compound remaining and oxidized by Ce(IV), even in the very heavy compounds, is enough to change the values found beyond the experimental error inherent in the procedure itself. LITERATURE CITED
(1) Cherney, P. J., Crafts, B., Hager-
moser, H. H., Boyle, A. J., Hardin, R., Zak, B., ANAL.CHEM.26, 1806 (1954).
(2) Erdey, L., Mazor, L., Meisel, T., Mikrochim. Acta 1958, 140. (3) Goodwin, J. F., Hahn, R. B., Boyle, A. J., B s a ~CHEM. . 29,1681 (1957). (4) Honetz, S., Kotzaurek, R., Klin. Wochschr. 38,494 (1960). (5) Maclagan, N. F., Bowden, C. H., Wilkerson, J. H., Rec. trav. chim. 74, 632 (1955). (6) Magee, R. J., Spitzy, H., Mikrochint. Acta 1959, 101. (7) Schoniger, W., Ibid., 1955, 123.
( 8 ) Smith, G. F., “Cerate Oxidimetry,”
pp. 88-117, G. F. Smith Chemical Co., Columbus. Ohio. 1942. (9) Strickland, R.’ D., Maloney, C. M., ANAL.CHEX.29,1870 (1957). (10) Zak, B., Boyle, A. J., J . Amer. Pharm. Assoc. Sci. Ed. 41, 260 (1952). (11) Zak, B., Holland, J.. Williams,’ L. A , dlin. Chem., to be pubiished.
RECEIVED for review August 25, 1961. Accepted December 8, 1961. Work was supported in part by a Grant-in-Aid from the Receiving Hospital Research Corporation.
SimuItaneous Determination of Tin and Indium Using Anodic Stripping Voltammetry RICHARD D. DeMARS Thomas 1. Watson Research Center, International Business Machines Corp., Yorktown Heighfs, N. Y.
b A method i s described for the simultaneous determination of tin and indium in binary alloys, using the techniques and advantages of anodic stripping voltammetry with linearly varying potential, The method i s particularly suitable for the analysis of thin films and samples containing low indium concentrations. Its use for the determination of tin i s extremely convenient, since the oxidation state of tin is not critical. Current-voltage curves were obtained from an electrolyte containing pyrogallol and ammonium thiocyanate. The accuracy and precision of the method are comparable to those obtained with cathodic voltammetry with linearly varying potential. The sensitivity of the method makes it possible to determine concentrations o f indium as low as 10 p.p.m. in tin.
R
work in this laboratory has pointed out the need for a rapid, precise, and accurate method of determining low concentrations of indium in tin-indium alloys. From the requirements of the analysis, it appeared that an electrochemical method would be most suitable. The determination of these elements either separately or in complex mixtures has been investigated, using a variety of electrochemical methods. A vast amount of literature is available on the polarographic analysis of these elements. The most recent describes the simultaneous determination of these elements without prior separation (11). Other electrochemical methods applied to the determination of these elements include a x . polarography (1, 7 ) , square-wave polarography (4, 6 ) , and oscillographic polarography (8, 16, 17, 18). ECENT
The major disadvantage of the polarographic methods, when used to analyze tin-indium alloys, is that the small wave due to the indium reduction is masked completely by the tin reduction wave if the indium concentration is less than 5%. In addition, the system exhibits a peculiar behavior in many electrolyte systems, which further complicates the analysis. This behavior is illustrated in Figure 1. The peculiar shape of the wave is dependent on the relative Concentrations of tin and indium in the solution. At low indium concentrations (less than 5%) it becomes impossible to get reproducible standardization curves. Recently, the method of stripping analysis has been applied to the determination of tin in the presence of zinc (IO) and tin in steel alloys (la). The method of anodic stripping analysis for the simultaneous determination of tin and indium in binary alloys appeared promising, because tinindium amalgams should exhibit two distinct peaks \\hen oxidized under the conditions of voltammetry with linearly varying potential. In addition, the indium peak should appear first, making it possible to measure extremely small amounts of indium in the presence of large amounts of tin. The main advantage of stripping analysis for the determination of tin lies in the fact that the oxidation state of the tin in the solution to be analyzed is not critical. This advantage arises because the tin in the sample solution is reduced to the metal in the preelectrolysis step and the analytical curve obtained in the st.iyping piocess involves only the oxidation of the tin amalgam to the stannous state. For these reasons, a method utilizing anodic stripping voltammetry was applied t o the determination of tin
