THIN FILM CuS ELECTRODE FOR THE DETERMINATION OF Cu2+, Cr3+ & Fe3+ IN AQUEOUS SOLUTION

A dissertation submitted to the Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, NEPAL.

In partial fulfillment of the requirement for the Master’s Degree in Chemistry

By Basu Dev panthi

Central Department of Chemistry Institute of Science and Technology

Tribhuvan University Kirtipur, Kathmandu, NEPAL

2002

TRIBHUVAN UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY

CENTRAL DEPARTMENT OF CHEMISTRY KIRTIPUR, KATHMANDU

The dissertation entitled THIN FILM CuS ELECTRODE FOR THE DETERMINATION

OF Cu2+, Cr3+ & Fe3+ IN AQUEOUS SOLUTION

Submitted by Basu Dev Panthi

has been accepted as a partial fulfillment of the requirement for the Master’s Degree in Chemistry

_______________ _____________________________ External examiner Supervisor/Head of The Department Prof. Dr. Raja Ram Pradhananga Central Department of Chemistry

FOREWORD The entire work presented in this thesis has been carried out by Mr. Basu Dev Panthi under my supervision in the academic year 2000-2002. During the research period, he has performed his work sincerely and satisfactorily. No part of this thesis has been submitted for any other degree.

----------------------------------- Prof. Dr. Raja Ram Pradhananga Head of the Department Central Department of Chemistry Tribhuvan University. Kirtipur, Kathmandu, NEPAL

ACKNOWLEDGEMENT I would like to express my sincere gratitude to my respected supervisor Prof. Dr. Raja Ram Pradhananga, Head, Central Department of Chemistry, T.U., for his kind help, valuable guidance and encouragement throughout the research work without which this work would not be possible. I am also grateful to Prof. Dr. Mangala Devi Manandhar, former Head, Central Department of Chemistry, T.U., for providing me the required laboratory facilities to carry out this work. My sincere thanks goes to my respected teachers Prof. Dr. Shiva Prasad Dhoubhadel, Prof. Dr. Chhabi Lal Gajurel, Prof. Jaya Krishna Shrestha, Prof. Dr. Mohan Bikram Gewali, Dr. Jagadish Bhattarai, Dr. Sabita Shrestha, Dr. Deb Bahadur Khadka, Dr. Tara Niraula, and all other teachers of the Central Department of Chemistry for their co-operation and encouragement during the work. I am also thankful to senior brothers and sisters Mr. Subin Adhikari, Mrs. Kshama Parajuli, Mr. Lok Kumar Shrestha ‘Rajesh’ and Ms. Shikchya Tandukar for their kind help and co-operation during the work. I am thankful to my colleagues Ms. Durga Parajuli, Mr. Yam Bahadur Poudel, Mr. Raj Kumar Malla, Mr. Birendra Babu Adhikari, Ms. Neeva Karmacharya, Mr. Netra Subedi, Mr. Lekh Nath Sharma and Mr. Deepak Gupta for their kind co-operation during the work. I would like to thank all the supporting staffs of the Department. I want to thank Royal Nepal Academy of Science and Technology (RONAST) for providing me some financial support to carry out this work. Lastly, I want to express my gratitude to my parents and brother for their love and support.

Basu Dev Panthi

August, 2002.

ABSTRACT

The laboratory made CuS/Ag2S pressed pellet electrode containing 95

mole percentage of Ag2S and 5 mole percentage of CuS did not response

favorably for the determination of concentration of copper ion in aqueous

solution. But electrode prepared by depositing thin film of CuS on graphite rod

by chemical method was found to be sensitive and selective to the copper ions.

The electrode was found to obey the Nernstian behavior from 10-1 to 10-4

mole/L Cu++ concentration with slope equal to 30 mV at pH range 4-5. The

response time of this electrode was found to be less than 30 seconds. With the

ageing of the electrode, there is slight drift in electrode potential; however,

gradient per decade change in concentration of copper ions remains almost

constant. The electrode has been successfully used for the determination of Cu2+

by direct potentiometry and as an indicator electrode for the potentiometric

titration with EDTA. The electrode has also been successfully used as an

indicator electrode for the determination of Cr3+ and Fe3+ in aqueous solution by

indirect potentiometric titration with Cu2+. The results obtained are in good

agreement with the expected value within experimental error.

Table of contents Foreword Acknowledgement Abstract 1. Introduction ..................................................................................................... 1 1.1 General introduction ....................................................................................... 1 1.2 Types of ion-selective-electrodes ................................................................... 2 1.3 Theory of ion-selective-electrode ................................................................... 3 1.4 Selectivity coefficient ..................................................................................... 7 1.5 Advantages and limitations of ion-selective-electrodes ................................. 8 1.6 Literature survey ........................................................................................... 10 1.7 Objective of the present work ....................................................................... 13 2. Experimental ................................................................................................ 14 2.1.1 Preparation of Ag2S/CuS pressed pellet ISE ............................................. 14 2.1.2 Preparation of CuS-graphite ISE ............................................................... 15 2.2 Preparation of reagents ................................................................................ 16 2.3 Characterization of Cu-ion-selective-electrodes ........................................... 18 2.4 Determination of Cu++ by direct potentiometry ............................................ 20 2.6 Selectivity coefficient .................................................................................. 20 2.7 Determination of Fe3+ in aqueous solution ................................................... 20 2.8 Determination of Cr3+ in aqueous solution ................................................... 21 3. Results and Discussion .................................................................................. 22 3.1 Characterization of Cu-ion-selective-electrodes ........................................... 22 3.2 Determination of Cu++ by direct potentiometry ............................................ 25 3.3 Interferences by different metal ions ........................................................... 26 3.4 Potentiometric titration ................................................................................. 29 3.5 Determination of Fe3+ in aqueous solution ................................................... 30 3.6 Determination of Cr3+ in aqueous solution ................................................... 31 Conclusion ......................................................................................................... 33 Bibliography ....................................................................................................... 35 Appendix ............................................................................................................. 38

1

CHAPTER - ONE INTRODUCTION

1.1 General Introduction

Ion-selective electrodes are electrochemical sensors whose potential

depends on the activity or concentration of determinand ion according to Nernst

equation.

E = Eo ± RT/nF(ln ax) ............................(1)

Where Eo is the standard potential of the electrode, ax is the activity of

determinand ion 'x' and ‘n’ is the number of charges on 'x'. The sign is positive

when 'x' is a cation and negative when it is an anion.

The equation (1) describes the pure response of the electrode to 'x' and

thus assumes that there is no other ion in the solution to which the electrode can

respond. But this ideal situation is not a feature of a real electrode. The range

over which an electrode will give a Nernstian response is limited in practice,

even in the absence of other interfering ions to which the electrode responds and

depends on the surface characteristics, composition, nature of electrodes and

other various preparation parameters of electrodes.

