9. I., Zh. Fiz. Khim. 34, 1885 (1960); C.A. 55, 9116f (1961). (16) McMasters, D., Schaap, W. B. Proc. Indiana Acad. Sci. 67, 117 (1957). (17) Macero, D. J., Herman, H. B., Dukat A. J., Paper 65, Division of Analylical Chemistry, 141st Meeting, ACS, Washington, D. C., April 1962. (18) Maksimov, V., ?Jovoselov, A., Semenko, V., Zh. Fiz. Khim. 2, 997 (1957); C.A. 52, 1825b (1958). (19) Meites, L., Cover, R., Anal. Chim. Acta 25, 93 (1961). (20) Meites, L., Israel, Y., J . Ani. Chem. SOC.83, 4903 (1961). (21) Misumi, S., Ide, Y., Bull. Chern. SOC. Japan 33, 836 (1960). (22) Misumi, S., Masuda, Y., Anal. Chim. Acta 28, 188 (1963). (23) Moeller, T., Cullen, G. W., J . Znorg. Nucl. Chem. 10, 148 (1959). ( 2 4 ) Moeller, T., Galasyn, V., Ibid., 12, 259 (1959).
125) Moeller. T.. Zimmerman. P. A,.' J. Am. Chern: So;. 75, 3940 (1953). (26) Onstott, E. I., Zbid., 74, 3773 (1952). (27) Zbid., 77, 2129 (1955). (28) Ibid., 78, 2070 (1956). (29) Ibid., 81, 4451 11959) (30) Panda, S.,Patnaik, D., J . Indian Chem. SOC.33,877 (1956). (31) Purushottarn, Bh. S. V., Raghava, Rao, Anal. Chim.Acta 12,589 (1955). (32) Randles, J. E. B., "Progress in Polarography," P. Zuman, I. M. Kolthoff, eds., Vol. I, p. 133, Interscience, New York, 1962. (33) Rinehart, R. W., ANAL.CHEW 26, 1820 (1954). (34) Schaap, W. B., et al., Rec. Chem. Progr. 22, 197 (1961). (35) Schober. G.. Gutmann., V.., Monats. Chem. 88,i06 (1957). (36) Schober, G., Gutmann, V., 2. Elektrochem. 63,274 (1959). '
(37) Schober, G., Gutmann, V., Nedbalek, E., 2. Anal. Chem. 186, 115 (1962). (38) Shults, W. D., ANAL.CHEX.31, 1095 I1 ,-9.59). - - - ,.
(39) ,Southworth, B., Osteryoung, R. A., Zbzd., 33, 208 (1961). (40) Treindl, L., Collection Czechoslov. Chem. Commun. 24, 3389 (1959). (41) VlEek, A. A.,Zbid., 20, 894 (1955). (42) Zbid., 24, 181 (1959). (43)Wise, E. N., Cokal, E. J., AKAL. CHEM.32, 1417 (1960). (44) Witt, J. P., Onstott, E. I., J . Inorg. Nucl. Chem. 24. 637 - - f 1962). (45) Yakubson,S. I., Kostromina, N. A,, Zh. Aremg. Khim. 2, 349 (1957): C.A. 51, 16149c (1957). 1
\ - - -
I
RECEIVEDfor review June 20, 1963. Accepted August 26, 1963. Work supported by the National Science Foundation, research grant N o . G-19122.
Anodic Stripping Voltammetry of Gold and Silver
with Carbon Paste Electrodes EMMETT S. JACOBS Jackson laboratory, E. I . du Pont de Nemours & Co., Inc., Wilmington 99, Del.
b Carbon paste electrodes have been applied to the determination of submicrogram quantities of gold and silver using the general technique of anodic stripping voltammetry. The effects of electrode material, electrode area, supporting electrolyte, deposition potential, deposition time, deposition solution volume, stirring rate, and anodic voltage scan rate are discussed, A simple, easily prepared carbon paste electrode provides the determination of as little as 1 .O p.p.b. of gold and 0.25 p.p.b. of silver with approximately 10% relative error. A modified Sargent Model XXI Polarograph was used for these analyses.
