Voltammetry with disc electrodes and its analytical application

ELECTROANALYT1CALCHEMISTRYAND INTERFACIALELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne Printedin The Netherlands

V O L T A M M E T R Y W I T H DISC E L E C T R O D E S A N D APPLICATION

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ITS A N A L Y T I C A L

II. ANODIC STRIPPING VOLTAMMETRY OF TRACE CONCENTRATIONS OF SILVER AND COPPER EMPLOYING A GLASSY CARBON ELECTRODE* MILOSLAV KOPANICA ANOFRANTISEK VYDRA J. Heyrovsk) Pohtrographie Institute, Czechoslovak Academy of Sciences, Prague "Czechoslovakia)

(Received 21st October 1970)

Electrochemical stripping techniques, and anodic stripping voltammetry especially, are widely used methods for the determination of trace amounts of metals 1. Recently great interest has been paid to the application of solid electrodes in anodic stripping voltammetry. Platinum2- 4, gold 5 - 8, silver 9,1 o, Copper 11 and graphite 12 15 electrodes have been investigated and it has been stated that the sensitivity of anodic stripping voltammetry is generally higher when solid electrodes are applied; in many cases the reproducibility is however very poor. The preparation and pretreatment of solid electrodes are most important because these factors determine the reproducibility and accuracy of the results. Various graphite forms were recently explored as electrode materials ~2-15 and electrodes prepared from pyrolytic graphite impregnated with wax were found most suitable for anodic stripping voltammetry 14. Eisner and Mark 14 have shown that wax impregnated pyrolytic graphite electrodes yielded voltammograms where silver exhibited one main and one or two secondary dissolution peaks in acidic, neutral and alkaline media. The application of glassy carbon rotating electrodes in voltammetry has been studied in this laboratory ~6. During this study the pretreatment step was found to be very simple when the glassy carbon electrode was used. To obtain reproducible voltammograms a simple polish with very fine sand paper was quite satisfactory. In this work the use of rotating glassy carbon electrode in anodic stripping voltammetry was examined. Compared with other graphite forms, the stripping voltammograms recorded with the use of the glassy carbon electrode showed one sharp and narrow dissolution peak when a solution of one metal was analyzed. The technique described may be also applied to the analysis of mixture of two metals. - -

EXPERIMENTAL Apparatus

The instrumentation used was constructed from operational amplifier modules by Dr. Kalvoda t7 from this institute. The apparatus enabled scan rates to be employed from 10-60 mV s-1 in the anodic dissolution. The voltammograms were recorded * Part I of the series: F. Vydra and P. Pefftk,J. Electroanal. Chem., 24 (1970) 379. J. Electroanal. Chem., 31 (1971) 175-181

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M. KOPANICA, F. VYDRA

with a Polarecord E 261 R (Metrohm, Switzerland) used as voltmeter. The indicator electrode was a rotating disc made from glassy carbon with surface area 7.06 and 19.62 mm 2 respectively. Glassy carbon G.C. 20 (Tokay Electrode Manufacturing Co., Tokyo, Japan) had a total pore volume of 0.36~o and pore diameter of 10-2-10-3 A. A glassy carbon rod of a given diameter was pressed into the Teflon tube (outside diameter 15 mm) at a higher temperature. The complete electrode was placed into the rotation mechanism which enabled the experiments to be carried out at rotation speeds of 500-10000 rev./min. The reference electrode was a saturated calomel electrode connected with the solution to be analyzed by a salt bridge (0.3 M KNO3). The cell used was a 150 ml quartz beaker. The solution analyzed had a volume of 100 ml.

Reagents All chemicals used were reagent grade. The supporting electrolytes were prepared from "Suprapure" sulphuric acid (E. Merck, Darmstadt, Germany) and from "Superpure" potassium hydroxide (Institute Inorganic Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia). Solutions of copper(II) sulphate were prepared from "Specpure" copper(II) oxide (Johnson, Matthey, England). Thrice distilled water was employed for the preparation of all solutions. Pretreatment of the electrode Pretreatment of the glassy carbon electrode (GCE) is simple. The electrode surface was renewed before each measurement by a short polish with metallographic papers, Nos. 3 and 5 (SIA, Switzerland). Procedure for the deposition The solution to be analyzed was first deaerated by a stream of pure nitrogen for 15 min. The constant stirring necessary for the electrodeposition of the metals was attained by rotation of the electrode itself. In some cases the solution was stirred simultaneously by means of a magnetic stirrer. The time of the deposition step was controlled with an electrical stop-watch. In case when the magnetic stirrer was used, this was switched off 30 s before the start of the stripping process. During the deposition and stripping process nitrogen was allowed to flow over the solution to prevent contact with the air. The deposition potential employed was - 0.60 V vs. the saturated calomel electrode (SCE) in all experiments. Procedure for the stripping process The deposit was anodically stripped by means of a linear anodic potential sweep with a rotating or a stationary GCE. Anodic stripping was started from the deposition potential (-0.60 V). The area under the peak, which is proportional to the number of coulombs consumed during the stripping step, was determined by weighing the recorder paper. The concentration of the metal to be analyzed was determined by the method of standard addition. RESULTS AND DISCUSSION