It is not possible to measure a single electrode potential; hence, the

electrode is coupled with reference electrode. The emf of the cell is given by:

Ecell = Eo ± 2.303 RT/nF (log ax) - Eref

= Eo’ ± k/n (log ax) …………….................... (2)

In the above equation Eo’ is equal to ( Eo- Eref) a constant and k =2.303

RT/F which is equal to 59 mV at 25oC for an ideal electrode. Thus, the emf of

the cell is a logarithm function of determinand ion activity. In real electrode the

value of k up to 55 mV are frequently referred to as Nernstian. The term 'sub-

Nernstian' is used if the value of k is lower than 55 mV per decade change in

2

concentration. Lower value of k usually indicates that the electrode is faulty and

the results may be irreproducible.

1.2 Types of ion-selective electrodes.

Most popular ion-selective electrodes of practical importance are

following types [1].

• Glass membrane electrodes.

• Solid-state membrane electrodes

• Liquid ion-exchange membrane electrodes

• Gas sensing probes

• Enzyme electrodes

• Ion-selective field effect transistor [ISFET]

• Miscellaneous electrodes

Solid-state membrane electrodes may be further classified into following

types:

• Single crystal electrodes

• Solidified salt melt electrodes

• Pressed pellet electrodes

• Thin film deposited electrodes

• Coated wire electrodes

A thin film deposited electrodes are the electrodes prepared by depositing

a thin film of ion-sensitive membrane in an inert supporting material (e.g.

CuS/Ag2S deposited on graphite rod).

3

1.3 Theory of ion-selective electrode

1.3.1 Glass electrodes

The theory and response mechanism of glass electrodes for pH

measurement and working principle of glass electrode for the measurement of

other monovalent cations is thoroughly described by Eisenman [2]. When a

glass electrode is immersed in an aqueous solution, the glass membrane absorbs

water and a swollen hydrated layer is formed on the inner and outer surface,

which may be represented as:

0.1M HCl

Solution.

Inner

Hydrated

Layer.

Dry

Glass

Layer.

Outer

Hydrated

Layer.

Unknown

(Test)

Solution.

Figure 1.1: Representation of hydrated glass membrane.

The thickness of this hydrated layer varies considerably, according to the

composition of the glass and the nature (i.e. pH) of the solution in which it is

immersed. At the interface of the hydrated layer with a sample solution the

monovalent cations on the glass, sodium or the lithium ions usually, are

exchanged and the sites are involved in ion-exchange equilibrium with the ion

to be measured. The exchange of hydrogen ion between solution and glass

membrane takes place in both inner and outer hydrated layers of the glass

membrane which gives rise to electrode potential which is proportional to the

H+ diffusion between inner and outer solution separated by the glass.

H+ HG+

Nicolsky in 1935-37 first gave this ion exchange theory for the

development of electrode potential which was extended and refined by many

scientists [1]. With the fundamental work of B.P. Nicolsky concerning the

4

interpretation of potential response of glass electrode through ion-exchange

theory, a new idea has been introduced in the development of a novel ion-

selective electrode. This ion-exchange theory is generally accepted till today.

The electrode potential of the pH glass electrode is given by:

EG = Eo ± K.pH ……………………… (3)

Where K=2.303RT/F and Eo is the standard glass electrode potential. The

standard potential of glass electrode depends upon the composition, surface

characteristics and history of the glass electrodes. In the equation (3) the sign is

positive when the solution is alkaline (i.e. when pH>7) and negative when the

solution is acidic (i.e. when pH<7).

1.3.2 Inorganic precipitated based ion-selective electrodes:

In inorganic precipitated based ion-selective electrodes the response

mechanisms appear, in general, to be more straightforward than those of glass

electrodes, due to the absence of the hydrated surface layer.

Buck and Shepard [3] developed a more complete theory, relating the

standard potential of silver-halide based electrodes to the stoichiometry of the

active material in the membrane. The formula of silver halide may be precisely

written as Ag1+∂X; thus, the compound is slightly nonstoichiometric and while

at equilibrium contain a uniform excess of elemental silver (∂ positive) or

halogen X(∂ negative). When silver metal back contact is made, the diffusion of

silver to the membrane takes place until the stoichiometry has been altered to

give unit silver activity and the potential is produced from the ion-exchange

process [1]. The same argument may be applied to the silver sulfide membranes.

Hence, the electrode potential is dependent on the ion-exchange reaction

between membrane and test solution because ion exchange at membrane and

metal contact interface is constant.

5

In electrodes of second kinds, the silver halide takes no important part in

the potential determining process. The potential of the silver is established as

the result of the electron exchange between the silver metal and ions in the bulk

of the metal and ion exchange between the silver ions in the metal and the silver

ions at the silver-solution interface. The activity of silver at the interface is

unity; hence, the potential of the electrode depends on the ion exchange at

silver-solution interface.

In the case of electrodes with an internal reference solution and silver

based internal reference electrode leads to the same as for electrodes of the

second kinds.

If in a solid-state electrodes, the membrane is contacted with noble

metal then the silver activity in the membrane is fixed by the stoichiometry of

the silver salt and electron exchange replaces ion exchange for the development

of electrode potential at the membrane contact interface. The ion exchange

between membrane and solution interface is as described above.

When the electrode membrane is contacted with carbon, unit activity in

the solid phase will remain, because no chemical reactions occur and the

potential is developed by the ion exchange between the active material in the

membrane and test solution. The standard electrode potential of this kind of

electrode varies according to the method of manufacture and the stoichiometry

of the active material in the membrane [1].

When a membrane is contacted by a reactive metal such as lead,

cadmium or solder, the potential is initially similar to that of membranes

contacted by a noble metal and only electron exchange occurs at the membrane

contact interface. However, the membrane, AgX, reacts with the contact metal,

L, to form salts LXXY and free elemental silver. This silver will eventually act as

contact metal itself and the standard potential will shift, as the membrane

stoichiometry shifts, to that expected for a silver contact.

6

Buck and Shepard [3] extended their arguments from the silver halide and

silver sulfide membranes to cover the mixed sulfide membranes used in lead,

cadmium and copper electrodes (PbS/Ag2S, CdS/Ag2S, CuS/Ag2S). Lead,

cadmium or copper cannot be used as contact metals because of corrosion

caused by the silver salts. Hence, the range of potentials is set by the limits of

stoichiometry when aAg = 1 or aS = 1 referred to the standard state of aM2+ = 1.

The equilibrium in the CuS/Ag2S system are much more complicated

than in the other two systems because the range of copper sulfides of grossly

different stoichiometry (CuS, Cu1.8S, Cu1.9S, Cu1.95S and Cu2S) which can exist.

The value of the electrode standard potential depends on the form of sulfide that

is present at equilibrium. When the membrane is contacted with silver (aAg = 1),

the silver is in equilibrium with sulfur in the system via the silver sulfide; thus,

the chemical potential of the sulfur and, hence, the chemical potential of the

copper sulfur system is fixed.

Silver sulfide acts as a binder in the heavy metal sulfide ion-selective

electrode. Besides this silver sulfide help to keep the activity of the metal, M,

constant at the membrane solution interface. Silver in the solid will diffuse

relatively fast. Hence, in any reaction on the surface silver sulfide tends to act as

a buffer in the equilibria involved with any silver that is lost or gained on the

surface being rapidly equilibrated with the bulk of the membrane by diffusion

process.