A
stripping techniques have been a subject of renewed interest during the past few years, due in part to the remarkable sensitivity that can be obtained with this electroanalytical method. I n these analytical applications the metal to be determined is electrodeposited from a dilute solution onto a microelectrode and is then stripped from this electrode by anodic oxidation. The stripping has been carried out using linear voltage scan ( d ) , current-step procedures ( 7 ) , and square wave polarography (3). Generally some form of a mercury electrode has been used. Platinum and gold electrodes have been employed (6) to provide greater sensitivity because of more complete recovery of the deposited SODIC
21 12
ANALYTICAL CHEMISTRY
metal and a t the same time provide a more positive potential range. One real difficulty with the noble metal electrodes has been the inability to treat them properly, so as to give reproducible results, either qualitative or quantitative. This is due to the oxide films formed in oxidizing and noncomplexing media ( 2 ) and chloride films formed by platinum in hydrochloric acid solutions (6). This paper reports on the use of a carbon paste electrode (C.P.E.) for use in the anodic stripping voltammetry of the noble metals, gold and silver. Adams (1) first reported the use of a C.P.E. for organic polarography in 1958; however, its application to inorganic stripping analysis reported in this paper is new. The carbon paste electrode has a number of characteristics which make it very good for anodic stripping voltammetry. It is a relatively inert electrode, whose surface can be easily renewed to give reproducible results and the electroSWITCH F h
\ 2 5 0 n 1z5on(250ri~25OIl
i0 .
IT
6-VOLT BATTERY
0-1 - t
- t
- t
Figure 1.
\rn8 ""LI.
-
t
Electroplating controller
deposited metal can be completely recovered by anodic stripping. The electrode is almost entirely free of residual currents and has a wide range of anodic potential use. EXPERIMENTAL
Apparatus. A Sargent Model XXI Polarograph was used for all anodic stripping analyses. The recorder was modified to give a 2.5-second fullscale pen response and the linear voltage drive was increased for most of the work from 2 to 15 r.p.m. T o run more analyses per day, a simple four-position electroplater (Figure 1) was used during the plating cycle. This freed the polarograph for recording the anodic stripping current-voltage curves. The electrolysis cell shown in Figure 2 used a saturated calomel electrode (S.C.E.) as a reference electrode. This cell had a resistance of about 800 ohms with a solution of 0.1N hydrochloric acid. A small magnetic stirrer was used to agitate the solutions. The cell was not thermostated for this work. The various carbon paste electrodes used in this study are shown in Figure 3. A11 were easily prepared from female glass joints of different sizes, and the area of each size proved to be reproducible from one joint to another. Electrical contact is made to the carbon paste by means of a copper wire sealed to the glass with D e Khotinsky cement. Materials. All chemicals mere reagent grade and were used without further purification. Stock solutions of each metal ion were prepared at and l O - 5 M concentrationq, and diluted
immediately prior to All solutions were made with
LLc,,=YUmLSr
use.
dou hle-deionized water ....~.~~ .~~~~~ ~~~~~
~~
~
A nitrogen gas purge was used to remove oxygen from the solutions before determining silver. Because of the positive reduction potentials used for gold, it was unnecessary to remove the oxygen before gold determinations. All glass apparatus and Teflon stirrers were cleaned prior to each determination with aqua regia and rinsed with double-deionized water. The tip of the calomel electrode was cleaned with chromate-sulfuric acid cleaning solution and then rinsed with double-deionized water nrior to each determination. The'carhon paste was prepared by mixing 50 grams of Acheson's No. 38 graphite powder with 20 ml. of Nujol to obtain a thick uniform paste. If the amount of Nujol was decreased, the sensitivity of the electrode was increased, hut the paste became too dry to permit the preparation of reproducible surfaces. The carbon paste electrode was resurfaced with fresh paste prior to each determination. Usually about 3 days' experience was needed before a technician became proficient enough to nreaare electrodes which would eive ;es;lts with only 2 to 3% rel&ve standard deviat,ion.
paste electrode from a stirred solution for a predetermined length of time, usually 15 minutes. The stirring must be reproducible; thus, placement of the electrodes, volume of solution, and stirring rate must be fairly reproducible. The potential of the C.P.E. during the plating cycle was held at a predetermined value, usually f0.1 volt for gold and -0.3 volt us. S.C.E. for silver and mixtiires of silver and gold.
