV; depositiontime, 15 s; pulse amplitude, 50 mV; scan rate, 20 mV s−1. (Repro-duced by courtesy from Anal. Chim. Acta 372 (1998) 307).

tion of 0.2 M HCl to the plant electrolyte [19].However, as noted above, antimony(V) is not com-pletely electroinactive under these conditions and themethod is subject to a small positive but calcula-ble (<10%) error for antimony(III) determinations[19]. In contrast, the DPAdSV method for the de-termination of antimony(III) has been found to becompletely interference free as is the DPASV methodfor total antimony after a 1 : 1 dilution of plantelectrolyte and 10 M hydrochloric acid [19]. Thus,it is now recommended that antimony(III) shouldbe determined by the DPAdSV method, total anti-mony by the DPASV method and antimony(V) bydifference.


7. Off-line voltammetric methods for thedetermination of other elements in zinc plantelectrolyte using a range of separation methods toachieve adequate resolution

As noted in most of the examples described in Sec-tions 5 and 6, the ‘trick’ in trace metal determinationin zinc plant electrolyte by voltammetry is to elimi-nate interferences that arise from the very high zincconcentration and other elements present that produceoverlapping responses. In Section 7, novel methods ofachieving the required resolution in the determinationof germanium and arsenic are emphasised.

7.1. Determination of germanium withelectrochemical separation of interfering ions8

Levels as low as parts per billion of germaniumdecrease the current efficiency for zinc deposition,causing the total or partial dissolution of zinc duringelectrolysis and/or lowering of the purity of the zincdeposited. One of the main reasons for the decreasein current efficiency is the catalysis of the reductionof hydrogen ions from the acidic electrolyte media,which causes the evolution of hydrogen. Mackinnonet al. reported that impurities such as germanium arehydride formers and can facilitate the reduction of hy-drogen ions [59] so that monitoring of germanium iscrucial in maintaining efficient plant operation. Whilstthe irreversible electrochemical characteristics associ-ated with the direct reduction of germanium(IV) [60]have usually made DPASV unsuitable for determiningtrace concentrations of this element, recent publica-tions have shown that germanium can be determinedaccurately by DPAdSV down to sub parts per billionlevels. The main difficulty when determining low con-centrations of germanium by DPAdSV in complex ma-trices such as zinc plant electrolyte is the necessityto avoid interferences from Cd(II), Pb(II) and Zn(II)ions. Thus, methods for the removal of these poten-tially interfering elements encountered in zinc plantelectrolyte have been developed.

An important criterion for ligands to be suitable forthe AdSV determination of germanium appears to bethe presence of at least two hydroxy groups whichare at positionsortho to each other [61–63]. These

8 Adapted from Electroanalysis 10 (1998) 387.

Fig. 22. Structure of ligands examined for the determination ofgermanium in zinc plant electrolyte. (a) Catechol, (b) fluoroneblack, (c) pyrogallol, (d) PCV, (e) alizarin red, (f) quercetin.(Reproduced by courtesy from Electroanalysis 10 (1998) 38).

hydroxy sites are used to complex germanium priorto adsorption of the metal complex onto the HMDE.Fig. 22 contains the structures of six ligands examinedfor the determination of Ge(IV) by DPAdSV. Cate-chol and pyrogallol have been used previously [61,62]while four new ligands, alizarin red, pyrocatechol vi-olet (PCV), fluorone black and quercetin, have beenconsidered in our work. The suitability of a ligand inthe context of these studies was based on the abilityto achieve a well defined AdSV signal, which is notinterfered by cadmium, lead and zinc ions present inthe zinc plant electrolyte.

A low pH regime also has to be used with zincplant electrolyte as precipitation occurs at high pHvalues. After a wide range of experiments comparingthe DPAdSV response in the presence of the six lig-ands, PCV was eventually chosen as the ligand to be


Table 8Recommended conditions for the determination of germanium(IV)by DPAdSV in zinc plant electrolytea

Parameter Optimal condition

PCV concentration 1.5× 10−5 MpH value pH 0.43 (adjusted with H2SO4)Pulse amplitude −0.05 VPulse duration 200 msScan rate 0.02 V s−1

Deposition potential 0 mV (vs. Ag/AgCl)Deposition time 20 sPotential sweep range 0–−700 mV (vs. Ag/AgCl)Peak potential −450 mV (vs. Ag/AgCl)Linear concentration range 3× 10−7–4× 10−5 M

a Data obtained from [21].

used for the determination of germanium in zinc plantelectrolyte on the basis of having optimal sensitivityand the greatest peak potential separation from cad-mium under the conditions where the pH is adjustedto 0.43. Furthermore, at this pH, the peak potential iswell removed from the zinc reduction process, as isrequired for the determination of germanium in zincplant electrolyte.

The optimal experimental conditions for the deter-mination of germanium in zinc plant electrolyte aresummarised in Table 8. A typical germanium–PCVDPAdSV response under the optimal conditions isshown in Fig. 23 for a 1 mg l−1 solution of Ge(IV) in1 M ZnSO4 at a pH of 0.43 containing 1.5× 10−5 MPCV with 2× 0.3 mg l−1 Ge(IV) standard additions.The linear peak height versus concentration rangeunder these conditions was found to be 2.75× 10−7

(0.02 mg l−1) to 4.13× 10−5 M (3 mg l−1) whichcovers the range of practical interest in zinc plantelectrolyte.