E. Pungor put forward a entirely different view regarding the response

mechanism of ion-selective electrodes which is yet to be universally accepted.

According to E. Pungor [4] ion exchange does not take place in the membrane

but the electrode potential is caused by the charge separation at the surface of

the electrode. The interpretation is connected with the chemisorptions of the

primary ion from the solution phase onto the surface of the electrode. In this

case counter ions accumulate in the solution phase and this is the charge

separation.

7

1.4 Selectivity coefficient, .,

PotBAK :

No ion-selective electrode responds exclusively to the ion which it is

designed to measure, although it is often more responsive to this primary ion

than to others. If another, interfering, ion is present at a concentration that is

large with respect to primary ion, the electrode response will have contributions

from both the primary and interfering ions. The degree of selectivity of the

electrode for the primary ion, A, with respect to an interfering ion, B, is

expressed by the potentiometric selectivity coefficient, .

,Pot

BAK ; this is defined by

the general equation for the electrode potential;

)log(303.2 .,∑+±=

B

nnB

potBAA

A

o BAaKaFnRTEE ………… (4)

Where nA & nB are the charges on ions A & B and Eo is the standard

potential of the electrode. The sign in the equation is positive for cation and

negative for anion. This equation is applicable to nearly all electrodes. This

equation is often referred to as Nicolsky equation.

When an electrode is very selective for A in comparison with B, then .

,Pot

BAK will be much less than unity. Conversely, if, as occasionally happens, the

electrode responds preferentially to B rather than A, .,

PotBAK will be greater than

unity. For example, the selectivity coefficient of the Orion calcium electrode for

barium ions is 0.01; thus, the electrode is 100 times more responsive to calcium

ions than barium ions. However, chloride electrode is 300 times responsive to

bromide ions than chloride ions.

The value of .,

PotBAK is never constant for all activities of A & B, although

it is sometimes constant for a given ratio of the activity of A to B. For the

experimental determination of selectivity coefficients, separate solution method

and mixed solution methods are used.

8

1.5 Advantages and limitations of ion-selective electrode.

Ion-selective electrodes have been the subjects of rapidly increasing

interest over the past three decades and their development has opened a large

new field of potentiometry and provides the average analyst for rapid, accurate

and low cost analysis. The growth of ion-selective electrodes and their

applications has been fast in relation to the growth of other techniques partly

because the sensors themselves became widely available commercially soon

after their development and in some cases, quite easy to make in the laboratory.

The construction and chemical design of the sensors have been steadily

improved but the range of ions, which may be directly sensed by ion-selective

electrodes, will not be greatly enlarged.

Ion-selective electrodes sense the activity, rather than the concentration of

ions in solution. The ideal feature of the electrode described by the Nernst

equation is not a feature of real electrode and interference by other ions will be

observed. Electrodes with sub-Nernstian slope and irreproducible readings are

also anticipated. Since the electrodes give a voltage signal, which is

proportional to the logarithm of the determinand ion activity, the accuracy and

precision of the determinations are sometimes poorer than for other techniques,

especially when high determinand activities are measured. However, the

accuracy and precision of the determinations may remain essentially unaltered

over several decades of activity. The accuracy and precision depend on the

degree of control exercised over the experimental conditions, such as

temperature, stirring rate and on the measurement technique.

One of the most attractive features of the ion-selective electrode is the

speed with which they permit a sample to be analyzed, and the ease with which

the methods may be made semi-automatic or automatic. About 20-30 samples

per hour may be analyzed manually; if the sensor is incorporated in a flow

system, discrete samples may be analyzed at rates upto about 60 per hour

9

or a sample stream may be analyzed continuously. The methods of analysis may

be non-destructive and are adaptable to very small sample volumes.

Furthermore, analysis may be made without difficulty of highly colored, viscous

samples containing a high concentration of suspended solids. However,

problems do arise in the analysis of some non-aqueous and partially non-

aqueous samples. Non-correspondent ions may also be determined by using ion-

selective electrodes.

Research on ion-selective electrode is progressing very rapidly which is

reflected by the publication of hundreds of paper every year in different aspects

of ion-selective electrodes.

10

1.6 Literature survey:

The glass electrode fabricated by Haber and Klemensiewicz [5] in 1909

for the determination of hydrogen ion concentration in aqueous solution was the

first ISE of practical importance. In 1934 Lengyl and Blum [6] prepared a glass

electrode with different composition, which was sensitive to sodium ions. Latter

glass membrane was replaced by single crystal of inorganic salt, which were

sensitive to different ions. Kolthoff and Sanders [7] prepared halide-selective

electrodes from polycrystalline pellets of silver halides. After that Pungor, et al.

[8,9] developed several ion-selective electrodes based on precipitates of

inorganic salts. The electrodes prepared by Pungor and his co-workers were the

first truly selective ion-selective electrodes (non-glass). These electrodes display

favorable mechanical properties and good chemical durability. Many

researchers changed the composition of electrode membrane material for the

improvement of sensitivity, linear range, stability, response time etc. Birger [10]

deposited Ag2S on silver rod electrolytically to produce Ag+ & S-- ion-selective

electrode. Electrodes with thin film & liquid membrane were also reported [11].

Further research on ion-selective electrodes developed the ion-selective

electrodes for the determination of F-, Cl-, Br-, I-, S2-, SCN-, CN-, Pb2+, Cu2+,

Cd2+, Ag+, Hg2+, Ca2+.

In the years from 1969-1972, Orion research published a magazine

devoted to developments and applications of ion-selective electrode [12]. Orion

Research Inc. has played a pioneering role in the development of the majority of

the inorganic salt-based ion-selective electrodes including heavy metal ions [1].

The copper ion-selective electrode is the most successful of the divalent

heavy metal ion selective electrodes. This electrode has several applications. In

1970-72 Ruzika and co-workers [13,14,15] designed a new ion-selective

electrode, which they termed selectrode. A selectrode consists of a graphite rod

covered with an ion-selective membrane. This type of electrode is extremely

easy to fabricate and use but its greatest disadvantage is that this electrode gives

11

reproducible & reliable results only in some selected systems. Cu (II)–ion-

selective electrodes with CuS/Ag2S heterogeneous membrane were prepared

[16,17,18] by simultaneous precipitation of metal sulfides. The applications of

these electrodes were also studied. Pcik et al.[19] described Cu(II)-ion-selective

electrodes based on copper sulfide in silicon rubber. Nomura and Genkichi [20]

studied the properties of a membrane electrode made of vinyl chloride polymer

containing CuI precipitate. A wire ion-selective electrode was made by sulfiding

copper deposited on copper wire [21]. Cu (II)-ion-selective electrode with

Ag1.5Cu0.5S doped with Ag2Se sensors [22] were also constructed.

