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). ( 9 j 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 by 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
with linearly varying potential. A rate of voltage scan of 33.3 mv. per second was used in this work. The hanging mercury drop electrode, cell, and associated equipment were similar to those described previously (4, except that the reference electrode was a large saturated calomel electrode, connected to the cell by a salt bridge containing saturated potassium chloride. All potentials are referred to this reference electrode. The cell resistance was on the order of 100 ohms. The radius of the hanging drop electrode was 0.052 cm. Reproducible stirring was provided by a synchronous magnetic stirrer, made by mounting a small magnet on a Sargent synchronous rotator. Materials. All materials were reagent grade and were used without further purification. The principal interfering impurity, lead, was not present in either the hydrochloric acid or the ascorbic acid. High purity nitrogen was passed through a gas washing bottle containing 1.2M hydrochloric acid, and then was used to remove oxygen from the cell. To maintain the stirring conditions as constant as possible during the standard addition analyses, only small volumes of the standard tin solution were added to the cell. The normal aliquot was 0.500 ml., and a Gilmont microburet was used for this purpose. No attempt was made to thermostat the electrolysis cell, since the relatively small errors from this source were minimized by use of the standard addition method. RESULTS AND DISCUSSION
Reduction of Stannic Ion. As a preliminary study, the reduction of stannic ion was investigated a t a hanging mercury drop electrode using voltammetry with linearly varying potential. I n general, the behavior paralleled the equivalent reactions a t a dropping mercury electrode (IS). The curves (Figure 1) were drawn out, and the current corresponding to the reduction of stannic ion to stannous ion did not exhibit a well defined peak. For the second wave, the peak current increased as expected on increasing the hydrochloric acid concentration. Although previous workers, using dropping mercury electrodes, recommended the use of high concentrations of hydrochloric acid (up to 5 M ) as the indifferent electrolyte (5, 13, I?’), such high concentrations could not be used with stationary electrodes. The hydrogen overvoltage on a tin amalgam electrode appeared to be significantly less than on a pure mercury electrode, and as the hydrochloric acid concentration was increased, the hydrogen wave moved closer to the tin wave. In 2.434 hydrochloric acid containing 5 x 1 0 - 5 ~ stannic ion, the tin wave was only a shoulder on the hydrogen wave, and in 5M acid the waves were completely merged. iittempts to reduce this in-
-
5t
I1 I
vj
a I
3 t-.. z w U U
3 0
VOLTS vs. S.C.E. Figure 1. Current voltage curves for reduction of stannic ion in 1.2M hydrochloric acid Rate of voltage scan, 33.3 mv./sec. Concentration of stannic lonr
A.
2.00 x 1 0 - 4 ~
B. 5.00 X 10-6M C.
Blank
terference by using an electrolyte of 0.1M hydrochloric acid and 3M potassium chloride were not successful, and the cathodic peak current was significantly less than in the 1.2M hydrochloric acid solution. These results indicate that the direct application of voltammetry with linearly varying potential t o the determination of tin in hydrochloric acid solution would be difficult a t the rates of voltage scan used here. The higher rates of voltage scan used in cathode ray polarography would enhance the tin peak without particularly affecting the hydrogen current ( I @ , and this is reflected in the results reported by Scholes (17). Stripping Analysis of Tin. From the above data it is apparent that a t the pre-electrolysis potential selected for the stripping analysis (-0.60 volt) significant hydrogen evolution accompanied the deposition of the tin. T o determine the effect of this simultaneous hydrogen evolution on the preelectrolysis step, a series of stripping analyses was performed on solutions containing 5 X 10-5LTf stannic ion and varying concentrations of hydrochloric acid. The results (Table I) indicate that although the potential selected for the pre-electrolysis is significantly cathodic of the tin peak potential, the rate of electrodeposition of tin is still a function of the hydrochloric acid concentration. At acid concentrations