The anodic stripping voltammograms obtained with the rotating GCE showed J. Electroanal. Chem., 31 (1971) 171-181

ANODIC STRIPPINGOF SILVERAND COPPER WITH RDE

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sharp mono-peaks when solutions of single metal ion were analyzed. Figure 1 shows a typical anodic stripping voltammogram of dilute solutions of silver and copper(II) ions. The area under the dissolution peak is linearly proportional to the concentration of the metal to be analyzed. The experiments carried out with the rotating GCE showed that the concentration of the sample is also a linear function of the peak height ip. It is obvious that for analytical utilization of anodic stripping vdltammetry the GCE is preferable to other graphite forms. ip 80 lO7 %/root t-' Ckl

60

5

Z5

10

125

I 1

I 2

I 3

I 4

40

20.

I

0

I

i

+o.6 +04 ~ 2

i

0

i

i

-0.2 -0.4 E/v

lO6 Ccu/mot t-'

Fig. 1. Anodic stripping voltammogramsof dilute silver and copper solns. Rotating GCE ; 2800 rev./min; scan rate 33 mVs-1; deposition time 3 min (2.5 rain with magnetic stirrer); deposition potential -0.60 V (SCE); supporting electrolyte0.01 M H2SO4; CAg=5x 10-7 M; Ccu=2 × 1 0 - 6 MFig. 2. Dependenceof the peak potentials correspondingto anodic dissolution of silver (1) and copper (2) on the concn, of these metals. Rotating GCE ; 2800 rev./min; scan rate 33 mV s- 1; depositiontime 3 min; deposition potential -0.60 V (SCE); supporting electrolyte0.01 M H2SO4.

During the experiments carried out with submicro solutions of silver(I) and copper(II) ions (10-6-10 -9 M) the area under the dissolution peaks or the peak heights were found to depend linearly on the concentration of silver(I) and copper(II) ions. With increasing concentration of silver or copper, the potentials of the corresponding dissolution peaks Ep shifted to more positive values. The experimental data showed that log Ep depended linearly on the concentration of silver or copper. The corresponding data are presented in Fig. 2. In the analytical application of anodic stripping voltammetry with the rotating GCE it is recommended that the recording of the anodic stripping voltammogram is repeated; the second voltammogram is equal to the voltammogram of the pure electrolyte (blank). In the analysis of more concentrated silver solutions (silver concentration above 5 x 10-7 M, deposition time 3 min a t - 0 . 6 0 V) the second voltammogram showed a small dissolution peak. To determine the concentration of silver accurately it is necessary to add together the heights of the first and second peaks. It is, however, more convenient to repeat the analysis with a shorter deposition time. The shape of the dissolution peaks of silver and copper does not depend on the composition of the supporting electrolyte. The peak potentials however depend J. Electroanal. Chem.,

31 (1971) 175-181

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M. KOPANICA, F. VYDRA

TABLE 1 EFFECT OF THE MEDIUM ON THE ANODIC STRIPPING VOLTAMMOGRAMS OF SILVER AND COPPER

Electrolyte medium

Peak potential Ep/ V (SCE)

0.01 M H2SO 4 0.01 M H2SO 4 and 0.05 M K2SO 4 0.05 M K N O a 0.05 M N H 3 and 0.05 M N H 4 N O 3

Copper

Silver

+0.118 +0.080

+0.313 +0.450

+0.060

+0.335 + 0.093

no peak

Rotating GCE, 2800 rev./min; Cc, 2

× 10 - 6 M ; CAg 5 × 10

7 M.

on the quality of the supporting electrolyte; the basic data are summarized in Table 1. From an analytical point of view the linear dependence of the peak heights on the concentration of the metal analyzed is most important. Another important factor is the fact that the peak height depends linearly on the deposition time; this enables a comparison of the results obtained by the measurements with different deposition times. Other important factors connected with the deposition process are the value of deposition potential Ed and the rotation velocity of the electrode during the deposition step. In agreement with earlier studies it has been observed that the amount of material deposited increases with increasing rotation velocity of the electrode. Additional stirring with a magnetic stirrer causes only a slight increase in the amount of the deposit. During the analysis of silver and copper solutions this increase was found to be approximately 10 %. The results of the analysis of silver and copper samples have also shown that the amount of deposited metals increases with decreasing deposition potential, as shown in Fig. 3. '