After the appropriate electrolysis time had elapsed, the electrodes were disconnected from the electroplater and connected to the polarograph. The C.P.E. was connected to the normal D.M.E. lead and made positive with respect to the S.C.E. After about 20 seconds for the current t o decay to a constant value, the anodic stripping wave was recorded by linearly varying the voltage over a range of +0.3 to 1.3 volt us. S.C.E. for gold, -0.3 t o +0.7 volt us. S.C.E. for silver, and -0.3 to +1.3 for mixtures of gold and silver. The optimum rate of voltage scan was 1200 mv. per minute. Typical current-voltage dissolution curves for silver and gold at different concentrations are shown in Figure 4.
-0.5
t1.5
F i g u r e 4. Current-voltage curves for anodic stripping of gold and silver on C.P.E.
. .
10-'M _ _ _ 1 O@M - , - Background . .
B
A
Figure 3.
D
C
Carbon porte electrodes A.
0.196
19. cm.
8. 0.283 sq. cm. C.
0.502 rq. cm.
D. 1.32 sq.
cm.
These determinations aere made with a 0.283-sq. em. C.P.E. from 100 ml. of solution stirred at 1400 r.p.m. and electrolyzed for 15 minutes. The background for each metal ion was determined by anodic stripping after a 15minute electrolysis of the electrolyte solution. The BP (peak potential) became sliehtlv more Dositive with the smaller dissolution \%-ares. To investigate the variahles, precision, and applicability of this method, gold and silver ion concentrations were lneasured over a range of 5 X IO-' to 5 x 10-9~. ~Y
I
VARIABLES
Eledrometric cell
VOLTS VS.SCE
~~~~~~~
Figure 2.
c
Besides the electrode material at least seven other variables affect the results of an anodic stripping determination. Supporting Electrolyte. The potential range for the C.P.E. as well as t h e plating potential for each metal will differ for each supporting electrolyte. A solution of 0.1N nitric acid proved t o be the optimum electrolyte for silver, since it gave t h e greatest plating rate and t h e most reproducible results. A solution of 0.1N hydrochloric acid proved to he
.
t h e best medium for gold tor the same reasons. Nitric acid was used for analysis of mixtures of these metals. The useful pote in either of thes approuimately S.C.E. Deposition rule of thumb !or anodic stripping voltammetry is t o operate t h e potential of the working electrode during the plating cycle at a value about 0.2 volt more cathodic t h a n t h e polarographic half-wave potential. Since the reduction half-wave cannot he it is measured for values below 10W6MM, necessary to predict the reduction potential from the Nernst equation. .4t values of IO-EM and lower this prediction may not be strictly obeyed on all electrode surfaces, so it was necessary to determine the optimum plating potential experimentally. In general, the reduction potentials of both silver and gold became more negative as the concentration of the metals decreased. Finure 5 shows a d o t of the Dlatiip potential us. the reiative amount of metal plated within 30 minutes for solutions of gold and silver at several concentrations. The amount of each metal plated was determined by anodic stripping determinations made on each metal at the various indicated potentials. The amount plated at -0.4 volt us. S.C.E. was taken as 100%. The conclusions drawn from this figure are that it is possible to plate gold and not silver at the expense of losing some sensitivity for determining gold. Actually in practice we were not able to plate gold without plating some silver. This is possible only by very carefully controlling the cathodic potential, which was not attempted. From the curves VOL 35, NO. 13, DECEMBER 1963
2113
4 (r w
VOLTS VS. SCE I l l I I t 0 . 4 t0.2 0.0 -0.2 -0.4 DEPOSITION POTENTIAL
Figure 5. Effect of deposition potential on amount of metal plated
-i o - 7 ~ --- 10-*hi for these metals a value of $0.1 volt
was selected for gold and -0.3 volt v s . S.C.E. for silver and mixtures of gold and silver as operating plating potentials. Deposition Time. The amount of metal plated from a solution of a certain concentration is directly proportional to the time of electrolysis, a8 shown by Figure 6. The amount plated was determined from the coulombs of electricity represented by the area under the anodic stripping peak. The per cent plated was determined by comparing these anodic stripping analysis values to the theoretical amount initially present in the plated solution. These results were obtained from determinations made on 10-'M solutions with a O.5-sq.-cm. area
Table I, Peak Current as a Function of Concentration
(Electrolysis time, 15 minutes) Concn., i,, Ion mole/liter pa. Au+S(O.lM HCI) 5.0 X 10-9 0.12 1.00 x 10-8 0.21 5.00 X lo-* 1.34 1.00 x 10-7 2.78 7.00 2.60 x 10' Ag+ (0.1M HKOa) 5.00 X 10-9 0.19 1.00 X 0.36 5.00 x 10-8 1 81 1.00 X lo-? 3.68 2.50 x 10-7 9 39
Table II.