Palanivel and Riyazuddin [23] prepared Cu (II)-ion-selective electrode by

depositing thin film of CuS & Ag2S on the same graphite rod. The graphite rod

was extracted from used dry cell. This type of electrode is extremely cheap and

one can construct such electrodes in the laboratory quite easily. But such

electrodes need to be characterizing before taking them into operation. The low

detection limits quoted for the copper electrodes are difficult to achieve in

practice. The membranes are prone to oxidation in the presence of acids and

oxidants, such as dissolved oxygen in the electrolyte solution [24,25]. The effect

of oxygenated solutions on the surface composition of the jalpite-based (

Ag1.5Cu0.5S ) Cu-ion-selective electrode membrane was examined by De Macro,

Roland, et al. [26]. These authors demonstrated that oxidation of the surface

occurs with a consequence shift in the Eo of the system. Tsingarelli and

Nikolenko [27] determined the selectivity coefficients for a solid Cu (II)-ion

selective electrode with respect to Ni(II), Zn(II), Cd(II), Pd(II), Fe(III), Cr(III),

Al(III), Hg(II) & Ag(I) by using different methods. The Cu (II)-ion-selective

electrode have been used in determination of secondary aliphatic amines in

micro quantities, stability constants of copper complexes and potentiometric

determination of Fe (III) etc [28,29,30]

Different types of modifications and improvements on Cu ion-selective

electrodes were continuously going on in last few decades [31-37]. The main

12

thrusts of the recent investigations were focused on the development of simple,

low cost electrodes with good sensitivity, which gives reproducible results. In

this context, it is worth mentioning the recent investigations of Cu (II)-ion-

selective electrodes, which were fabricated by depositing CuS/AgS on graphite

rod.

13

1.7 Objective of the present work:

From the literature survey, it is quite clear that the several researchers

were working on the development, improvement, characterization and

applications of different types of copper ion-selective electrodes. Modifications

and improvements in the composition of electrode membrane materials have

been investigated for the improvement of sensitivity, stability, linear range and

response time etc. These improved electrodes are being used for various

analytical works as well. Much of the current research interest is in the

fabrication of simple, low cost electrodes having good sensitivity that give

reproducible results. Thus the aim of present work is:

(1) To fabricate heterogeneous CuS/Ag2S based pressed pellet and thin film

copper sulfide deposited on graphite rod copper ion-selective electrodes.

(2) To compare the performance of these electrodes for the determination of

concentration of Cu++ in aqueous solution.

(3) To study the applicability of the copper ion-selective electrodes as an

indicator electrode for the quantitative estimation of heavy metal ions such as

Cu (II), Cr (III), Fe (III), by potentiometric titration with EDTA.

14

CHAPTER TWO

EXPERIMENTAL

2.1.1 Preparation of CuS/Ag2S pressed pellet electrode

Pressed pellet copper ion-selective electrodes based on CuS/Ag2S were

prepared from the intimate mixture of CuS & Ag2S powder prepared by

coprecipitation of copper and silver sulfide from aqueous solution [mole ratio of

CuS:Ag2S=5:95]. 8.133 g AgNO3 dissolved in water & to it 12.5 ml of 0.102

mole/L Cu(NO3)2 [=0.236 g of Cu(NO3)2]solution was added. To that mixture

solution H2S gas was passed till complete precipitation took place.

Cu(NO3)2 + 2AgNO3 + 2H2S CuS + Ag2S + 4HNO3

The CuS/Ag2S precipitate thus obtained was filtered, washed with

distilled water and finally washed with acetone. It was then dried in air oven at

1200c for an hour. The dried material was ground to fine powder in an agate

mortar. The powder was sieved from Mesh no. 140 size. About 2g of that fine

powder was taken in IR pellet making die and pressed under twelve tons of

pressure in vacuum for five minutes [In the Perkin Elmer IR accessory

hydraulic press]. The pressed pellet thus obtained was dried for half an hour in

an air oven at 120oC. Two such pellets were made. In one pellet back contact

was made with silver and in another with copper.

For copper back contact copper metal was deposited in one side of the

pellet by electro-deposition from copper sulfate solution. For this electroplating

bath is described elsewhere [38]. For silver back contact, one side of the pellet

was painted with electro-conductive silver paint (Dotite, Japan).

A polythene rod of 10 cm long & 1.5 cm diameter was taken and a

groove of 1.4 cm diameter and 4 mm depth was made. A hole of 5 mm diameter

was bored at the center. Copper back contacted CuS/Ag2S pellet was inserted

15

into polyethylene rod which is rest on copper disc and copper wire was soldered

for electrical connection.

Similarly, silver back contacted CuS/Ag2S pellet was inserted into

polyethylene rod which rest on silver disc and copper wire was soldered for

electrical connection. Finally, the pressed pellet membranes were fixed in the

groove by araldite. In this way a complete solid-state polycrystalline (pressed

pellet) membrane of CuS/Ag2S, copper ion-selective electrodes were fabricated

in the laboratory (Figure 2.1). The stoichiometry of CuS and composition of

electrode was not analyzed.

Figure- 2.1: Pressed pellet Ag2S/CuS copper ion-selective electrode

2.1.2 Preparation of CuS-graphite electrode:

Thin film copper ion-selective electrode was fabricated by depositing thin

film of copper sulfide to the porous surface of graphite rod by chemical method.

For this, a 2.5 cm long graphite rod with cap extracted from a dry cell was

boiled for about two hours in distilled water. It was then dried in hot air oven at

the temperature 120oC for two hours. Then that rod was dipped into saturated

solution of copper sulfate for an hour. It was then dipped into a saturated

solution of sodium sulfide for an hour. The rod was removed from the solution

Copper wire

Electrode body

Ag or Cu metal disc

Araldite

Ag2S/CuS pressed pellet

16

and was washed with distilled water and finally washed with ethanol. A copper

wire was soldered in the metal cap of that graphite rod for electrical connection.

It was then fitted into the glass tube using araldite, in such a position that about

2 cm of the graphite rod coated with CuS was protruding. The schematic

diagram of the electrode is shown in Figure 2.2. Thus, prepared copper ion-

selective electrode was characterized and used for different analytical purposes.

Figure –2.2: Thin film CuS-graphite ion-selective electrode

2.2 Preparation of reagents:

All the stock solutions were prepared in distilled water using LR/AR

grade reagents without further purification and for the purpose of daily use

those stock solutions were appropriately diluted.

Copper wire

Electrode body

Metal cap of graphite rod

Araldite

Graphite rod

Thin film of CuS.

17

I. 0.1M AgNO3 solution:

It is prepared by dissolving 4.25g of LR grade AgNO3 in 250 ml distilled water.

II. 0.1 M Na2S solution:

It is prepared by dissolving 3.752 g of LR grade Na2S (minimum assay 52%) in

250 ml distilled water.

III. 5 M NaNO3 solution:

It is prepared by dissolving 42.95 g of LR grade NaNO3 in 100 ml distilled

water.

IV. 1M CuSO4 solution:

It is prepared by dissolving 24.968 g of LR grade CuSO4.5H2O in 100 ml

distilled water.

V. 0.1N K2Cr2O7 solution:

It is prepared by dissolving 1.226 g of LR grade K2Cr2O7 in 250 ml distilled

water.

VI. 0.1M Na2S2O3 solution:

It is prepared by dissolving 6.204 g of LR grade Na2S2O3 in 250 ml distilled

water.