7.1.1. Removal of interferencesThe germanium–PCV DPAdSV technique was ap-

plied to zinc plant electrolyte samples using the con-ditions contained in Table 8. However, for some plantelectrolyte samples, the Ge–PCV response contained ashoulder or a peak at−0.42 V (versus Ag/AgCl) whichis attributed to reduction of lead ions in the sample.In order to establish a completely general method forgermanium determination in zinc plant electrolyte, itwas verified that lead and cadmium ions can be sepa-rated easily from germanium by bulk electrolysis prior

Fig. 23. DPAdSV curves obtained for a (a) 1 mg l−1 solution ofgermanium(IV) in 1 M ZnSO4 at pH 0.43 containing 1.5× 10−5 MPCV with (b)–(c) 2× 0.3 mg l−1 germanium(IV) standard addi-tions. Details of experimental conditions are available in [21].(Reproduced by courtesy from Electroanalysis 10 (1998) 387).

to addition of the complexing ligand. In contrast, theadsorbed germanium complex is reduced only if theligand PCV is present. Thus, initial bulk electrolysisof zinc plant electrolyte enables cadmium and leadions to be reduced to their metallic state and henceremoved while the germanium ions remain in solu-tion. Subsequently, germanium determination can beundertaken by addition of PCV followed by the use ofthe recommended DPAdSV method at an HMDE.

7.1.2. Selection of optimal working electrodematerial for removal of lead and cadmium by bulkelectrolysis

The reduction of Pb(II) and Cd(II) to their elemen-tal form from a 1 M ZnSO4 solution adjusted to pH0.43 with sulfuric acid was studied at glassy carbon,mercury pool and platinum working electrodes withall working electrodes having a diameter of 22 mm.The percentage of Cd(II) and Pb(II) removed from 1 MZnSO4 containing 2.5 ppm Cd(II) and Pb(II) was mea-sured after 8 min of electrolysis at−1800 mV (versus


Ag/AgCl). The very negative reduction potential waschosen because of the large overpotential required forthe reduction of cadmium at non-mercury electrodes.

63% of cadmium was found to be removed fromsolution after 8 min of electrolysis when mercury wasused as the working electrode. In contrast, when usingglassy carbon or platinum working electrodes, only 38and 20%, respectively, were removed. The differenceis attributed to the fact that, at a mercury pool elec-trode, the reduced form of cadmium forms an amal-gam with mercury, whereas, at the solid glassy carbonand platinum electrodes, metallic cadmium or lead re-main on the surface, which can decrease the removalefficiency. Removal efficiencies for lead were found tobe 38, 34 and 32% at mercury, glassy carbon and plat-inum electrodes, respectively, after 8 min of electroly-sis. 20 min long electrolysis times at the mercury poolelectrode can be used to ensure removal of Pb(II) toconcentrations below the voltammetric detection levelby DPV at an HMDE. Furthermore, at this electrode, adeposition potential of only−0.8 V (versus Ag/AgCl)is required, which avoids any problems with hydrogenevolution and reduction of zinc ions to elemental zinc.

Fig. 24 shows voltammograms obtained from a 1 MZnSO4 solution containing 2.5 ppm Cd(II), Pb(II) and1 ppm Ge(IV). The voltammogram obtained prior toelectrolysis at a mercury pool electrode in Fig. 24acontains the expected Cd(II)/(Hg) and Pb(II)/(Hg) re-sponses, but of course, no germanium response sincethe PCV ligand is absent. Fig. 24b shows the voltam-mogram after a 20 min electrolysis at a mercury poolelectrode held at a potential of−1800 mV (versusAg/AgCl). After electrolysis, the lead level has beenreduced below the detection limit and the cadmiumresponse to an acceptable level for the determinationof germanium. Fig. 24c shows the germanium–PCVDPAdSV curve after addition of 1.5× 10−5 M PCV.Germanium determination is now readily achieved.

7.1.3. The germanium–pyrocatechol violet reductionmechanism

The germanium ions initially complex with the lig-and, presumably to form a 2 : 1 ligand to metal com-plex [64] which then adsorbs onto the HMDE. Afteradsorption, it has been reported that the complexedGe(IV) ion undergoes reduction to its elemental form[64]. However, it is more likely that the reduction pro-

Fig. 24. Voltammograms obtained for a 1 M ZnSO4 solution con-taining (a) 2.5 mg l−1 cadmium(II), lead(II) and 1 mg l−1 germa-nium(IV) and (b) after 20 min of electrolysis at−1800 mV vs.Ag/AgCl at a mercury pool electrode and (c) as in (b), but afteraddition of 1.5× 10−5 M PCV. Details of experimental conditionsare available in [21]. (Reproduced by courtesy from Electroanal-ysis 10 (1998) 287).

cess is ligand based since PCV is itself very easilyreduced at a similar potential to that of the adsorbedgermanium complex [21].