I IOOC
I 180
I
60
120
I
DEPOSITION TIME. MIN.
Figure 6. Effect of plating time on amount of metal plated
nients were just as precise as the area measurements. Deposition Solution Volume. While the rate of plating is proportional to the concentration, it is possible, in the case of extremely long deposition periods, to increase the amount of metal plated by plating from very large volumes of solution. The rate of change of concentration during electrolysis is changed by going t o larger volumes, since the ratio of the amount of metal plated out to the amount remaining in solution is smaller. For example, the amount of gold plated from 100 ml. of a 10-*M solution of gold during a I-hour period can be doubled by plating from 1200 ml.
Effect of Electrode Area on Residual Current and Amount of Gold Found at Constant Plating Time
Electrode area, sq. cm.
Peak height, pa.
0.196 0.283 0.502 1.32
10.7
21 14
C.P.E. However, these are general curves and they represent the rate of plating for these metals a t any concentration onto a carbon paste electrode. It is obvious from this plot that it is impractical to plate all of the metal in an analytical procedure. For a gold concentration of 10-7M a period of 15 minutes is sufficient t o plate about 0.1 pg, of gold and this is equivalent t o 50 microcoulombs or a flow of 5 pa. for 10 seconds, which can be measured by the apparatus used in this study. The results for anodic voltammetric determination of gold and silver represented by peak height us. concentration for a 15-minute electrolysis time (Table I) show that the peak current is linearly related to the concentration. GeneraIly the peak area in coulombs is used to measure the amount plated, but in these analyses the peak height measure-
8.52
18.2
43,4
ANALYTICAL CHEMISTRY
Peak height/ electrode area 43.5 38.1 36.2 32.8
Residual current/ electrode area 0.68 0.49 0.60 0.64
VOLTS VS. SCE
Figure 7. Effect of stirring during anodic stripping of gold
- - - Unsfirred -Stirred of the same solution. Since all oi our work was done with a t least 100 ml. of solution, the volume required to achieve a worthwhile increase in the amount plated was much larger than practical for general use. Electrode Area. The larger the area of a n electrode the greater vias the amount of metal deposited within a given time period. To determine the optimum electrode area for plating and stripping, gold was plated from a 2.5 X 10-iilI solution onto C.P.E.'s whose apparent surface area varied from 0.196 to 1.32 sq. cm. for 30 minutes a t a potential of +0.1 volt us. S.C.E. The amount plated was determined by measuring the anodic stripping peak height. Under these conditions one would expect the ratio of the amount plated (peak height) t o the electrode area to be a constant, and the ratio of the residual current during the stripping cycle t o the electrode area to be constant. This was only partly true, as shown by the results in Table 11. The smallest electrode had a larger plating rate and a greater residual current in proportion t o its area. Also, the largest electrode had the lowest plating rate and a high residual current in proportion to its area. Thus it would seem that there is a lorer and a higher limit to the electrode area and the optimum area for analysis should be about 0.3 sy. em. Stirring Rate. The sensitivity of this method depends on the amount of metal plated, which in turn depends on the diffusion of the metal ion to the cathode during plating. Stirring the solution will increase the rate of diffusion, as shown in Table 111. A stroboscope was used to measure revolutions per minute. Although higher values for the peak height are obtained with each successive increase in the rate, 1400 r.p.m. is the practical limit, Above this value the peak height was not reproducible, because of formation of bubbles in the
Figure 8. Effect of linear voltage scan rate. on anodic stripping of silver A.