above about 4Tf, very erratic results were obtained because small bubbles of hydrogen formed on the electrode, which interfered with the deposition of the tin. On the basis of these results a hydrochloric acid concentration of 1.2M was selected as the indifferent electrolyte. To test the stoichiometry of the stripping analysis of tin, a series of analyses was run on standard solutions of stannic ion in 1.2M hydrochloric acid. In these experiments, the normal procedure for stripping analysis was followed: The pre-electrolysis step was carried out a t -0.60 volt us. S.C.E. in stirred solution for the indicated time interval; then the stirring was stopped, a 30-second interval was allowed for the solution to come to rest, and the anodic scan was then recorded. The results (Figure 2, Table 11) indicate that in spite of the complex nature of the electroreduction of stannic ion, a reproducible portion of the tin in the solution can be concentrated in the hanging mercury drop electrode, and that the subsequent stripping of the tin amalgam (to form stannous ion) is relatively straightforward. These analyses are less accurate than normally encountered in stripping analysis of this type, primarily because of the rather large cathodic residual current caused by hydrogen evolution a t the pre-electrolysis potential. Furthermore, Figure 2 indicates that the hydrogen evolution current L much larger on a tin amalgam electrode than on a pure mercury electrode. Thus, a residual current curve obtained on a solution containing 1.2M hydrochloric acid could be used to estimate the base l i e only when the tin concentration was relatively high, and analyses of tin solutions below lO-’M were not attempted. Application to Steel Samples. I n the extension of this method t o the determination of traces of tin in steel samples, several additional sources of
Table 1. Stripping Analysis of Tin as Function of Hydrochloric Acid Concentration
Stannic ion concentration: 5 x 10-6M Pre-electrolysistime: 5 minutes HC1 Peak Std. RelaConcn., Current,‘ Dev.,. tive iPlb Moles/Liter pa. % % 0.24 0.48 1.2 2.4 3.6
22.0 32.6 39.6 40.8 43.5
0.3 1.0 1.3 1.5 1.8
55
82 100 103 109
a Mean and standard deviation of six replicate determinations on same solution. b Per cent of peak current observed for 1.2M hydrochloric acid solution.
VOL 34, NO, 2, FEBRUARY 1 9 6 2
0
263
-
0.5
0vj
CL
5
-0.5-
a
+- - I .o z W
E
-1.5-
3 0
-2.0 -
-2.5 -
0
-0.2 -0.4
-0.6
VOLTS vs. S.C.E. Figure 2. Stripping analysis of 1 O+M tin in 1.2M hydrochloric acid Pre-electrolysis, 5 min. at - 0.6 volt A. Anodic stripping at 33.3 mv./sec.
8. Blank
interference were encountered. The presence of iron caused no particular difficulty, since the large cathodic residual current caused by the ferric ion could be eliminated with ascorbic acid. The other interferences were of a nature which ultimately necessitated the use of a standard addition technique. Specifically, molybdenum, tungsten, and copper each changed the calibration reported in Table I1 in a complex manner. Scholes (17) reported that the cathodic peak for stannic ion using cathode ray polarography was reduced when molybdenum was present. Similar observations were made in this work a t lower rates of cathodic potential scan. Stripping analyses also were performed with a solution containing 5 X 1O-P1f stannic ion, 10-4M molybdate ion, and 1.2M hydrochloric acid. The results indicated that the molybdenum also interfered with the electrodeposition of
Table II.
tin a t -0.60 volt. The anodic peak currents were 7 to 10% low, and thus calibration data obtained in solutions not containing molybdenum could not be used. The source of the interference may be related to precipitation of the reduction product of the molybdenum. Scholes (17) also reported an interference by tungsten, which is reduced a t about -0.6 volt, just cathodic of the tin wave. It was not expected that simultaneous reduction of the tungsten a t -0.6 volt during the pre-electrolysis step of a stripping analysis would interfere with the electrodeposition of tin. However, anodic stripping peaks for the tin were reduced by half in the presence of 10-4M tungstate ion. It is probable that the reduction products of the tungsten also interfere by precipitate formation, although no precipitate could be observed a t this concentration level for either tungstate or molybdate. The presence of copper affected the tin calibration in several ways. First, the stripping current for tin is markedly reduced when copper is codeposited. This effect is probably caused by the formation of various intermetallic compounds between tin and copper. Such intermetallic compounds were studied by Russell, Cazalet, and Irvin (16) using classical techniques. More recently, Kemula (7, 8) has pointed out that anodic stripping of mixed amalgams is a powerful tool for the study of intermetallic compounds. A quantitative study of the phenomenon was not made here, but the major effect on the tin peak was a reduction of the current without significant shift in the tin peak potential. No additional peaks for the oxidation of any possible intermetallic compounds were observed a t potentials cathodic of the mercury dissolution potential. The presence of copper also affected the hydrogen overvoltage as indicated by observing the cathodic residual current a t -0.60 volt us. S.C.E. Copper alone (at low concentrations) had little effect on the hydrogen evolution current, but tin and copper, when present simultaneously, increased the current. When present in relatively high concentrations (tenfold higher than tin)
Stripping Analysis of Tin in 1.2M Hydrochloric Acid
Moles/Liter
z p , Fa.