7.2. Determination of total arsenic and removal ofinterferences by reductillation and differential pulsecathodic stripping voltammetry9

Arsenic is naturally abundant at levels in the range0.1–0.5% (m/m) in zinc concentrates. The bulk of ar-senic removal occurs by co-precipitation or adsorption



on to precipitated jarosite [NH4Fe3(SO4)2(OH)6] andiron(III) hydroxide during the iron removal stage. Be-fore entering the zinc dust purification stages, the ar-senic content must be less than 5 mg l−1 in order to pre-vent formation and evolution of poisonous arsine gas.Consequently, monitoring of the arsenic concentrationin solutions entering the purification stages is desirablein order to avoid the occurrence of such a hazardoussituation. Additionally, arsenic concentrations greaterthan 1 mg l−1 in zinc electrolysis feed solutions resultin arsenic being deposited on the cathode, causing thezinc metal to redissolve, swell and break away, therebylowering the quality and productivity of the zinc prod-uct. To avoid either of the above-mentioned problems,the arsenic concentration is monitored in the neutralleach clarifier overflow. The need for further arsenicpurification is based on the analytical results for thesesolutions.

Determination of arsenic in zinc plant electrolytewas reported by Monama and Duyckaerts [65], whofound that the element could be determined at concen-trations down to 1mg l−1 by using CSV in 2.4 M hy-drochloric acid. However, the samples studied requiredextensive sample pre-treatment to form the electroac-tive As(III), and interfering species such as copperand cadmium were present only in trace amounts inthese highly pure zinc electrolytes. Efforts have beenmade to determine As(V) by voltammetric methodsand use of complexing agents such as pyrogallol [66]and catechol [67] or by indirect methods based on12-molybdoarsenic acid [68]. These methods eitherlack the required sensitivity, are prone to interfer-ence from copper and cadmium or require complexsolvent-extraction steps. Davis et al. [69] overcameproblems of interfering ions and different oxidationstates of the arsenic species by simultaneously reduc-ing As(V) to As(III) and separating arsenic from thesample by distillation before using a voltammetric pro-cedure. In this combined reduction–distillation proce-dure, called reductillation, any As(V) is reduced toAs(III) with Cu(I), and the volatile arsenic(III) chlo-ride vapour formed in the presence of hydrochloricacid is distilled into a separate solution.

Davis et al. [69] performed the voltammetric part ofarsenic determination on gold-film electrodes whereproblems are associated with the past history andpre-treatment of the electrode. Subsequently, Sadana[70] observed that problems with the use of solid

electrodes could be overcome by performing the de-termination at an HMDE. Further, amplification of thearsenic signal was observed in the presence of copper.Reductillation followed by addition of copper andvoltammetric determination at an HMDE was, there-fore, the procedure developed for the determinationof arsenic in zinc plant electrolytes.

7.2.1. Recommended reductillation–cathodicstripping voltammetric procedure

A 300ml sample of plant electrolyte is added to8 ml of reductillation solution in the chamber of thereductillation apparatus (Fig. 25a), and the head isfitted. The chamber is heated for 2 min at 105◦C tobring the solution up to the operating temperature andthen for a further 15 min with passage of nitrogengas at a flow rate of 100 ml min−1. The outlet tip ofthe condenser delivery arm into which the volatile ar-senic(III) chloride distils is partially (75%) immersedin a solution consisting of 20 ml of 5 mg l−1 Cu(II).After the 17 min reductillation period, the solution intowhich the As(III) has been distilled is analysed di-rectly for the arsenic content by DPCSV at an HMDEwith the following parameters. Nitrogen purge time,60 s; accumulation and initial potential,−0.52 V ver-sus Ag/AgCl; accumulation time, 90 s; final potential,−0.9 V versus Ag/AgCl; scan rate, 10 mV s−1; pulseamplitude,−50 mV; ramp step,−2 mV, pulse width,50 ms. The peak potential for the determination of ar-senic is−0.70± 0.02 V versus Ag/AgCl.

7.2.2. Voltammetric determination of total arsenic inneutral leach after reductillation

Typically, neutral leach samples have high levels ofcopper and cadmium, which interfere with the voltam-metric determination of arsenic. Isolation of arsenicfrom the plant electrolyte sample by reductillationproved to be an extremely efficient way of eliminatingproblems of interference from copper and cadmium.Further, with this method, reduction of the electroin-active As(V) species to the required As(III) state isachieved. Eqs. (13) and (14) describe the importantfeatures of the reductillation method:

As(V) + 2Cu(I) → As(III ) + 2Cu(II ) (13)

As(III ) + HCl → AsCl3–HCl(vapour) (14)


Fig. 25. Determination of total arsenic by the reductillation sepa-ration method and detection by DPCSV. (a) Schematic diagram ofthe reductillation apparatus. The ‘Spanish dungeon trap’ is a solu-tion trapping device. (b) DPCSV voltammograms obtained for thedetermination of total arsenic in a neutral leach clarifier overflowsample. (A) Without reductillation and using 300ml of sample in16% (v/v) HCl containing 5 mg l−1 of Cu(II), the arsenic signal isengulfed by that from the large cadmium concentration present,(B) after reductillation with an arsenic concentration in the cellof 4mg l−1. (Reproduced by courtesy from Analyst 119 (1994)1051).