1 5 0 mv./min.
B.
562 mv./min. 1200 m\./min.
C.
solution and on the surface of the carbon paste electrode. ;\lost of the investigations with anodic stripping voltammetry reported in the
Table 111. Effect of Stirring Rate on Amount of Gold Plated
Stirring rate, r.p.m.
Peak height,
400 600 so0 1200 1400
4.5 6.2 7.5 8.7 9.6
1600
pa.
10.2
literature have used quiet solutions during the dissolution cycle, in order to avoid vibrations and obtain the best precision. However, the currentvoltage curves shon-n in Figure 7 indicate that stirring during the anodic stripping cycle Kith carbon paste electrodes gave greater sensitivity, more symmetrical peaks, and more negative peak potentials. =1 relative precision of 2 to 37, n a s maintained in our analyses. The stirring rate used during the dissolution cycle was thP same as that used for plating the metal. Voltage Scan Rate. The effect of the voltage scan rate during the stripping cycle is demonstrated by the curves shown i n Figure 8. These are t h e results of a 30-minute plating period from a 10-iL14 silver solution onto a 0.5-sq.-cm. area C.P.E. Not only does the peak height increase with the scan rate but the shape of the peak becomes more symmetrical. While it is probable that greater sensitivity for silver could be achieved with faster linear scan rates, this represents the fastest practical rate for gold, since a t faster rates the gold peak cannot be separated from the background. Calibration. T h e method was calibrated by plating each metal ion onto a 0.283-sq.-cm. C.P.E. from 100 ml. of a standard solution for 15 minutes. T h e solution was stirred a t 1400 r.p.m. during the plating and stripping cycles. T h e results of peak height measurement of t h e anodic wave obtained by stripping t h e metal a t 1200 mv. per minute as compared to the molar concentration are given in Table I Precision. The results of anodic stripping analyses on eight separate,
1
Ep’tO.28 SILVER
VOLTS
VS. SCE
Figure 9. Current-voltage curve for anodic stripping of a mixture of gold and silver Gold concn. = 1 X 10%4 Silver concn. = 1 X 1 V M Ploting time = 15 min.
but identical samples, run by two operators, using three different eleotrodes in separate cells, over a period of 2 days are given in Table IV. T h e precision of the method at this level of concentration is extremely good. Mixed Compounds. The currentvoltage curve shown in Figure 9 illustrates the possibility of determining two components simultaneously. The little pip at 0.06 volt vs. S.C.E. is due to a trace impurity of copper. ACKNOWLEDGMENT
I thank Doris L. Carter, who performed the major portion of the analyses reported here and also contributed the idea for using the glass joints as carbon paste electrodes. LITERATURE CITED
Table IV.
Precision of Anodic Stripping Analysis with Carbon Paste Electrode
(Gold concn. = 2.5 X 10-7M)
CPE1 So. of (letus.
Av. peak height, pa. Std. dev., Ma. Rel. std. dev., yo
CPE-2
8
8
11.60 f0.60 f5.17
Over-all AV. peak height, pa. Std. dev., pa. Rel. std. cev., %
=
=
11.44 xk0.37 f3.21 11.64 f0.39 f3.34
CPE-3 8 11.86
h0 29 h2.44
(1) Adams, R. X., ASAL. CHEW 30,
1576 (1958). (2) Anson, F. C., Lingane, J. J., J. Am. Chem. Sac. 79, 4901 (1957). (3) Barker, G. C., Anal. Chim. Acta 18, 11s (19,5Sl ( 4 ) L&dJd,8’S., Jr., O’Neill, R. C., Rogers, L. B., ANAL.CNEM.24, 209 (1952). ( 5 ) Nicholson. M. M.. Ibid., 32, 1058 (1960). (6) Peters, D. G., Lingane, J. J., J. Electroanal. Chem. 4, 193 (1962). (7) Porter, J. T., Cooke, W. D., J . Am. Chen. SOC.77, 148 (1955).
RECEIVEDfor review June 5, 1963. Accepted September 18, 1963.
V O L 35, NO. 13,
DECEMBER 1963
21 15
