Determination of Arsenic(III) in Acetonitrile by Stripping...
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Indian Journal of Chemistry Vol. 24A, November 1985, pp. 957-959 Determination of Arsenic(III) in Acetonitrile by Stripping Voltammetry on Gold Electrode A A RAMADAN* & H MANDIL Department of Chemistry, University of Aleppo, Syria Received 25 February 1985; revised and accepted 3 June 1985 The electrochemical behaviour of As(IlI) on gold electrode has been studied in acetonitrile and LiCI0 4 . Reduction of As(JII) is carried out at a potential of -2.5 V. An oxidation wave corresponding to the equation, As - 5e->AsH, has a peak potential of 1.42 V. The occurrence of a higher and sharper peak on Au electrode than that on Pt electrode makes As determination on the former electrode more accurate and sensitive. Based on this, a method has been devised which allows the determination of As(III) at very low concentrations (4.0 x 10 -10 M), not reported before. Extensive studies have been carried out on the determination of As(I1I) by voltammetry. Ii was found that its reduction takes place in steps and its determination in very dilute solution can be accurately done up to 10 -7 M on a Pt electrode 1,2 by stripping voltammetry. Still lower concentrations were determined (10 -9 M) by inversion pulse polarog- raphy '. In the present work one reduction wave has been noticed which corresponds to the following equation, As3+ +3e->As Different solvents influence the half-wave potential and the magnitude of the diffusion currenr':". The half-wave potential (or alternatively the peak potential) was found to be + 2.06 V on a Pt electrode" and + 1.42 V on a gold electrode. It was also noted that gold electrode produces a higher and sharper peak than the platinum electrode. This difference in behaviour made arsenic de- termination on gold electrode more accurate with greater sensitivity which enabled its determination at lower concentrations than those reported earlier. ~7 fig. l=-Electrolysis vessel [(I) for the experimental solution, (2)auxiliary electrode vessel, (3)contains CHJCN to prevent contamination of experimental solution with water, (4) reference electrode vessel, (5) NaHCO, solution, (6) N, tube, (7) Au electrode] Materials and Methods A polarograph, model TIPOL, provided with a recorder model EPLI was used for the present studies. Electrolysis vessel (Fig. I) was provided with three electrodes (triple electrode). A disc like Au electrode (I mm diameter) was used. The solution was stirred with a magnetic stirrer and it was kept in a thermostat at 25.0° ± 0.5°e. All the chemicals and solvents used were of AR grade. Pure N2 gas was used for all the experiments. Procedure The electrolysis cell used is shown in Fig. (I). A standard As(III) solution in acetonitrile, 0.1 M in LiCI0 4 , was placed in the compartment. Nitrogen gas was bubbled through tube 6 for 15 to 20 min with continuous stirring. Reduction of As(llI) was carried out at a potential of -2.5 V. At the end of reduction, the potential was increased from + 0.45 V at a rate of 600 m V /min until an anode peak was obtained at a potential varying between + 1.39 and + 1.43 V. This caused the oxidation of As to As 5 + according to the following equation, As-5e->As5+ Results and Discussion Arsenic diffusion constant (K) is related inversely to the viscosity (11) of the solvent by the following equation, The reduction of As(III) on Pt and Au shows that in the former the working potential varies from - 1.0 to - 2.8 V and for the latter the working potential varies from -1.4 to -3.2 V. In the present work a potential of - 2.5 V was selected for the Au electrode, and 957
Transcript of Determination of Arsenic(III) in Acetonitrile by Stripping...
Indian Journal of ChemistryVol. 24A, November 1985, pp. 957-959
Determination of Arsenic(III) in Acetonitrile by StrippingVoltammetry on Gold Electrode
A A RAMADAN* & H MANDIL
Department of Chemistry, University of Aleppo, Syria
Received 25 February 1985; revised and accepted 3 June 1985
The electrochemical behaviour of As(IlI) on gold electrode has been studied in acetonitrile and LiCI04. Reduction ofAs(JII) is carried out at a potential of -2.5 V. An oxidation wave corresponding to the equation, As - 5e->AsH, has a peakpotential of 1.42 V. The occurrence of a higher and sharper peak on Au electrode than that on Pt electrode makes Asdetermination on the former electrode more accurate and sensitive. Based on this, a method has been devised which allows thedetermination of As(III) at very low concentrations (4.0 x 10 -10 M), not reported before.
Extensive studies have been carried out on thedetermination of As(I1I) by voltammetry. Iiwas foundthat its reduction takes place in steps and itsdetermination in very dilute solution can be accuratelydone up to 10 -7 M on a Pt electrode 1,2 by strippingvoltammetry. Still lower concentrations weredetermined (10 -9 M) by inversion pulse polarog-raphy '.
In the present work one reduction wave has beennoticed which corresponds to the following equation,
As3+ +3e->As
Different solvents influence the half-wave potentialand the magnitude of the diffusion currenr':". Thehalf-wave potential (or alternatively the peakpotential) was found to be + 2.06 V on a Pt electrode"and + 1.42 V on a gold electrode.
It was also noted that gold electrode produces ahigher and sharper peak than the platinum electrode.This difference in behaviour made arsenic de-termination on gold electrode more accurate withgreater sensitivity which enabled its determination atlower concentrations than those reported earlier.