VII. 1M Cu(NO3)2 solution:

The compound is highly hygroscopic, hence, 150 g of LR grade Cu(NO3)2.3H2O

is dissolved in 500 ml distilled water. 10 ml of this solution is 10 times diluted

and the concentration of the resulting solution is determined by titrating with

standard sodium thiosulfate solution. Then exact concentration of stock solution

is calculated. The required volume of stock solution is diluted to make 250 ml

of exactly 1M Cu(NO3)2 solution.

18

VIII. 2.5M acetic acid & 2.5M sodium acetate solution (buffer solution):

It is prepared by dissolving 14.7 ml of LR grade CH3COOH & 34 g of LR grade

sodium acetate in 100 ml distilled water. The pH of this solution was checked

with pH-meter.

IX. 0.1 M EDTA solution:

It is prepared by dissolving 9.306 g of LR grade

[CH2.N(CH2.COOH)CH2.COONa]2.2H2O in 250 ml distilled water.

2.3 Characterization of copper ion-selective electrodes:

2.3.1 Response of Cu-ion-selective electrode to Cu++ ions:

For the characterization of copper ion-selective electrode Ion Analyzer

(Model L1-126) produced by Elico Ltd, Hyderabad, India was used. Ion

analyzer is a digital millimeter, which can measure emf, concentration, and

temperature. Ion analyzer contains three channels for reference electrodes &

three for ion-selective electrodes. One reference electrode can be used for three

ion-selective electrodes i.e. the reference electrode channels are inter-connected.

For reference electrode Ag/AgCl combined with glass electrode can be used and

salt bridge is not necessary for coupling with copper ion-selective electrode.

Both electrodes can be deep into the test solution.

The electrochemical cell is represented as:

Ag/AgCl, KCl (0.1M) // test solution /Cu-ion-selective electrode.

The Cu++ solution of 10-1mole/L to 10-6 mole/L concentrations were

prepared by serial dilution of 1 mole/L Cu++ solution. 50 ml of each solution

was taken into 100 ml beaker and 1 ml 5M NaNO3 solution was added to keep

the constant ionic strength at 0.1 M. The pH of the solution was fixed by adding

1 ml acetic acid-sodium acetate buffer solution. Then emf of each solution was

measured by stirring the solution continuously using magnetic stirrer. A

19

calibration curve of observed emf versus negative logarithm of [Cu++] was

plotted.

2.3.2 Response of Cu-ion-selective electrode to S-- ions:

To study the response of Cu ion-selective electrode for S-- ions the same

procedure as described for Cu++ ions (section 2.3.1) was adopted except the

addition of NaNO3 solution and buffer solution. Here the entire S-- solutions

were prepared in 0.1 M NaOH and 1 mg/ml ascorbic acid was added as an

antioxidant.

2.3.3 Response of Cu-ion-selective electrode (CuS/Ag2S pressed pellet

electrode) to Ag+ ions:

To study the response of Cu ion-selective electrode for Ag+ ions the same

procedure as described for Cu++ ions (section 2.3.1) was adopted except the

addition of buffer solution.

2.4 Determination of Cu++ in aqueous solution by direct potentiometry:

For the determination of Cu++ in aqueous solution by direct potentiometry

method, 50 ml Cu++ solution of the concentration within the linear range of

electrode performance was taken in a 100-ml beaker. 1 ml 5M NaNO3 and 1 ml

acetic acid-sodium acetate buffer was added. Then emf was measured using Cu-

ion-selective electrode by continuously stirring the solution. From the observed

emf the concentration of the solution was determined using calibration curve.

20

2.5 Potentiometric titration:

The applicability of the Cu-ion-selective electrode as an indicator

electrode for the potentiometric titration was examined by the titration of:

(i) 50 ml of 0.01M Cu++ solution with 0.05M EDTA solution.

(ii) 10 ml of 0.01M EDTA with 0.01M Cu++ solution.

(iii) 25 ml of 0.01M Cu++ solution with 0.05M EDTA solution.

(iv) 10 ml of 0.01M Cu++ solution with 0.01M EDTA solution.

In case of (ii) & (iv) the volume was made more than 25 ml by adding distilled

water.

2.6 Selectivity coefficient, pot

BCuK , :

The selectivity coefficient pot

BCuK , with different metal ions was

determined by mixed solution method following standard addition technique. In

this method the concentration of copper ions was varied from 10-1 mole/L to 10-

5 mole/L and the concentration of interfering ions (B) was kept constant. The

potential of the Cu-ion-selective electrode under investigation was plotted

against -log [Cu++] and for determination of selectivity coefficient substituted in

equation (4).

2.7 Determination of Fe3+ in aqueous solution:

For the quantitative estimation of Fe3+ in aqueous solution by indirect

potentiometric titration, 5 ml of 0.01M Fe3+ solution was taken in a beaker.

Then 10 ml of 0.01M EDTA solution and 1 ml acetic acid-sodium acetate buffer

solution was added to that solution. The unreacted EDTA was then back titrated

with 0.01M Cu++ solution using Cu-ion-selective electrode as an indicator

electrode.

21

2.8 Determination of Cr3+ in aqueous solution:

For the quantitative estimation of Cr3+ in aqueous solution by indirect

potentiometric titration, 10 ml of 0.01M EDTA solution was taken with 5 ml of

0.01M Cr3+ solution and 1 ml acetic acid-sodium acetate buffer solution. Total

volume was made 25 ml by adding distilled water. The mixture was boiled for

15 minutes and cooled. Then the unreacted EDTA was back titrated with 0.01M

Cu++ solution using Cu-ion-selective electrode as an indicator electrode.

22

CHAPTER-THREE

RESULTS AND DISCUSSION

3.1 Characterization of Cu-ion-selective electrodes:

Among the pressed pellet membrane ion-selective electrodes, silver

sulfide based electrodes are most well-behaved and extensively investigated

electrodes. By incorporation of appropriate silver salts, electrodes selective to

Ag+, Cl-, Br-, I- & S-- have been fabricated and widely used for the analysis of

these ions in different analytical purposes [1,7,8,9,12]. There has been a report

in the literature, by incorporation of sulfides of heavy metal in silver sulfide,

ion-selective electrodes selective to the heavy metal ions can be fabricated

[1,12]. In this direction, attempt was made to construct Cu-ion-selective

electrode by incorporation of copper sulfide into silver sulfide matrix. Since the

pressed pellet contains both silver sulfide and copper sulfide, so there is a

choice to use silver or copper metal contact between inner surface of the

membrane and cable. The internal potentials produced will depend upon the

nature of the metal. For this two electrodes were constructed, in one copper

metal and in another silver metal were used for internal contact.