Table 9Comparison of data obtained for the determination of total arsenicin neutral leach clarifier overflow by reductillation and a DPCSVor AASa

Sample Voltammetryb (mg l−1) AASc (mg l−1)

Neutral leach 1 0.30 0.33Neutral leach 2 0.50 0.50Neutral leach 3 1.2 1.3Neutral leach 4 0.60 0.68Neutral leach 5 1.4 1.4

a Data obtained from [18].b RSD = 6%,n= 5.c RSD = 3%,n= 5.

The efficiency of the reductillation process is influ-enced by a range of parameters, which are listed in[7]. For the determination of arsenic in the zinc plantsamples, the hydrochloric acid concentration in thereductillation chamber for 100% efficiency must be37% (v/v). The volume of reductillation solution cho-sen was 8 ml, with addition of 300ml of sample. Withthis volume, the optimum concentration of reducingagent was found to be 0.5% (m/v) copper(I) chloridewhen using a reductillation time of 15 min with a 2 minheating period before the nitrogen gas purge.

A volume of 20 ml allows convenient operation ofthe voltammetric cell. Thus, distillation of arsenic(III)chloride–hydrochloric acid vapour into 20 ml of5 mg l−1 Cu(II) solution yielded an acid concentra-tion of 5.5% (v/v), which was found to be ideal forthe determination of arsenic by cathodic DPCSV. Thecontrolled Cu(II) concentration provides an enhancedresponse and is used to provide adequate sensitivity.An example of the voltammetric response obtainedfor a neutral leach sample, before and after reduc-tillation, is included in Fig. 25b and illustrates theneed for the reductillation pre-treatment. Analyticaldata for arsenic in various samples obtained by acalibration-graph method where the linear region oc-curs from 1 (the detection limit) to 100mg l−1, witha correlation coefficient of 0.999, are summarised inTable 9. After correction for dilution, the detectionlimit is 0.07 mg l−1. The results for the voltammetricdeterminations are in excellent agreement with thosefor determinations of arsenic by AAS. Since the lattermethod is known to detect total arsenic, the fact thatexcellent agreement is obtained implies that total ar-senic is also determined voltammetrically and that allthe As(V) has been successfully reduced to As(III).


8. Use of ion selective electrodes in the monitoringof metal ions present in zinc plant electrolyte

The zinc plant electrolyte analytical monitoringprogram has to be implemented under conditions ofvery high ionic strength, high temperature, variableacidity and severe instrument operating conditionsassociated with large electrical fields and corrosiveconditions. All of these factors mitigate against boththe implementation of voltammetric techniques andconventional instrumentation for ISEs. However, aswill be shown in Section 8, some applications ofISEs are possible when carefully chosen strategiesare implemented [10].

8.1. Continuous monitoring of copper and cadmiumin zinc plant electrolyte using a microprocessor-basedbattery-operated data acquisition system, multipleion-selective electrodes and redundancy principles10

In order to use ISEs to monitor copper and cad-mium, concentrations need to be greater than about10−5 M. At lower concentrations , the response timesof the ISEs are very slow and the limits of detectionare approached. Consequently, ISE applications haveto be restricted to feed to the purification plant ratherthan purified solutions, where the much more sensi-tive DPASV and related voltammetric methods de-scribed above have to be employed. De Bellefroid [71]mentioned the use of ‘a selective copper electrode’ tocontrol coarse zinc dust addition at the Balen plant,Vieille-Montague, Belgium. Geissler and Kunze [72]also have recently examined the use of an ISE for de-termining copper in zinc plant electrolyte. Whilst theirmethod was off-line and conventional analog instru-mentation was used, their work highlights the diffi-culties which exist with ISE methods. They discussedthe problems of long-term reproducibility, the needfor periodic cleaning of the electrode surface and dif-ficulties with the reference electrode in the high ionicstrength environment.

In the work undertaken in the author’s laboratories,battery-operated microprocessor-based instrumenta-tion based on complementary metal oxide semicon-ductor (CMOS) circuitry, with multi-channel mul-tiplexing capabilities, was developed for use with


copper and cadmium ISEs. The battery-operated sys-tem was designed to be portable, and easily shieldedfrom the harsh electronic and chemical environment.The multiplexing facility coupled with a speciallydesigned cell enabled four identical electrodes or arange of different electrodes to be used. The instal-lation of a number of identical electrodes has threeadvantages: (i) the signal can be amplified; (ii) de-liberate application can be made of a redundancyphilosophy; and (iii) long-term continuous operationcan be provided with minimal operator interven-tion. The equipment design, therefore, incorporatesthe maximum of flexibility, ranging in applicationfrom a multiprobe unit completely dedicated to thedetermination of one variable and involving the re-dundancy principle, to a multi variable instrument.This instrumentation, when combined with a standardmains-powered computer-controlled liquor supplyline, has been applied to zinc electrolyte from thePasminco Hobart plant.

8.1.1. Multichannel instrumentation for ion-selectiveelectrode monitoring

Fig. 26a illustrates the completely integrated rela-tionship between the data acquisition system (DAS)and the plant circuit solutions in the copper purifica-tion stage. The microcomputer data acquisition sys-tem uses CMOS integrated circuitry exclusively and isbased on a multiplexed address rather than a data bus.The unit is based on a field data acquisition systemdescribed in [73]. In the development of this instru-ment, the following design objectives were achieved:(a) use of low-power CMOS technology to providea battery-operated, self-contained, portable, low-noisedevice that can operate in a harsh environment; (b) sixinput channels, four for ISEs, one for the referenceelectrode and a separate one for the temperature sen-sor; (c) use of a sealed metal box to protect the instru-ment from the hostile chemical and physical environ-ment of an industrial plant; (d) a monitoring programwhich prompts the operator regarding the state of theinstrument and condition of the batteries; (e) an RS232interface to a mains-powered printer, plant computer,microcomputer or control room. Concentrations werecalculated with these mains-powered devices in theconventional manner.