50

50-

/-,0-

40

30-

30 20

10-

10

I -200

-400

-600

10

I

I

20 30 v/mv s -1

I

I

40

50

Fig. 3. Effect of the deposition potential Ea on the peak height of silver and copper. Scan rate 33 mV s - 1 ; deposition time 3 min; supporting electrolyte 0.01 M H2SO4; (1)Anodic stripping of silver, rotating G C E 2800 rev./min; (2) anodic stripping of silver, stationary G C E ; (3) anodic stripping of copper, rotating GCE, 2800 rev./min. Fig. 4. Dependence of peak height on the scan rate for anodic stripping of silver. Rotating GCE, 2800 rev./min; deposition time 3 min; deposition potential - 0 . 6 0 V (SCE); supporting electrolyte 0.01 M H 2 S O 4 ; CAge-5 X

10 -7 M.

J. Electroanal. Chem., 31 (1971) 175-181

ANODIC STRIPPING OF SILVER AND COPPER WITH RDE

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As was expected, the amount of deposited material depends on the surface area of the GCE used. When solutions of copper and silver were analyzed under otherwise identical conditions (deposition potential, deposition time, identical stripping process) but with two different GCE, the peak heights or the areas under the peaks were found to be proportional to the surface areas of each GCE used. The anodic stripping of the deposited material was carried out with a rotating and a stationary GCE. Stripping carried out with the rotating electrode produced higher peaks or peaks of larger area, compared with the peaks obtained with the stationary GCE under otherwise identical conditions. The height of the peaks is further influenced by the rotation speed of the electrode. This effect is, however, not very pronounced; for anodic stripping of silver and copper the optimum rotation velocity was 2000-3000 rev./min. The dissolution peaks are more affected by variation of the scan rate. The peak heights corresponding to anodic dissolution of silver and copper with increasing scan rates are shown in Fig. 4. The recording of the dissolution peaks of silver and copper at higher scan rates (> 50 mVs-1) is not accurate enough owing to the limited velocity of the recorder pen used. Scan rates of 30-45 mVs- 1 were therefore employed for analytical work. Finally the effect of interrupting the current after the deposition step on the nature of the dissolution peaks was studied. When the circuit was broken for 30 s after the deposition step, the voltammogram recorded after this period was identical with that recorded without any interruption of the current. This finding is very important for analytical applications, it enables the solution analyzed to be exchanged after the deposition step and the stripping of the deposit to be carried out into another supporting electrolyte.

Analytical application The experimental results described in this work show that anodic stripping voltammetry with the GCE may be used for the determination of trace amounts of silver or copper in any of the supporting electrolytes studied. The analysis, however, becomes more complicated when the solution contains both these metals. Copper and silver, when present in comparable concentrations, may be determined simultaneously when sulphuric acid (mixture of sulphuric acid and potassium sulphate) is used as the supporting electrolyte. If, on the other hand, one of the metals is present in excess of the second, the resulting dissolution peaks may not be identical with that obtained by the analysis of the solution of a single metal. This effect was very pronounced when potassium nitrate was used as supporting electrolyte. An interesting result was found in the analysis of mixtures containing silver and copper with constant silver and increasing copper concentration. The height of the resulting dissolution peak of silver increased linearly with increasing concentration of copper and the dissolution peak of copper appeared when the ratio Ag : Cu reached the value 1 : 5. Analytically promising results were obtained when buffer solution ammoniaammonium nitrate (pH =9.3) was used as supporting electrolyte. In ammoniacal solution copper is not deposited on the rotating GCE at the deposition potential -0.60 V (SCE). An earlier voltammetric study 18 showed that Cu(NH3) 2+ yielded, in ammoniacal medium, a one-electron reduction step on the rotating GCE with a half-wave potential of + 0.10 V (SCE). This result agrees with the finding that copper does not exhibit a dissolution peak in ammoniacal medium. Only at high copper J. Electroanal. Chem., 31 (1971) 175-181