Irrespective of the nature of metal contact, the response of electrodes with

respect to Cu++ ion is far from satisfactory. The electrode responses in Nernstian

manner only in the concentration range from 10 -1 mole/L to 10 -2 mole/L of

Cu++ ion. In this range the results are quite reproducible. The useful range of the

electrode is so limited that the electrode is of not much practical importance for

the determination of Cu++ in aqueous solution. The mole ratio of the Ag2S &

CuS in the commercial Cu-ion-selective electrode is not reported in the

literature. When the percentage of CuS in the Ag2S/ CuS electrode is high the

electrode piles up in layer and disintegrate quite easily. Therefore, the mole

percentage of CuS in Ag2S and CuS was made small. Copper sulfide being a

23

non-stoichiometric compound, the activity of copper ions in copper sulfide

depends upon the method of preparation of copper sulfide and treatment of the

precipitate. Owing to this standard electrode potential of copper sulfide based

ion-selective electrode may vary from electrode to electrode.

Surprisingly, both the electrodes did not show stable and reproducible

result for S-- ions and for Ag+ ions. Both the electrodes did not follow Nernstian

behavior. Hence the electrode, fabricated in the laboratory containing mole ratio

95:5 of Ag2S and CuS were not useful for the determination of Ag+ or S-- ions.

Pure silver sulfide pressed pellet electrode had been found to be an

excellent ion-selective electrode for monitoring Ag+ or S-- in aqueous solution

[39]. The purpose of incorporation of CuS in Ag2S was to make the electrode

sensitive to Cu++. In fact incorporation of CuS, made the electrode sensitive to

Cu++ but insensitive to Ag+ or S--. The objective of the present investigation was

to make Cu-ion-selective electrode. Hence, no further investigation was carried

out to make the electrodes sensitive to Ag+ or S--. During coprecipitation of CuS

and Ag2S from the solution excess of S-- was added. Hence, there is a lack of

excess of silver in the precipitate but an excess of S-- and CuS being non-

stoichiometric compound made the electrode insensitive to silver ions. The

pressed pellet Ag2S/CuS electrode though response to Cu++ but response range

was limited to decimolar range. Perhaps by strict controlling the stoichiometry

of CuS and by appropriately controlling the mole ratio of Ag2S:CuS, electrode

more sensitive to Cu++ may be possible to fabricate. But this work had been left

for further investigation.

Another electrode sensitive to Cu++ is a thin film CuS deposited in

different material. These days thin film CuS ion-selective electrode is drawing

greater attention for the analysis of Cu++ in solutions. Such thin film electrodes

are extremely cheap but response range, stability, reproducibility and response

time depends upon the method of fabrication and pre-treatment of the

electrodes. The life of CuS thin film deposited on graphite support may be low

24

but construction of such electrode is not difficult. It can be readily fabricated in

the laboratory. Such electrodes must be well characterized before putting them

into analytical purpose with respect to reproducibility, Nernstian behavior,

response time etc.

After unsuccessful attempt to fabricate pressed pellet Cu-ion-selective

electrode, an attempt was made to fabricate Cu-ion-selective electrode by

depositing thin film of CuS on the graphite rod taken from used dry cell. Ruzika

type graphite-CuS selectrode and other thin film deposited electrode had been

fabricated and characterized by a number of authors [41-43]. A thin film of CuS

was deposited on graphite rod by carrying out the precipitation reaction at the

graphite rod itself. Such electrode is quite easy to fabricate, extremely simple

and can be constructed within a day.

The typical plot of observed emf of the cell (CuS-graphite Cu-ion-

selective electrode coupled with Ag/AgCl reference electrode) as a function of

negative logarithm of Cu++ concentration is shown in figure (A.1). The points in

the graph are experimental values and the line is the line of best fit obtained by

regression analysis. The slope of the line is found to be 30 mV per decade

change in concentration of Cu++ with standard deviation ±0.7. The observed emf

of the cell was reproducible for one day and on the following day a drift in the

emf was observed. However, the electrodes still follow the Nernstian behavior

with constant slope. During six months the standard electrode potential of the

cell (CuS-graphite Cu-ion-selective electrode coupled with Ag/AgCl reference

electrode) varied from 259 mV to 284 mV but the slope remained almost

constant. So, the electrode can be use for the determination of Cu++ in aqueous

solution but must be calibrated before use like glass electrode. The slight change

in standard electrode potential of the cell may be due to the oxidation of

measuring surface of the membrane material in the CuS-graphite Cu-ion-

selective electrode. If the electrode is stored in dry, the electrode need to be

activated by immersing the electrode in 10-2 mole/L Cu++ solution for about five

25

minutes. Storing the electrodes in 10-2 mole/L Cu++ solution did not improve the

performance of the electrode.

The CuS-graphite Cu-ion selective electrode followed the Nernstian

behavior from 10-1 mole/L to 10-4 mole/L Cu++ in aqueous solution and attained

equilibrium potential within 30 seconds. The dynamic response of the electrode

is shown in figure (A.2). When the concentration of Cu++ is less than 10-4

mole/L, the response of the electrode is quite sluggish and takes 15 minutes or

more to attain equilibrium potential.

3.2 Determination of Cu++ in solution by direct potentiometry:

Ion-selective electrodes have extended the field of direct potentiometry.

The direct method is, as the name implies, the most straightforward technique

and is the technique of choice whenever possible. With calibration standards

that are appropriate for the particular test sample, it is possible to make

measurement of concentration by means of ion-selective electrode. The one

requirement is that the concentration of test sample must be within the linear

response range of the electrode. The sample to be analyzed is pretreated as

required and the electrodes are immersed in it. The equilibrium cell potential is

then measured and related to the determinand ion concentration by means of

calibration curve.

Alternatively, the concentration of the determinand solution can be

symmetrically calculated. The potential of the cell is given by the equation:

]log[303.2 +++= CunF

RTEEcell ……… (5)

Or ]log[. +++= CuKEEcell

Where E is the standard electrode potential of the cell which can be

evaluated from the intercept of Ecell versus -log[Cu++] plot and K is a constant,

the value of which can be obtained from the slope of the plot.

26

Or KEEAntiCu cell −=++ log][ ……………………. (6)

To determine the concentration of Cu++, the Cu-ion-selective electrode is

deeped into the solution and emf of the solutions is measured. The concentration

of Cu++ solution was then calculated using equation (6). Table 3.1 shows the

actual concentration of Cu++ solution and the concentration determined using

thin film CuS ion-selective electrode along with percentage error. In the

concentration range from 10-2 to 10-4 mole/L Cu++ the experimental error is not

more than 5%. For divalent metal ions this degree of accuracy by direct

potentiometry is generally considered as an excellent result.

Table3.1- Expected and calculated concentration of some Cu++ solutions.

S.

No.

Concentration of Cu++

taken. (mole/L)

Concentration of Cu++

determined from equation

(13). (mole/L)

Error

%

1. 4 × 10-2 3.95 × 10-2 1.25

2. 8 × 10-3 7.871 × 10-3 1.62

3. 4 × 10-3 3.807 × 10-3 4.82

4. 8 × 10-4 7.575 × 10-4 5.31

27

3.3 Interferences by different metal ions:

The plot of emf of the cell versus –log[Cu++] shown in figure (A.1) is

linear upto the 1 ×10-4 mole/L Cu++ concentration with slope equal to 30 mV.

This means that CuS-graphite Cu-ion-selective electrode responses in a

Nernstian manner upto 1 ×10-4 mole/L Cu++ concentration. When the

concentration of Cu++ is less than 1 ×10-4 mole/L the curve becomes non-linear.