and indium in binary alloys. The electrolyte system used to obtain satisfactory deposition of the tin and indium in the pre-electrolysis step consisted of pyrogallol and ammonium thiocyanate. The deposition reactions proceed smoothly and reversibly in this medium (11). The pre-electrolysis of the sample was carried out a t -0.800 volt z’s. S.C.E.for a total time of 5 minutes. This total time included 4.5 minutes with stirring and 30 seconds without, to allow the solution to come to rest. The voltage was then scanned in the anodic direction and the currentvoltage curve was recorded. For alloys containing 20 to 50% indium, one sweep was sufficient to determine both components. For indium concentrations lower than 2070, the accuracy and precision are increased if two separate determinations are made, one a t a higher sensitivity for the indium and another a t a lower sensitivity for the tin. The method was applied to the determination of alloys in the range from 0.01 to 20% of indium in tin. The accuracy and precision obtained mere comparable to those obtained with cathodic voltammetry with linearly varying potential. EXPERIMENTAL
Apparatus. The instrumental setup and electrode system have been described (2). The cell utilized in these experiments was a 200-ml. tall-form borosilicate glass beaker. The working electrode was a conventional hanging mercury drop (14) with a radius of 0.0650 cm. and an area of 0.0531 sq. em. The counter electrode was a large platinum sheet immersed in the solution. The reference electrode was a B e c h a n fiber-type calomel, connected to the solution by a Saltbridge-Luggin capilVOL. 34, N O , 2, FEBRUARY 1962
259
I
T
50
-
404
3
.
b
I-
z
w a K 3 o
30. r
0
40
20-
a
I
I
O'
-0'7
I
-0 6
-0.5
VOLTS vs. S.C E.
Figure 2. Current-voltage curve for anodic stripping of indium under conditions of voltammetry with linearly varying potential
-0.58
-0.48
-0.68
VOLTS VS. S.CE.
Figure 1. Polarograms of tin and indium in 0.1OM ammonium thiocyanate and 0.020M pyrogallol at PH 1 A.
E. C.
6.00 X 1 O-%l In 6.00X 1 0 - 6 M I n + 1 . 0 0 X 1.00 X IO%I Sn
lary arrangement. The anode to cathode resistance of the cell was less than 50 ohms. All potentials were measured with a Rubicon portable precision potentiometer. The sweep voltage was 33.3 mv. per second. Reproducible stirring was obtained using a synchronous magnetic stirrer with a speed of 600 r.p.m. The cell was maintained a t 25.0" f 0.1" C. for all work. Materials. Standard solutions of tin and indium were prepared from the pure metals by dissolving weighed quantities in hydrochloric acid. Pyrogallol and ammonium thiocyanate were reagent grade (Baker's analyzed) and were not purified further. High purity nitrogen was used to remove oxygen from the solutions. All solutions were prepared with doubly distilled n-ater. PROCEDURE
Standard Solutions. Place desired quantities of standard tin and indium solutions in a 250-ml. beaker. Add 25 ml. of 1 M ammonium thiocyanate and 380 mg. of pyrogallol. Dilute to about 200 ml. and adjust the p H to 1.0. Transfer the solution quantitatively to a 250-ml. volumetric
260
ANALYTiCAL CHEMISTRY
Solution. 2.00 X 10-W In+3, 0.1 M NH4CNS, 0.02M CaHAOH), Rate of voltage scan, 33.3 mv./sec. Pre-electrolysis time, 5 min. a t -0.8 volt VI. S.C.E. Theoretical 0 Experimental
-
IO-sMSn
flask and dilute to volume. Rinse the cell with a portion of the solution, refill with the solution, and deaerate 10 minutes with nitrogen. Remove nitrogen inlet and allow nitrogen to pass over the solution during the course of the experiment. Analyze according to standard stripping techniques (3, 9). Unknown Samples. Dissolve samples in 25 ml. of 6 N hydrochloric acid, using dropwise addition of nitric acid if necessary, and dilute to appropriate concentration range. Proceed as described above. RESULTS AND DISCUSSION