Fig. 26b is a schematic diagram of the operatingsystem in which the DAS is used for the determination


Fig. 26. Determination of copper and cadmium in zinc electrolyte by the ISE method. (a) Relationship between the DAS and the plantcircuit solution at the cementation stage of the zinc electrowinning process, (b) schematic diagram of the operating system for thedetermination of copper and cadmium in zinc plant electrolyte. E, electrodes; H/E, a heat exchanger cooling the plant liquor sample; P,centrifugal pump; M/C, microcomputer for local plant control; PL/C, major plant computer; T, temperature probe; V, control valves, (c)simple circuit diagram of the multiplexing unit, (d) schematic diagram of the flow-through electrochemical cell. Height, 14 cm, length,15 cm. (Reproduced by courtesy from Anal. Chim. Acta 260 (1987) 213).

of copper and cadmium in zinc plant electrolyte. Animportant feature of this instrument is the multiplex-ing capability which enables redundancy principles tobe included in the analytical scheme. Eight-channelmultiplexing is used to enable selection of the multi-variable input modules under software control so thata number of similar electrodes can be used to ensurecontinuous operation without interruption to the mon-itoring process (Fig. 26c).

8.1.2. The flow-through electrochemical cellFig. 26d is a schematic diagram of the ISE cell.

Large volumes of plant circuit electrolyte are avail-

able for sampling and the output from the cell can bereturned conveniently to the circuit stream, so that theuse of a large flow cell with high throughput, multi-sensing capacity and which operates on a single-passflow is a viable proposition.

The stability of the potentiometric measurementtechnique is enhanced by the existence of flowingsolutions. Additionally, the flowing solution also en-sures that the sensing surfaces of the electrodes areexposed continually to fresh electrolyte and kept asclean as possible. The capacity of the 10 cm wide cellshown in Fig. 26d is 300 ml, and is operated with atypical flow rate of 1500 ml min−1. The sensing elec-


trodes are orientated in the same plane at right anglesto the direction of flow. This plane is situated at theturn of the 90◦ elbow of the cell. The transverse align-ment of the electrodes permits all probes to sense thesolution simultaneously. The electrodes are immersedto a depth of 6 mm in the electrolyte. The referenceelectrode is placed downstream of the sensing elec-trodes, thus avoiding any contamination or physicalinterference to the stream flow patterns. Experimen-tally, it was shown that the reference electrode can bea considerable distance away from the sensing probes.

In addition to incorporating the ISEs and the ref-erence electrode in the cell design, the temperaturecan be monitored continuously. The temperature ofthe plant circuit liquor is about 70–80◦C. In order toensure reproducibility of results, the analytical proce-dure includes cooling of the electrolyte as a prelim-inary step to measurement in the cell. The tempera-ture of the solution can be controlled by varying theflow rate or solution path length and by introducing astainless-steel cooling coil before the electrolyte en-ters the electrochemical cell, as shown in Fig. 26a.The monitoring temperature was typically 25◦C andstandards were matched to the actual measured tem-perature.

The cell was built of poly(vinyl chloride) (PVC)with conventional PCV glue used in the joints. Thisplastic, as the material of construction, is preferable toglass, being not only easier and cheaper to machine,but far more robust and capable of withstanding the se-vere operating conditions in industry. The carbon con-tent in grey PVC discharges any stray currents whichmight otherwise accumulate on the cell.

8.1.3. Instrument configurationFor operation in a zinc refinery, use is made of

a two-stage process consisting of a battery-poweredISE data acquisition system monitoring copper andcadmium coupled with a conventional mains-poweredliquor delivery unit. The battery-powered DAS is usedto minimise the problems in the data measurement byavoiding the impact of extraneous currents which arecommon in any industrial electrolytic process.

8.1.4. Copper electrode response in plant electrolyteIn ‘synthetic ZnSO4 solution’ made from analytical

reagent-grade zinc sulphate in distilled water over the

concentration range 100–150 g Zn l−1 (1.53–2.29 Mzinc solution), plots of potential against log concen-tration of copper were linear with a Nernstian slopeof 29± 1 mV at 25◦C over the copper concentrationrange of interest which is 10−4–10−2 M.

When ‘low-level’ purified plant liquor was used,Nernstian responses over the same copper concentra-tion range mentioned above were observed. As thisliquor is an excellent match for the matrix of the plantsolution to be monitored, this liquor is recommendedfor calibration purposes rather than the synthetic solu-tions of zinc sulphate. An additional advantage in us-ing this plant solution is that a ready supply is at handand can be removed from the process circuit down-stream of the zinc dust cementation stage. In addition,the waste from the flow-through cell can be returnedto the process stream.