Pre-electrolysis Time, t, Minutes
5.00 x 8.00 x 5.00 x 5.00 x 2.00 x
32.55 5.20 3.26 0.330 0.390
5 5 5 5 15
Sn Concn.,
10-5 10-5 10-7 10-7
Peak
Current,"
( x 10-5)
Dev.,"
1.30 1.30 1.31 1.32 1.30
0.5 0.6 0.8 1.0 1.1
Mean and standard deviation of six replicate determinations.
264
ANALYTICAL CHEMISTRY
Std.
ip/tc
%
O6 vj
a
0.4
z
5 +-
0.2
t
I
W
E E
= =
0
-0.2 O I
-0.41
,
I
,
, I
-0.2 -0.4 -0.6 VOLTS vs. S.C.E. Figure 3. Stripping analysis of NBS sample 1 1 g Pre-electrolysis. 5 min. at -0.6 volt in 1.2M hydrochloric acid A. Anodic stripping in same solution 6. Blank
the copper interfered in still another way, since the foot of the copper wave extends slightly under the tin peak and causes high results. The net result of these comparisons is that when the tin concentration is equal to or greater than the copper (or when the tin concentration is fairly high, regardless of the copper) the best blank is that obtained on a solution containing only 1.2M hydrochloric acid. On the other hand, when the tin concentration is low and the copper concentration is high, blanks which contain copper must be used. An estimate of the copper concentration can be obtained directly from the anodic stripping curve. (The anodic peak potential for copper in 1.2M hydrochloric acid is -0.15 volt us. S.C.E.) In some cases the concentrations of copper and tin were such that extremely high hydrogen evolution currents were obtained, presumably because of the formation of insoluble intermetallic compounds. In such cases, a method was developed in which the anodic stripping portion of the analysis was performed in a less acid solution. Procedure. I n view of the peculiar effects of the various interferences, the following standard addition technique was developed. -4 steel sample of 0.100 gram was dissolved in a mixture of 10 ml. of concentrated hydrochloric acid and about 10 ml. of distilled water contained in a 100-ml. borosilicate volumetric flask. On dilution to volume, this resulted in an acid concentration of 1.2M, the amount of acid used up in the dissolution procedure being negligible. After rinsing the cell with several aliquots of the sample
solution, a specific volume (depending on the type of cell used) was pipetted into the cell, and several milligrams of ascorbic acid were added. Nitrogen was bubbled through the solution for about 10 minutes, and then the solution was tested for completeness of reduction of the ferric ion by monitoring the residual current a t -0.1 volt us. S.C.E. before and after adding 1 drop of a freshly prepared concentrated solution of ascorbic acid. The analysis was then carried out as described above for the pure tin solutions. After obtaining replicate determinations (three to six, depending on the precision required), a 0.500-ml. aliquot of a standard tin solution was added. Replicate determinations were again obtained, and the tin concentration was calculated from the ratio of the currents:
where Aip is the change in peak current corresponding to the standard addition (11). The method was applied to National Bureau of Standards sample 152, an open hearth (tin bearing) steel. The results (Table 111) indicate that the accuracy and precision of this method are satisfactory. Since chemical separation of the tin was not required, the analyses were relatively rapid, and the possibility of either loss of tin or contamination of the sample was reduced. Electrode Transfer Procedure. I n trying to extend this method to other steel samples in v-hich the tin content was an order of magnitude lower, and which also contained copper, the interference caused by the cathodic residual current resulted in significantly larger errors. This was encountered in applying the method to National Bureau of Standards sample llg, which contains about tenfold less tin than does sample 152, and which also contains about 0.046% copper. A typical anodic stripping curve for this sample, with a residual current curve obtained for 1.2M hydrochloric acid, are shown in Figure 3. The tin peak is very ill defined, appearing between the cathodic hydrogen evolution current, and the anodic copper amalgam dissolution current. At this low tin concentration, attempts to estimate the base line from curyes obtained on copper solutions in 1.2M hydrochloric acid solution were not successful. Thus, a means of reducing the hydrogen evolution current was sought. Although the acid concentration cannot be reduced in the pre-electrolysis step, the stripping can be performed in a variety of solutions. A series of qualitative experiments was performed in which the tin amalgam was formed as described above; but after the preelectrolysis step, the hanging mercury drop electrode was quickly transferred
un i o
3
i
-0.1
i
0
-0.2 -0.4 -0.6
VOLTS vs. S.C.E. Figure 4. Stripping analysis of NBS sample 1 1 g Pre-electrolysis, 5 min. a t -0.6 volt in 1.2M hydrochloric acid A. Anodic stripping in 0.1 M potassium chloride 0.01 M hydrochloric acid B. Blank containing some copper concentration as sample C. Indifferent electrolyte only
t.0another electrolysis cell for the anodic stripping step. The method was analogous to that used by Taylor and Smith (18) with a large mercury pool electrode. These experiments indicated that the oxidation of tin amalgams to stannous ion proceeds without major complication in various electrolytes, and the results were generally similar to those obtained by Cooper (2) who used dropping tin amalgam electrodes. In the electrode transfer process, how-
Table 111.