In presence of interfering ion, the curvature in the emf of the cell versus –

log[Cu++] starts early and departure becomes more and more pronounced with

increase in concentration of interfering ion. This deviation from the linear

response of Cu-ion-selective electrode towards the Cu++ determines the

selectivity towards Cu++ against other interfering ions. Figure (A.11) shows the

interference of different metal ion in the determination of Cu++ by Cu-ion-

selective electrode.

The selectivity coefficient (pot

BCuK , ) is a parameter which describes the

relative response of an ion-selective electrode towards interfering ion in

comparison with the main determinand ion. An empirical equation given by

Nicolsky and Eisemann is written as:

∑+±=B

nnB

potBAA

A

BAaKaFnRTEE ).log(303.2

,0

………..(4)

Where nA and nB are the charges on the primary and interfering ions A &

B respectively and aA and aB are their respective activities.

When both ions are contributing to electrode response, then

BA nnB

potBAA aKa .,= ………………………..(7)

If the activity of A at which this equality occurs is a'A and the activity a'B,

then the electrode potential is given by,

)'.'log(303.2,

0 BA nnB

potBAA

A

aKaFnRTEE +±=

28

Or )'2(303.20A

A

aFnRTEE ±= ……………………………... (8)

The difference between the electrode potentials in solution of A with and

without B at activity a'B is therefore given by,

)2log(303.2 ''AA

A

aaFnRTE −=Δ

2log303.2FnRT

A

=

mVnA

18= At 25oC …………………………………….(9)

Thus, finding the value of a'A from the plot of emf of the cell, E versus –

log[Cu++] at the point where the experimental curve deviates from the

extrapolation of Nernstian line by 9 mV and substituting the value in equation

(10) the selectivity coefficient is then calculated.

BA nn

Bpot

BAA aKa '.' ,= …………………………….. (10)

The value of pot

BCuK , obtained by mixed solution method for Cu-ion-

selective electrode are tabulated in table 3.2. The presence of 0.1 M Ca++, Zn++,

Co++ did not interfere in the determination of Cu++ by the present CuS-graphite

ion-selective electrode.

29

Table 3.2: Selectivity coefficient of some metal ions.

S.

No.

Foreign

ion

Concentration of foreign

ions. (mole/L)

potBCuK ,

1 Pb++ 0.1 1 ×10-2

2 Cd++ 0.1 5.62 ×10-4

3 Ag+ 1 ×10-4 4.46 ×104

Significance ofpot

BCuK , :

The selectivity coefficient pot

PbCuK , =1×10-2 signifies that the electrode is

100 times more responsive to copper ion than lead ions. If concentration of Pb++

in the Cu++ solution is 100 times less than that of Cu++ then the Pb ++ does not

interfere the response of the electrode to Cu++ . The selectivity coefficient pot

CdCuK , = 5.62 ×10-4 means the electrode is much more responsive for copper

ions and cadmium ion only slightly interferes in the electrode response. The

selectivity coefficient pot

AgCuK , = 4.46×104 is high value which means the

electrode is much more responsive to silver ions than copper ions i.e. the

electrode responses like silver selective electrode. Thus, there should be absence

of silver ions in the solution for the determination of copper ion.

3.4 Potentiometric titration:

In potentiometric titrations absolute potentials are not usually required,

and measurements are made while the titration is in progress. The equivalence

point of the reaction will be revealed by a sudden change in potential in the plot

30

of emf readings against the volume of the titrating solution. The solution must

be stirring during titration.

The applicability of the CuS-graphite ion-selective electrode as an

indicator electrode for the potentiometric titration is examined by titrating

different concentration of Cu++ solution with EDTA solution and EDTA

solution with Cu++ solution. When Cu++ solution is titrated with EDTA solution,

the observed emf decreases in each addition of EDTA solution due to decrease

in free Cu++ in the solution and at equivalence point there is dramatically

decrease in observed emf. At that point nearly all the Cu++ has been titrated by

the EDTA solution. To locate the end point precisely a differential curve was

drawn.

A typical potentiometric titration curve for the titration of 10 ml of 0.01

M Cu++ solution with 0.01 M EDTA solution is shown in figure (A.3) and the

differential curve of that titration is shown in figure (A.4).

The titration was also repeated by titrating EDTA solution with Cu++

solution. The observed emf increases in each addition of Cu++ solution due to

increase in free Cu++ in the solution and at equivalence point there is

dramatically increase in observed emf. At that point nearly all the EDTA

solution has been titrated by the Cu++ solution. After that the increase in emf is

due to the increase in free Cu++ in the solution. A typical potentiometric titration

curve for the titration of 10 ml of 0.01 M EDTA solution with 0.01 M Cu++

solution is shown in figure (A.5) and the differential curve of that titration is

shown in figure (A.6). The results of the potentiometric titration are in good

agreement with expected value within 2% experimental error.

3.5 Determination of Fe3+ in aqueous solution:

The concentration of Fe3+ in aqueous solution has been determined by

indirect potentiometric titration using Cu-ion selective electrode. First the Fe3+

is reacted with excess EDTA and unreacted EDTA is then estimated by back

31

titration with Cu++ solution using Cu-ion-selective electrode. The stability

constant of the Fe(III)-EDTA complex is 25.1 and that of Cu(II)-EDTA is 18.8.

When excess EDTA is reacted with Fe3+ solution in acidic medium, the Fe3+

forms 1:1 complex with its equivalent amount of EDTA. The Cu++ does not

replace the Fe3+ from the Fe(III)-EDTA complex due to high stability than

Cu(II)-EDTA complex. Thus there is no possibility of displacement reaction in

the mixture on addition of Cu++ solution and added Cu++ forms complex only

with unreacted EDTA. The excess EDTA can be determined by potentiometric

titration with Cu++ using Cu-ion-selective electrode as an indicator electrode.

First the cell emf remains almost constant in each addition of Cu++ due to

reaction of added Cu++ with EDTA in the solution and at equivalence point the

cell emf increases dramatically. At that point all the EDTA has been reacted

with Cu++ and added Cu++ remains as free Cu++ in solution. Hence, after the

equivalence point the cell emf increases continuously in each addition of Cu++

due to increase in free Cu++ in the solution.

The typical plot of potentiometric titration of 10 ml of 0.01M EDTA and

5 ml of 0.01 M Fe3+ mixture with 0.01M Cu++ solution is shown in figure (A.7)

and the differential curve of that titration is shown in figure (A.8). The results

obtained by this method are in agreement with expected value within less than

4% experimental error.

3.6 Determination of Cr3+ in aqueous solution:

The concentration of Cr3+ in aqueous solution has been determined by

indirect potentiometric titration using Cu-ion selective electrode. First the Cr3+

is reacted with excess EDTA and unreacted EDTA is then estimated by back

titration with Cu++ solution using Cu-ion-selective electrode. The stability

constant of the Cr(III)-EDTA complex is 24.0 and that of Cu(II)-EDTA is 18.8.