Geissler and Kunze [72] reported the formation ofsurface layers on the copper-sensing surface when theelectrode was left standing in zinc sulphate solutions.We have also observed that, after a period of time, adark grey layer forms on the surface of the electrodesused here. With time and the appearance of the greysurface layer, the slope of a plot of potential againstlog of the copper concentration decreases. For exam-ple, in the high-volume flow-through cell and OrionCuS/Ag2S ISEs, the Nernstian slope (±2 mV) couldbe maintained for 3 days. With longer solution expo-sure times, the slope decreased, but could be restoredto Nernstian behaviour by polishing. As a diagnos-tic criterion, the electrode should not be used unlessit exhibits the expected Nernstian value±2 mV. This

Table 10Comparison of copper(II) concentrations in synthetic zinc elec-trolyte as determined by the ISE and AAS methodsa

Nominal Cu(II) ISE AASconcentration determinationb determination

(mg l−1) (mM) (mM) (mM)

120 1.9 1.9± 0.2 1.9210 3.3 3.3± 0.3 3.5220 3.5 3.4± 0.3 –300 4.7 4.7± 0.5 5.0320 5.0 4.7± 0.5 –410 6.4 6.5± 0.6 6.8435 6.8 6.4± 0.6 –

a Data derived from [10].b Error is the standard deviation for continuous measurements

made over 30 min.


diagnostic test forms part of the automation routine.The solution flow rate is also important in determiningthe period for which the electrode exhibits Nernstianbehaviour. Results obtained on the monitoring of cop-per in zinc electrolyte are summarised in Table 10 anddemonstrate the relationship of the system when cop-per concentrations are high. To achieve any long-termstability, Geissler and Kunze [72] introduced regularpolishing with a vibrator and brush. In our work, itproved possible to avoid this extra complication in thefinal ISE cell design, presumably via the use of rapidlyflowing solutions.

8.1.5. Cadmium electrode response in plantelectrolyte

The cadmium-selective electrode responded in asimilar fashion to the copper-selective electrode. Inboth synthetic ZnSO4 solution and low-level plantelectrolyte, a slope of 25± 2 mV could be maintainedfor a 3-day period for cadmium concentrations in therange 10−4–10−2 M.

8.1.6. Operation with four multiplexed copperelectrodes

The recommended operating mode involves the si-multaneous use of four copper electrodes with multi-plexed instrumentation. The computer takes readingswith respect to the reference electrode in the follow-ing sequence. First, the potential of copper electrodeno. 1 is sampled 30 times over a time interval of ap-proximately 3 ms. The arithmetic mean value of thereadings is computed and stored in memory. Althoughthere is a time interval of only 720 ns between inputchannel selection and start of measurement, a stabi-lization period of approximately 1 s was allowed be-tween readings from one electrode to the next, i.e., a 1 sdelay was allowed between the next channel selectionand commencement of the measurement. This timeinterval avoids any unwanted perturbations and givesthe reference electrode and the selected ISE time toequilibrate. Secondly, this process is repeated sequen-tially for copper electrodes no. 2, 3 and 4. Thirdly, oncompletion of this cycle, the stored data for the elec-trodes are transferred to a local visual display unit fordata examination and concentration evaluation. Fur-ther data treatment can be done by a down-line com-puter if required.

If the apparent concentration values are all within10% of the average, the average value is accepted.If one value is erratic and clearly different from theother three, then this response is discarded. If onthree consecutive occasions, the same probe pro-vides apparently anomalous data, then it is declarednon-operational and the results from it are ignored dur-ing the evaluation sequence. The malfunction of thiselectrode is further verified if it gives non-Nernstianresponses from the two-point calibration procedurespecified above. Typically, this means a very lowslope. From this evidence, a warning is issued that thiselectrode requires attention. However, the applicationof the redundancy concept still enables the measuringsystem to continue. If another electrode malfunctionsprior to the routine maintenance every 3 days, themeasuring system reverts to a two-electrode system.

In general, if all four copper electrodes suddenlyproduce erratic responses, then the computer assumesthat the reference electrode is faulty and a warning sig-nal is issued to the effect that the reference electrodeneeds replacement. Measurements are terminated un-der these conditions.

If the upper acceptable limit of copper is apparentlyexceeded, for example 10−2 M copper, a recalibrationis immediately introduced into the measurement pro-cedure and the copper concentration is determinedagain. If the upper acceptable limit is still exceeded,then it is assumed that the plant needs attention. At theend of each 3 days of operation, routine maintenanceincorporates polishing all electrodes, refilling the ref-erence electrode and after an inspection, cleaning thecell and calibration. In the above mode, the systemproved to be very reliable for 3-day periods, which isnot true for a single-electrode system. The effect ofdeliberately building redundancy into the system com-bined with the cell design ensures much longer-termstability and reliability than is normally associatedwith ISEs.

8.1.7. Operation with four cadmium electrodesEssentially, the operation of the cadmium monitor-

ing program follows the same pattern as for copper andneed not be discussed in any detail. However, insteadof the Nernstian slope for a plot of potential againstlog cadmium concentration, slightly lower slopes wereobserved reproducibly, e.g., 26 mV at 25◦C instead ofthe theoretically expected 29 mV. All operational and


calibration procedures are the same for cadmium andfor copper with a maintenance-free 3-day operationalperiod being attainable.