Stripping Analysis of Tin
Pre-electrolysis: 5 minutes in 1.2M hydrochloric acid NBS sample 152O Tin Standard Found, Deviation,b Wt. % 70 0.0356 0.0390 0.0377 0.0368 0.0367 0.0383 Av. 0.037
1.2 0.5 1.1 1.1 0.7 0.9 1.4
NBS sample 11gevd 0.00283 0.00306 0.00280 0.00314 0.00291 Av. 0.0030
4.3
3.i 2.8 3.8 2.0 2.8
NBS average: 0.036y0 range: 0.035-
0.039%.
Each result is a separately taken sample. Mean and standard deviation of six replicate determinations. NBS values: 0.003, 0.004%. Anodic stripping from O.1M potassium chloride, 0.01M hydrochloric acid.
ever, about 10% of the tin was lost, either by air oxidation when the electrode was transferred from one cell to the other, or from attack by the 1.2M hydrochloric acid solution during the few seconds that the circuit was open. (Each cell was provided with its own reference electrode, connected to the Polarograph a t all times. When transferring the electrode, the connections were not removed, and thus the circuit was open only while the electrode was out of solution.) The loss of tin was related t o the length of time required to make the transfer, and with a little practice, the results were reproducible. The standard addition technique was also used to account for the loss of tin, and the method was applied to the determination of tin in National Bureau of Standards sample llg. The preelectrolysis procedure was the same as above; the stripping solution was 0.1M potassium chloride, 0.01M hydrochloric acid. The hydrogen evolution current was reduced markedly, because of the decreased acid concentration, and the copper dissolution wave was shifted to less cathodic potentials because of the decreased chloride concentration (Figure 4). A significant improvement in precision was obtained, and the results (Table 111) are probably more accurate than those reported with the sample. LITERATURE CITED
(1) Allsopp, Wr.E., Damerell, V. R., ANAL. CHEM.21, 677 (1949). (2) Cooper, W. C., J . Am. Chem. SOC.77, 2074 (1955). (3) Delrlam, R. D., ANAL.CHEM.34, 259 1962). ( 4 j DeMars, R. D., Shain, I., Ibid., 29, 1825 (1957). (5) Ferrett, D. J., Milner, G. W. C., Analyst 81, 193 (1956). (6) Goto, H., Ikeda, S., Watanabe, S., Japan Analyst 3, 320 (1954). (7) Kemula, W., Galus, Z., BUZZ. acad. polon. sei. 7, 553 (1959). (8) Kemula, W., Galus, Z., Kublik, Z., Ibid., 6, 661 (1958). (9) Kemula, R., Kublik, Z., Anal. Chim. Acta 18, 104 (1958). (10) Kemula, W.,Kublik, Z., Glodowski, S., J . Electroanal. Chem. 1, 91 (1959-60). (11) Kolthoff, I. M., Lingane, J. J., “Polarography,” Vol. I, p. 377, Interscience, New York, 1952. (12) Ibid., Vol. 11, pp. 523-8, 616. (13) Lingane, J. J., J . Am. Chem. SOC.67, 919 (1945). (14) Phillips, S. L., Morgan, E., ANAL. CHEW33, 1192 (1961). (15) Reinmuth, IT7. H., Ibid., 32, 1891 (1960). (16) Russell, A. S., Cazalet, P. V. F., Irvin, N. hl., J . Chem. SOC.1932, 841. (17) Scholes, P. H., Analyst 86, 392 (1961). (18) Taylor, J. K., Smith, S. W.,J . Research .Vatl. Bur. Standards 56, 301 (1956).
RECEIVEDfor review October 2, 1961. Accepted December 13, 1961. Work supported in part by funds received from the U. S. Atomic Energy Commission under Contract No. AT( 11-1)-1083. VOL. 34, NO. 2, FEBRUARY 1962
265