Stripping Curves for Tin and Indium. Reinmuth (13) has recently published a theoretical derivation of the stripping process under the conditions of voltammetry with linearly varying potential. His derivation took into account the limited volume of the hanging mercury drop electrode and the curvature of the electrode surface. Shain and Lewinson (16)have shown that under the experimental conditions normally employed in stripping analysis only the spherical nature of the electrode has to be considered. Reinmuth's derivation can be used to predict the shape and magnitude of the
current-voltage curve obtained in the stripping process, provided that the concentration of the substance in the mercury drop a t the beginning of the anodic sweep is known. Shain and Lewinson (16) have presented a method of calculating this concentration based on the number of coulombs involved in the electrodeposition process. This Concentration, Cg, is given by the following expression 3ict CR
= 4 x 3
where CB is in moles per cubic centimeter, i, is in amperes, and TO is in centimeters. Thus, from a knowledge of the deposition current and the time of electrolysis, it is possible t o calculate the concentration in the hanging mercury drop a t the beginning of the anodic sweep. An alternative and more convenient method of calculating the theoretical current-voltage curve, once C R is determined, is to use the numerical data presented by Frankenthal and Shain (5), noting that the spherical correction must be subtracted for the stripping case. Since the derivations presented assume a diffusion-controlled reaction in the oxidation step, it should be possible t o determine if the oxidation step
in the stripping process is truly diffusion-controlled by comparison between theoretical and experimental current-voltage curves. This was done for the stripping analysis of tin and indium in solutions containing ammonium thiocyanate and pyrogallol. The results are presented in Table I, where the calculated peak currents are compared with those obtained experimentally. A comparison of the entire current-voltage curve is presented in Figure 2 for the oxidation of indium amalgam under the conditions of linearly varying potential. The results indicate that the theory predicts adequately the stripping curves for both tin and indium. The effect of p H and pyrogallol concentration on these reactions was studied. The pH had a marked effect on the tin reduct,ion. A variation of the pH from 1 to 2 resulted in a decrease of 86% in the peak of the stripping curve. Changes in pH over this narrow range did not appreciably change the peak height for the indium stripping curvo. Variations of the pyrogallol concentration between 0.001M and 0.020M did not appreciably alter the peak heights for the stripping of either metal, However, the reproducibility of peak heights for the stripping of tin was improved from 5 to 2% by
working a t the higher rather than the lower concentration. Standardization Curves for Tin and Indium. T o determine the range of applicability of the stripping method to the determination of tin and indium a number of solutions were analyzed containing varying concentrations of the metals. The results of these runs are summarized in Table 11. For the purpose of comparing results among various concentrations and the two metals it is convenient to define a term S,the sensitivity, where
with i, in microamperes. t is in minutes and is the total electrolysis time and C is in moles per liter. The introduction of S is merely a convenience and holds no theoretical significance. The value of S remains constant over a rather wide range of concentration for both tin and indium. The relative values of S for tin and indium indicate t,hat indium is more sensitive than tin in the stripping process, as would be expected. The data also show that the stripping analysis cannot be extended to the higher concentration range investigated. The decrease in S can be caused by a number of factors, some of which have been discussed (1.2, 16).
8
Table 1. Comparison of Theoretical and Experimental Peak Currents
CR, Mole/Liter
Ion In+3 Snf2
Calculated.
* Experimental;
terminations. Table II.
i,,
3 . 4 8 X IOd3 3 . 4 4 x 10-4 5.22 X
pa:
105 10.4
106
i,
pa.*
103 10.2 103
average of three de-
Peak Current as a Function of Concentration
Concn., Mole/Liter
Av.