8.1.8. Redundancy principleStatistical arguments indicate that the advantage of

four electrodes over a single electrode represents adistinct advantage. To achieve maintenance-free reli-ability over 3 days with any confidence using indi-vidual or even two sensing elements is very difficult.From the present studies, it can be concluded that theadvantages of deliberate introduction of redundancywith the ISE electrodes offset the higher capital costof the instrument package.

Redundancy was not built into the reference elec-trode because, in principle, they are much more re-liable, particularly with a regular 3-day maintenanceschedule. In addition, during multiplexing, use of asingle reference electrode ensures that exactly thesame reference potential is being used continually.

8.1.9. InterferencesISEs are seldom specific so that interferences can

arise. Kivalo et al. [74] reported that copper inter-feres with cadmium detection in pure aqueous solu-tions. Similarly, iron(III) has been reported to inter-fere with determinations of cadmium [75] and copper[76–79]. None of the species at their known concen-trations present in zinc plant electrolyte after iron pu-rification caused interference.

In contrast to the observations of Kivalo et al.[74] for pure aqueous solutions, no interference withthe copper response by the presence of cadmium, orvice versa, was observed during determinations in aconcentrated zinc electrolyte. No change in potentialof the copper electrode was registered in the caseof 200 mg l−1 Cu(II) standard solution containing150 g l−1 zinc at pH 5.00. Similarly, on addition ofcomparable levels of copper during the determinationof cadmium, no interference occurred in the samematrix.

9. Determination of zinc in plant electrolyte bydiscontinuous flow analysis11

The flow-based instrumental technique, discontinu-ous flow analysis (DFA), can be applied [80–82] to any


automated titration for which a potentiometric sensor(e.g. pH or ISE) or spectrophotometric (not described)method is available [83].

Determinations that have been undertaken tradition-ally by manual titrations in electrolytic zinc productionare the zinc content of both the pre-electrolysis (feed)and post-electrolysis (spent electrolytes from the elec-trowinning plant) periods and the sulfuric acid contentof the spent electrolyte. However, with increasing em-phasis being placed on automation and quality controland real time analyses, a flow-based titration methodsuch as DFA with potentiometric detection offers theopportunity for continuous on-line monitoring of thezinc and acid concentrations.

9.1. Construction of a zinc ion-selective electrode(potentiometric) sensor

In order to use DFA in the potentiometric detectionmode, a zinc ISE was constructed, as described below.The zinc salt of bis(2-ethylhexyl)phosphoric acid wasprepared by dissolving 10 g of the acid in 100 ml of90% ethanol and by adding zinc carbonate, with shak-ing, until the evolution of carbon dioxide ceased. Thesolution was filtered and the residue was washed withboiling ethanol. Ethanol was evaporated on a rotaryevaporator, leaving a white waxy solid, which was re-crystallised from ethanol.

A membrane mixture was prepared by dissolvingthe following components in the minimum amountof purified tetrahydrofuran: 3.5% (m/m) zinc saltprepared as above, 4.0% bis(2-ethylhexyl)phosphoricacid, 65% bis(2-ethylhexyl) 1-ethylhexylphosphonateand 27.5% PVC. This mixture was coated onto thecopper substrate of the ISE (potentiometric) sensor.The sensor was allowed to cure for 24 h and was con-ditioned for 1 h in 1× 10−5 M zinc sulfate, followedby a further 1 h in 1.0 M zinc sulfate. The sensor re-sponded to Zn2+ over the concentration range from1× 10−5–1.0 M and was near-Nernstian at high con-centrations. Interference from ions such as Mn2+and Mg2+, which can be significant in zinc plantelectrolyte, was found to be minimal.

9.2. Discontinuous flow analyser

A three-piston Ionode DFA instrument and theflow cell described in [82] were used. Mixing in the


Fig. 27. DFA zinc electrolyte titration curves using 0.1 M EDTA as the titrant and potentiometric detection with a zinc sensor (a) 0.1 MZn2+, (b) feed zinc plant electrolyte solution, (c) calibration curve. (Adapted from Analyst 117 (1992) 1845).

potentiometric cell was achieved with a vibratingreed. The cam operated over a four-fold concentrationrange. Of the three pistons driven by the cam, onlytwo were used with the potentiometric flow cell. Thethird piston was used for spectrophotometric titrations(not discussed).

9.3. Ion selective electrode (potentiometric) detection

Zinc was titrated potentiometrically at pH 4.3 with0.1 M EDTA by using the flow cell [82] fitted witha zinc ISE. Typical DFA titration curves are shownin Fig. 27a and b for a zinc standard (0.08 M) and azinc feed sample (Feed 1), respectively. A reasonablysharp end-point is obtained for the standard with a welldefined derivative. Broader end-points are obtained forplant samples with less well defined derivatives, asshown in Fig. 27b for Feed 1.