When excess EDTA is boiled with Cr3+ solution for about 15 minutes in acidic

medium, the formation of Cr(III)-EDTA complex takes place. Cr3+ forms

32

complex with its equivalent amount of EDTA and there remains unreacted

EDTA in the solution. The Cu++ does not replace the Cr3+ from the Cr(III)-

EDTA complex due to high stability than Cu(II)-EDTA complex. Thus there is

no possibility of displacement reaction in the mixture on addition of Cu++

solution and added Cu++ forms complex only with unreacted EDTA. The excess

EDTA can be determined by potentiometric titration with Cu++ using Cu-ion-

selective electrode as an indicator electrode. First the cell emf remains almost

constant in each addition of Cu++ due to reaction of added Cu++ with EDTA in

the solution and at equivalence point the cell emf increases dramatically. At that

point, all the EDTA has been reacted with Cu++ and added Cu++ remains as free

Cu++ in the solution. Hence, after the equivalence point the cell emf increases

continuously in each addition of Cu++ due to increase in free Cu++ in the

solution.

The typical plot of potentiometric titration of 10 ml of 0.01M EDTA and

5 ml of 0.01 M Cr3+ mixture with 0.01M Cu++ solution is shown in figure (A.9)

and the differential curve of that titration is shown in figure (A.10). The results

obtained by this method are in good agreement with expected values within less

than 4% experimental error.

33

CONCLUSION

It is known that incorporation of silver halide on Ag2S, made the

electrode selective to halide ion [1,39,40]. In line with this CuS was

incorporated into Ag2S and pressed pellet electrode was fabricated with the

intention of making electrode selective to copper ions. The electrode prepared

by incorporation of 5 mole percentage of CuS in Ag2S did response to Cu++ in

aqueous solution but the detection range was far from satisfactory. The

unsatisfactory functioning of electrode may be due to the low percentage of CuS

in the mixed sulfide and inability to control the stoichiometry of the CuS

appropriately in the final electrode. If percentage of CuS could be increased and

stoichiometry of CuS could be controlled properly, then well behaved electrode

sensitive to Cu++ may be successfully prepared. But this aspect of investigation

is left for future study.

Ruzika had prepared Cu-ion-selective electrode called “Selectrode” by

smearing CuS precipitate in graphite surface of the graphite rod extracted from

dry cell. But the electrode potential of such electrode is affected by the redox

system of the electrolytic solution. Hence, thin film CuS-electrode was prepared

by depositing thin film of CuS by chemical reaction on the porous surface of

graphite rod. The graphite rod was taken from used dry cell. Thin film CuS-

electrode, thus prepared, was found to be sensitive and selective to Cu++

between pH range 4 to 5. The response of the electrode towards Cu++ was quick

and stable potential could be obtained within less than 30 seconds. The change

of electrode potential per decade change in concentration of Cu++ was found to

be equal to 30 mV with standard deviation ± 0.7. Slight drift in standard

potential was observed with ageing of the electrode but the slope was almost

constant. The electrode followed Nernstian behavior from 10-1 to 10-4 mole/L

34

Cu++ concentration. The electrode can be successfully used for the

determination of Cu++ in aqueous solution by direct potentiometry and

potentiometric titration with EDTA. The electrode was found to be suitable

indicator electrode for the determination of Cr3+ and Fe3+ in aqueous solution by

indirect potentiometric titration with Cu++.

The electrode response to Cu++ was not affected by the presence of Co3+,

Zn2+ & Ca2+ in the test solution, slightly affected by Pb2+ & Cd2+ and strongly

affected by Ag+. The selectivity coefficient with Ag+, Pb2+ and Cd2+ are found to

be 4.46x104, 1x10-2 and 5.62 x 10-4 respectively.

Thus the thin film CuS deposited on graphite rod by chemical reaction at

graphite surface is a low cost copper ion-selective electrode which can be used

to determine the concentration of Cu++ in the range 10-1 to 10-4 mole/L. The

electrode thus prepared can be used as an indicator electrode for indirect

potentiometric titration of Cr3+, Fe3+ with Cu++.

35

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Selective Electrode Membrane, Central department of chemistry, T.U. (1997).

41. V. M. Jovanovic & M. S. Jovanovic, Ion-Selective Electrode Rev., 8, 115-

129(1986). 42. Teixeira, De S. Macros Fernando, et al., An Assoc. Bras. Quim., 147(2),

108-115(1998) cited in Chemical Abstracts, 129, 144253q (1998). 43. Lu, Xiaoqiao; Yang Xinhao; Chen Zuliang, Electroanalysis, 9(16), 1278-

1282(Eng.), 1997 cited in Chemical Abstracts, 128,70026y (1998).

38

APPENDIX

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7

-Log[Cu++]

Emf o

f the

Cel

l in

mV

Fig. A.1 : Cell emf as a function of negative logarithm of Cu++ ion concentration.

39

150

170

190

210

230

250

270

290

0 50 100 150 200 250 300 350

Time in Seconds

Emf o

f the

Cel

l in

mV

Fig. A.2 : Dynamic Response of CuS-graphite Cu-ion selective electrode to Cu++ ion

concentration.

1×10-1M

1×10-2M

1×10-3M

1×10-4M

40

50

70

90

110

130

150

170

190

210

230

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Volume of EDTA Solution in ml.

Emf o

f the

Cel

l in

mV

Fig. A.3 : Potentiometric titration curve for titration of 10 ml of 0.01 M Cu++ with

0.01 M EDTA solution.

020406080

100120140160180200

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Volume of EDTA Solution in ml.

ΔE/ Δ

V, m

V/m

l

Fig. A. 4 : Differential Curve.

End point

41

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Volume of Cu++ Solution in ml

emf o

f the

Cel

l in

mV

Fig. A.5: Potentiometric titration curve for titration of 10 ml of 0.01 M EDTA with

0.01 M Cu++ solution.

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Volume of EDTA Solution in ml.

ΔE/Δ

V, m

V/m

l

Fig. A.6: Differential Curve.

End point

42

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Volume of Cu++ Solution in ml

emf o

f the

Cel

l in

mV

Fig. A.9: Potentiometric titration curve for titration of 5 ml of 0.01 M Cr3+ and 10ml

0.01 M EDTA mixture with 0.01 M Cu++ solution.

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Volume of Cu++ Solution in ml.

ΔE/Δ

V, m

V/m

l

Fig. A.10: Differential Curve.

End point

43

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Volume of Cu++ Solution in ml

emf o

f the

Cel

l in

mV

Fig. A.7: Potentiometric titration curve for titration of 5 ml of 0.01 M Fe3+ and 10ml

0.01 M EDTA mixture with 0.01 M Cu++ solution.

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Volume of Cu++ Solution in ml.

ΔE/Δ

V, m

V/m

l

Fig. A.8: Differential Curve.

End point

44

100

120

140

160

180

200

220

240

260

280

0 1 2 3 4 5 6

- Log [Cu++]

emf o

f the

Cel

l in

mV

Fig. A.11: The effect of foreign ions to the response of Cu-ISE: I, Cu++ only; II, 0.1

M Cd++; III, 1×10-4 M Ag+; IV 0.1M Pb++.

I

II

III

IV