Dev.,
i,, pa.'
%"
S*
In+J 2 . 0 0 X 10-6 2.00 X lo-' 6 . 0 0 X 10-6 2.00 X
5.28 52.6 157 441
0.8 1.0 1.5 1.8
5.28 5.26 5.23 4.41
Sni2 2 . 0 0 X 10" 2.00 X 2.00 X
2.29 1 . 2 2.29 23.1 1 . 0 2.31 137 1 . 6 1.60
Ion
Average and average deviation of at least four determinations. b = 2. x 10-6, tC Table 111.
Analysis of Unknown TinIndium Alloys
Taken, Mg. In Sn 28.5 X 29.8 28.5 x 10-2 29.8 28.5 X lo-* 2 . 9 8 57.0 X 2.98
Found, Mg." In Sn 29.0 X lo-* 29.4 2 8 . 2 x lod2 30.5 2 8 . 7 X loe2 3 . 0 1 56.0 X 2.96
Average and average deviation of at
least three determinations.
I
5.0 pa.
c z W a a I3 u
-0.80
-0.60 VOLTS
-0.40
- 0 20
VS. S . C . E
Figure 3. Current-voltage curves for oxidation of indium amalgam Solution and concentrations for curves A, 6, and C same as in Figure 1
The data show that the determination of these elements either separately or in mixtures should be feasible using the procedure outlined. The possible effect of high concentrations of tin on the determination of small amounts of indium was investigated by running indium solutions a t the concentrations indicated in Table I1 in the presence of 1.00 X 10-aM tin. In all cases, the results agreed with those presented, t o within experimental error. Typical curves for this analysis are shown in Figure 3. Unknown Analysis. The accuracy of the method was investigated using synthetic unknowns in the concentration range of 0.01 to 20% indium in tin. The results of these analyses are presented in Table 111. The data indicate that the method is valid for samples in the concentration range studied. The sensitivity obtained indicates that it should be possible to analyze samples in the range of O . O O l ~ o indium with little added difficulty. For samples containing very small VOL. 34, NO. 2, FEBRUARY 1962
261
amounts of indium (0.1 to 0.001%) a slightly modified procedure was used for the analysis. The sample m-as dissolved as described and divided into two aliquots. The first aliquot was treated as described in the procedure and used to determine the indium content. The second aliquot was diluted further before treatment to bring the tin into a suitable range for analysis. ACKNOWLEDGMENT
Thanks are due to Sidney Phillips and Irving Shain for allowing me to see their data before publication.
LITERATURE CITED
(1) Busev, A. I., Metody Analiza Redkikh i Tsvet. Metal. Sbornik 1956, 79-81. (2) DeMars, R. D., ANAL.CHEW33, 342
(1961). (3) DFMars, R. D., Shain, I., Zbid., 29, 1820 (1957). (4) Ferrett, D. J., Milner, G. W. C., Analyst 81, 193 (1956). (5) Frankenthal, R. P., Shain, I., J. Am. Chem. SOC.78, 2969 (1956). (6) Hamm, R., ANAL. CHEM. 30, 350 (1958). (7) Kalvoda, R., Anal. Chim. Acta 18, 132 (1958). (8) Kalvoda, R., Chem. listy 51, 696 i1957). (9j Kemula, W., Kublik, Z., Anal. Chim. Acta 18, 104 (1958).
(10) Kemula, W., Kublilr, Z . , Glodowski, S., J. Electroanal. Chem. 1,91(1969-60). (11) Phillips, S., Morgan, E., ANAL. CHEN.33, 1192 (1961). (12) Phillips, S., Shain, I., Zbid., 34, 262 (1962). (13) Reinmuth, IT. H., Ibzd., 33, 185 ( 1961). (14) Ross, J. W., DeMars, R. D., Shain, I., Ibid., 28, 1768 (1956). (15) Scholes, P. H., Analyst 86, 392 (1961). (16) Shain, I., Lewinson, J., . ~ N A L . CHEX 33, 187 (1961). (17) Shinagama, H. I., Sannhara, H., Nippon Kagatsu Zasshi 77, 1453 (1956). (18) Treindl, L., Chem. listy 50, 534 (1956). RECEIVEDfor review October 4, 1961. Accepted October 30, 1961.