The calibration graph for zinc standards in the con-centration range 2–8 g l−1 is shown in Fig. 27c. Two25 times diluted feed and spent zinc electrolyte sam-ples were determined using this calibration graph, andthe results are presented in Table 11. The results arein agreement, within experimental error, with those of

Table 11Results obtained by different techniques for the determination ofzinc in feed and spent zinc plant electrolyte liquorsa,b

Sample Zinc (g l−1)

Sensorc Spectro-photometric Manuald AAS

Feed 1 168(2) 167(2) – 169Feed 2 155(3) 151(2) 149 148

Spent 1 64.4(1) 62.9(0.5) – 61.8Spent 2 47.1(1) 51.7(0.7) 49.0 –

a Relative standard deviation (RSD%) shown in parenthesis(n= 5).

b Data obtained from [15].c DFA method with potentiometric detection of end point using

a zinc ISE.d Manual titration.

spectrophotometric, AAS and manual titration meth-ods. The precision with the potentiometric method isslightly poorer than with spectrophotometric detec-tion. This is attributed to the slower response of the po-tentiometric sensor and the resulting broader end-pointdetection. However, the results demonstrate that DFAis a convenient, fast and precise instrumental methodfor the determination of zinc in zinc plant electrolyte


when using a potentiometric sensor. A major advan-tage is that the DFA method is capable of carrying outmore than 80 titrations per hour with the use of lessthan 1 ml of sample per titration.

10. Conclusions and future perspectives

A 200-year history of electrochemistry means thatnumerous electroanalytical methods are now avail-able for the determination of almost every elementin the periodic table. The long history of electrowin-ning of zinc also means that the importance of tracemetals in electrolytic zinc production is now wellrecognised. Zinc plant electrolyte, therefore, almostrepresents the electrochemist’s dream as this mediumprovides the opportunity to develop electroanalyticaltechniques for the determination of numerous metalions present in the plant electrolyte and also to studythe fundamental as well as applied aspects of theelectrowinning process.

A major highlight to emerge from the overview ofthe author’s 25 years of studies on applying electroan-alytical methods to metal determinations in zinc elec-trolyte is the demonstration that a very high level ofautomation can be achieved even in the very harsh zincplant industrial environment. Furthermore, these stud-ies have shown that off-line and on-line (on-stream)methods can be developed for almost every metal ionof interest at trace concentrations after detailed studieshave been conducted to remove interferences. Thus, agreat deal can be achieved in the area of analysis ofzinc plant electrolytes by electroanalytical techniques.

The overview of achievements presented in this ar-ticle may appear to lead to the conclusion that all iswell in the land of electroanalytical chemistry. How-ever, it needs to be pointed out that the new millen-nium may see the continued emergence of at least onesignificant restriction which could severely mitigateagainst the continued widespread use of voltammet-ric methods. Data contained in this paper have em-phasised the fact that mercury electrodes are vastlysuperior to solid electrodes in almost all on-stream ap-plications of voltammetric methods in zinc plant elec-trolyte. However, occupational health considerations,in all probability, will ultimately severely restrict theuse of elemental mercury as an electrode material,and to date, a truly competitive alternative electrode

material type has yet to emerge for on-line, on-streamapplications.

It may also be noted from this overview that thedetermination of most elements requires the use ofspecific methods to overcome a range of problemsof selectivity or interference. Spectroscopic meth-ods, which commonly have genuine broadly basedmulti-element determination capability and whichalso can be used on-line, are steadily improving inperformance in applications related to extremely highsalt content matrices such as those encountered in zincplant electrolyte. The challenge for electroanalyticalchemists in the early stages of the new millenniummay, therefore, be to generate truly multi-elementaldeterminations with non-mercury electrodes that em-ploy instrumentation and procedures which may beused in operator unattended on-line modes for evenlonger periods of time than has been achieved to date.This represents a great challenge and may, for exam-ple, require significant refinement of chromatographicseparation methods coupled with electrochemical de-tection at, say, a renewable carbon rather than a mer-cury electrode. A far from perfected development ofthis kind was reported from our laboratories some timeago [9]. If the use of mercury does become bannedas an electrode material and voltammetric methodsare to remain competitive for metal determinations inindustrially harsh media such as those found in elec-trolytic zinc plants, then extensions of these hybridchromatographic separation–electrochemical detec-tion approaches, use of on-line disposable low costinert carbon electrodes or the introduction of fully au-tomated electrode polishing techniques to keep solidelectrodes clean may need to be explored and per-fected. However, as Heyrovsky indicated many yearsago, the easily renewable mercury electrode is a veryattractive tool of the electroanalytical chemist and willnot be readily replaced by equivalently performingelectrodes.

To conclude on a positive note, it should be em-phasised that the ability to readily detect different oxi-dation states (speciation) remains an attractive advan-tage of voltammetry which is not yet widely avail-able with many non-electrochemical alternative tech-niques, and this represents an advantage that may re-main for some time to come. Additionally, the fullpower of computer technology has yet to be truly ap-plied to electroanalytical techniques and systems with


far greater ‘intelligence’ may address many of thepresent problems, and indeed, provide the key to makethe use of solid electrodes as reliable as that of mer-cury electrodes.

Finally, the ability to construct portable battery-operated, computer based electroanalytical instrumen-tation, with essentially the same capabilities as mainspowered laboratory based instrumentation, representsa considerable advantage over most spectroscopictechniques. Thus, hand held glucose monitors, fieldbased ASV instruments with data logging capabili-ties for use in remote locations and ISE monitoringsystems on lakes and rivers are all available withno real competitive techniques on the horizon. Thebattery-operated Cu and Cd ISE monitor describedin Section 8 represents an advantage of the use of abattery-operated system in an electrically and chem-ically harsh environment and many developmentsin these kinds of areas can be expected in the nextdecade.

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