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M. KOPANICA, F. VYDRA

conCentrations, was an anodic wave observed during the stripping process. This wave corresponds probably to the oxidation of copper (I) ions formed during the deposition period. In the analysis of silver and copper mixtures the disturbing effect of the anodic wave described was observed when the Ag:Cu ratio was above 100. When silver was to be determined in the presence of a large excess of copper, the disturbing effect of the high copper concentration was eliminated by changing the supporting electrolyte after the deposition step. The deposition was carried out with the sample in ammoniacal solution. After deposition, the circuit between the GCE and reference electrode was switched off, the electrolyte was replaced with a deaerated solution of pure electrolyte (NHa+NH#NO 3, pH 9.3), the circuit was switched on and anodic stripping was started. The resulting dissolution peak of silver was identical with that recorded in the absence of copper. This result agrees with the earlier suggestion that copper(II) ions are not deposited on the rotating GCE under the given conditions and that the disturbing effect of higher copper concentration is connected with-the oxidation of copper(I) formed during the deposition step. This procedurewas applied for the determination of 5 x 10-7 M silver solution in the presence of 5 × 10- 3 M copper; the error of the determination was not greater than ___5~o rel. The application of GCE in anodic stripping voltammetry enables silver to be determined with sufficient accuracy at a concentration of 1 x 10-9 M, when the surface area of the electrode is 19.6 mm 2 and the deposition time 15 min at -0.60 V (SCE). For the determination of copper the technique described is less sensitive; under identical conditions copper may be determined at a concentration of 1 × 10 -s M. ACKNOWLEDGEMENT

The authors wish to thank Dr. R. Kalvoda, J. Heyrovsk3~ Polarographic Institute, Prague, for his collaboration with the construction of the instrumentation used. SUMMARY

The rotating GCE has been found to be a suitable electrode for anodic stripping voltammetry of submicro amounts of metal ions. Sharp mono peaks were obtained corresponding to the anodic dissolution of silver and copper in acidic, neutral and alkaline media. The effect of various factors (rotation velocity of the electrode during the deposition and stripping process ,scan rate, deposition potential and deposition time, composition of the supporting electrolyte) on the peak current and peak potential was studied. The peak height has been found to be a linear function of the concentration of the metal analyzed and the log of the peak potential to depend linearly on the concentration of silver and copper, respectively. The determination of trace amounts of silver ( 1 0 - 6 - 1 0 - 9 M) and copper ( 1 0 - 6 - 1 0 - 8 M) can be carried out in acidic ,neutral or alkaline media. A procedure was established for the determination of micro amounts of silver in the presence of a large excess of copper. Ammonia/ammonium nitrate solution was employed as supporting electrolyte and when the concentration of copper was too high, the sample solution was replaced by a pure solution of ammoniacal buffer after the deposition J. Electroanal. Chem., 31 (1971) 175-181

ANODIC STRIPPING OF SILVER AND COPPER WITH RDE

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step. 5 x 10-7 M silver solutions containing copper at a concentration of 5 x 10-3'M were analyzed successfully; the error did not exceed +_5%. The application of GCE in anodic stripping voltammetry is being further studied. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

R. NEEd, Inverse Polarographie und Voltametrie, Akademie Verlag, Berlin, 1969. J. T. BRYNE, L. B. ROGERS ANt) J. C. GRI~SS, JR., J. Electrochem. Soc., 98 (1951) 452. M. W. BR~ITER, J. Electrochem. Soe., 114 (1967) 1125. D. P. SANDOZ, R. M. PEEKEMA,H. FREUND AND C. F. MORRISON,J. Eleetroanal. Chem., 24 (1970) 165. R. C. PROPS'r, ,L ElectroanaL Chem., 16 (1968) 319. E. SCHMIDT AND H. R. QYGAX, J. Electroanal. Chem., 13 (1967) 378. E. SCHraIDT, P. MOSER AND W. RmSEN, Helv. Chim. Acta, 46 (1963) 2285. E. SCHMIDT AND H. R. Q'CGAX, J. Electroanal. Chem., 12 (1966) 300. D. J. ASTLEY, J. A. HARRISON ANt) H. R. THIRSK, J. Electroanal. Chem., 19 (1968) 325. E. SCHMIDT AND H. R. GFGAX, Helv. Chim. Acta, 48 (1965) 1584. E. SCh'MIDT AND H. R. GYGAX, Helv. Chim. Acta, 49 (1966) 1105. S. P. PERONE, Anal. Chem., 35 (1963) 2091. B. H. VASSOSAND H. B. MARK, JR., J. Electroanal. Chem., 13 (1967) 1. U. EISNER AND H. B. MARK, JR., J. Electroanal. Chem., 24 (1970) 345. M. ROYZENBLAT AND Kt-L E. BRAININA, Elektrokhimiya, 5 (1969) 396. F. VYDRA AND P. PvrkK, J. Eleetroanal. Chem., 24 (1970) 379. R. KALVODA, Chem. Listy, 64 (1970) 1280. F. VYDRA AND M. STULIKOVk, unpublished results. J. Electroanal. Chem., 31 (1971) 175-181