Application of Stripping Analysis to the Trace Determination of Tin SIDNEY L.
PHILLIPS
and
IRVING SHAlN
Chemisfry Department, Universify o f Wisconsin, Madison, Wis.
b Stripping analysis with the hanging mercury drop electrode was applied to the determination of trace quantities of tin. During the pre-electrolysis step, a portion of the tin in the solution was reduced b y controlled potential electrolysis. The tin amalgam thus formed was subsequently analyzed by anodic stripping using voltammetry with linearly varying potential (fast sweep polarography). In addition, a technique was developed in which the preelectrolysis step was performed in one solution, and the stripping in another. This permitted the determination of tin in complex alloys, and the method was applied to steel samples.
T
HE DETERMINATION of trace quantities of tin in various samples has been investigated by several electrochemical techniques. Among the methods used for the determination of tin in steel have been polarography (1, 6 ) , square wave polarography (ti), and cathode ray polarography ( 1 7 ) . Stripping analysis with hanging mercury drop electrodes has been applied to the determination of tin in zinc (10) and in tin-indium alloys ( 3 ) . The work indicates that there are generally two factors which complicate the electrochemical determination of tin in alloy samples. First, the samples contain large amounts of other electroactive metals. In steel samples, for example, both ferric iron and copper are present; both are more easily
262
ANALYTICAL CHEMISTRY
reduced than tin, and generally they preyent direct application of polarography to the analysis. Thus, several of the previous studies (1, 6, 17) have required complicated and time-consuming separation procedures prior to the actual analysis. Such procedures must be carried out with extreme caution to prevent losses since the tin concentration may be on the order of a few thousandths of a per cent in the original sample. The second factor arises from the complex electrochemistry of tin (12). -4lthough stannous ion is readily reducible from various electrolytes, it is subject t o air oxidation. On the other hand, the electrochemical behavior of stannic ion is very complex. The polarographic waves are irreversible, and very sensitive to small changes in electrolyte composition. High concentrations of halide ion or other complesing agents such as pyrogallol (14) must be present to ensure reproducible behavior. These difficulties can be avoided in some cases by reducing the tin to stannous ion 11-ith aluminum (6). Kemula (10) has pointed out, hon-ever, that reagents normally thought to be pure cannot always be used with very sensitive methods of analysis because of the danger of adding impurities to the solution. I n particular, lead is a common contaminant in many reagents and produces current peaks a t potentials very close to tin. Thus, the application of stripping analysis (4, 9) to the determination of
traces of tin offers several important advantages. Prior chemical separation of the tin is not required. since in stripping analysis the pre-electrolysis step perfornis the same function. Furthermore, the oxidation state of tin in the test solution is immaterial since the analytical nieasuiement is made on the subsequent oxidation of the tin amalgam to stannous ion. The application of stripping analysis to the determination of tin in zinc was discussed briefly by Keniula (10). In the present n-ork, the stoichiometry of the stripping analysis of tin was examined carefully, and a :nethod was dereloped for the determination of tin in steel. To prevent contamination of the sample by reagents. emphasis Fas placed on determining the tin directly after dissolution of the 8 .inple in hydrochloric acid. ilfter addition of ascorbic acid to reduce the ferric ion, the sample was pre-electrolyzed a t -0.60 volt us. S.C.E. using the hanging mercury drop electrode. The tin amalgam thus formed n as anodically stripped using linear voltage scan voltammetry (fast siveep polarography). To simplify the proceduie and reduce the effect of interfeiences. a standard addition technique n a s used. The method was applicable to s3mples containing on the order of O . O O l ~ otin. EXPERIMENTAL
Apparatus. All data mere obtained on a Sargent Model XV Polarograph, suitably modified for voltammetry