~7
fig. l=-Electrolysis vessel [(I) for the experimental solution,(2)auxiliary electrode vessel, (3)contains CHJCN to preventcontamination of experimental solution with water, (4) referenceelectrode vessel, (5) NaHCO, solution, (6) N, tube, (7) Au electrode]
Materials and MethodsA polarograph, model TIPOL, provided with a
recorder model EPLI was used for the present studies.Electrolysis vessel (Fig. I) was provided with three
electrodes (triple electrode). A disc like Au electrode(I mm diameter) was used. The solution was stirredwith a magnetic stirrer and it was kept in a thermostatat 25.0° ±0.5°e.
All the chemicals and solvents used were of ARgrade. Pure N2 gas was used for all the experiments.
ProcedureThe electrolysis cell used is shown in Fig. (I). A
standard As(III) solution in acetonitrile, 0.1 M inLiCI04, was placed in the compartment. Nitrogen gaswas bubbled through tube 6 for 15 to 20 min withcontinuous stirring. Reduction of As(llI) was carriedout at a potential of -2.5 V. At the end of reduction,the potential was increased from + 0.45 V at a rate of600 mV /min until an anode peak was obtained at apotential varying between + 1.39 and + 1.43 V. Thiscaused the oxidation of As to As5 + according to thefollowing equation,
As-5e->As5+
Results and DiscussionArsenic diffusion constant (K) is related inversely to
the viscosity (11) of the solvent by the followingequation,
The reduction of As(III) on Pt and Au shows that inthe former the working potential varies from - 1.0 to- 2.8 V and for the latter the working potential variesfrom -1.4 to -3.2 V. In the present work a potentialof - 2.5 V was selected for the Au electrode, and
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INDIA.N 1. CHEM., VOL. 24A, NOVEMBER 1985
+E ,v 2,05'E' ,
0.1.51,25,
IIjII III
Fig. 2-Anodic peaks for As(III) at various concentrations [(I) 8xlO-,oM,(2)6xlO-,oM.(3)4xI0 10M]
[p, UA• 0 -
40 96o
o
)0 71
20 46
10 24
15 50 0
.La---<>---+2 O->--4 ...•0-+---:-<6~f---+-~8+:-0---<----:-:, O~O~.~,Sec
Fig. 3-Effect of Ir) on Ip for As(lII) [Curve I, for 10 "to 109MAs(lll), Curve 2, for 10 9 to 4 x 10 10M As(III)]
- 2.3 V was chosen for the Pt electrode, This enabledthe determination of As on the Au electrode to a newconcentration range of 4.0 x 10 -IOta 8.0 x 10 -10Mwhich was not reported before (see Fig. 2).
The present work shows a linear dependence of thetime of deposition (Tc = 5 to 30 see) for theconcentrations 10 x to 10 -y M. Likewise (TJ showedlinear dependence for a period 5 to 70 see atconcentrations 1.0 x 10 Y to 4.0 x 10 10M (Fig. 3). Asecond aspect of the present study was a correlation ofIp (peak current) and the arsenic ion concentration,
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o •
20 60
15
40
10
20
0.2 0.1. 0,6 0,8 1,0 1,2
o 2 6 8
1,4 0
10. C,roM
Fig. 4-Effect of varying concentration of As(lII) on Ip [(I) for (I to10) x 10 -9 M, (2) for (4 to 10) x 10 -10 M]
Table I-Determination of As(lII) in CH3CN in Presence0.1 M LiCI04 on Au Electrode
As(I1I) taken x 109 M As(I1I) found x 109 M, (nM)(nM)
*X S0.0320.0360.0320.0200.0110.0120.0140.011
8.006.004.002.001.00
0.800.600.40
8.006.004.001.991.000.790.600.40
LXi
*X (A verage mean) =~n
L (X_Xi)2
S (Standard deviation) = J:....; =....:.' _
n-I
SS, (Relative deviation) =co.
X
S,%0.400.600.801.001.101.522.402.80
CA" wrnch may be expressed by the followingequation,
i,=f(CAJ
The above relationship showed linear proportionalityover the concentration range 4,Ox 10-10 to 1.0x 10 -9 M (Fig. 4). In conclusion, we consider that thefollowing factors facilitated the submicro de-termination of As(III):
(i) Acetonitrile was a suitable solvent.
RAMADAN & MANDIL: DETERMINATION OF ARSENIC(I1D
(ii) Lithium perchlorate was a satisfactoryelectrolyte for the above determinations.
(iii) Use of a disc Au electrode operating at areduction potential of -2.5V. The optimal time ofdeposition ('<) was 5 to 30 see for the concentrationrange 1.0 x I0 ~9 to 1.0 x 10 ~8 M, and 5 to 70 see forthe concentration range 4.0 x 10 ~l 0 to 1.0 x I0 ~9 M.
Table (I) includes some of the samples analysed inthe present work, in which very low concentrations, ofthe order of 10 ~lO M, were determined in comparisonwith previous work which did not exceed 10 ~7 M.
ReferencesI Kaplin A A, Mordvinova N M & Veils N A, J anal Chern (USSR),
37 (1982) I 179.
2 Arnold J P & Johnson R M, Talanta, 16 (1969) 119I.
3 Forsberg G, O'Laughlin J W & Megargle R G, Anal Chern, 47(1975) 1586.
4 Ramadan A A,Agasyan P K & Petrov S I, Janal Chern (USSR), 31(1976) 322.
5 Ramadan A A, Res J Aleppo University (Syria), 1 (1976) 13.
6 Ramadan A A, Mandil H, Agasyan P K, Kamenev A 1& M ustafaI, J anal Chern (USSR), 39 (1984) 1234.
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