Polarography in a Sodium Chloride-Potassium ChIo ride M eIt Using Tu ngste n-in-Vycor Microelectrodes DONALD L. MARICLE and DAVID N. HUME Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge 39, Mass.
b The
polarography of nickel(ll), silver(l), iron(ll), and copper(1) has been investigated in a sodium chloride-potassium chloride melt at 738' C. using a newly developed tungsten sealed in Vycor electrode. The limiting currents were directly proportional to the concentration with a precision of 5 to 8%. The effect of electrode area, rate of potential scan, and temperature on the limiting current was investigated. The relationship between half wave potential and concentration did not agree well with theory. It was found that n values could be determined by analysis of the waves except for nickel(ll), which exhibited an anomalous wave shape.
E
in the field of fused salt polarography used very low melting salts and a dropping mercury electrode n-ith cells and other equipment similar to that employed at room temperatures (3, 16, 18, 22). To apply the polarographic technique a t higher temperatures, it is necessary to use either a solid electrode or a liquid metal with a boiling point higher than that of mercury. ii fised or a highly repro: ducible surface area is most desirable in a solid electrode. It is very difficult to achieve reproducibility of electrode urea by such means a5 periodic dipping with an argon bubbler (2, 6, 12, IS), and quantitative results are not obtainable when bare wire electrodes are simply dipped into the melts. The customary low temperature technique of sealing platinum or similar electrodes into glass fails a t higher temperatures. Platinum in soft-glass is not useful a t temperatures much above 450" C. (10, 11). Hills, Inman, and Young ( 7 ) have used tungsten sealed in borosilicate glass at temperatures up to 600" C. Above this point, however, borosilicate glasses begin to conduct electricity and are not suitable for sealing polarographic electrodes. Therefore, until now no molten salt polarography has been reported at temperatures above 600' C. and very little above 450' C., which utilizes a microelectrode with a precisely defined and reproducible area. With the aid of a recently developed tungsten-in-Vycor microelectrode and a plat,inum/platARLY WORKERS
1 188
ANALYTICAL CHEMISTRY
inum(I1) reference electrode, the authors have now succeeded in applying the polarographic method to the analysis of a sodium chloride-potassium chloride melt a t 740' C. The only prior instances of polarography in this medium appear to be the work of Delimarskii and Kuz'movich using platinum electrodes (4), presumably just dipped into the melt, and that of Stein (21) who mentions a platinum-quartz electrode but gives no description of how an electrode could be made from materials with such dissimilar coefficients of expansion, and shows no polarograms and no residual current curves. APPARATUS
The microelectrodes for polarography were fabricated by sealing tungsten wire into Vycor tubing as follows. A 4- to 5-em. length of tungsten wire, 0.5 to 0.7 mm. in diameter, is taken M G
R C N
P
I Figure 1 . trodes
Furnace, cell, and elec-
F. Furnace TI. Controlling thermocouple T?. Measuring thermocouple M. Metal ion generating electrode G. Gas inlet R Reference electrode C. Carbon rod electrode N. Nickel electrode P. Polarographic microelectrode S. Stainless steel beaker
and a length of nickel wire spot-welded to one end to serve as a lead. A bead of GSC-3 glass (about 2 mm. in diameter) is sealed to the center of the tungsten wire and the bead sealed into the end of a piece of 7-mm. Vycor tubing using a cane of GSC-1 glass to form a graded seal between the GSC-3 glass and the Vycor. (Canes of GSC-1 and GSC-3 glasses and graded seals in tubing form are available from the General Electric Co., Willoughby Quartz Plant, Willoughby, Ohio,) The protruding tungsten wire was cut off 1 to 5 mm. from the bead to form a n electrode of appropriate size for polarographic use and the heavy layer of oxide formed during the sealing operation was removed by polishing with 3/0 emery paper. The completed electrode is shown in Figure 1. The reference electrode was a modification of the type developed by Laitinen and coworkers (5, 9, 17). It consisted of a platinum foil in a sodium-potassium chloride melt containing platinum(II), the whole being contained in a Vycor tube (Corning No. 7900) the bottom of which was made of Pyrex (Corning No. 7740) and attached to the tube by a graded seal. The Vycor tube provides the necessary structural strength and the Pyrex (which at 740' C. is beginning to soften and has an electrical resistance of only 40 to 60 ohms) serves as a salt bridge. The electrode is easily constructed by cutting off a 13-mm. Pyrex to S'ycor graded seal (Cat. No. 6465, Corning Glass Works, Corning, N. Y.) so that about 10 mm. of Vycor remains, flanging a 25-em. length of 7-mm. Vycor tubing to 13 mm. and sealing i t on. The Pyrex end of the graded seal is then sealed off as close as possible to the grading so that only the very bottom of the electrode tube is Pyrex glass (Figure 1). The electrode tube is charged with an appropriate amount of a n equimolar mixture of sodium and potassium chlorides, and a platinum foil of about 2 sq. cm. area, spot-welded to a length of platinum wire, inserted. After the usual drying and preheating procedure (described below), the electrode is inserted into a cell containing molten sodium and potassium chloride and the platinum(I1) is generated by electrolytic oxidation. Because it had been found (14) that platinum(I1) was not generated with 1 0 0 ~ ocurrent efficiency in this melt (due a t least in part to the slight volatility of platinum chloride), the
volt5
Figure 2. Characteristics of automatically recorded polarograms and stripping curves A. B. C. 0. E.
volts
Figure 3.
Effect of scan rate on reduction wave of Ag(l)
0.623mm Ni(1ll 0.747mm Ag(l) 0.778mm Fe(ll) 1.245mm Cu(l) Residual current
potential of the reference electrode was measured against the chlorine electrode a t the start of each experiment. The melt was purified by treatment with chlorine gas so that the introduction of a spectrographic carbon rod was all that was needed to complete the chlorine electrode. The measured potential of the reference electrode against chlorine and the calculated [by extrapolation of log concentration us. potential plots after Laitinen, Liu, and Ferguson (IO)] value (-0.251 volt) of the potential of the molal platinum/platinum(II) electrode against the chlorine electrode in this melt a t 738" C. (14) permitted all measurements to be corrected to a scale on which the potential of the lmm ylatinum/platinum(II) electrode was taken as zero. All voltages recorded in this work were so obtained. These electrodes were sturdy enough to last through three or four runs, on the average. Reversibility was indicated by almost immediate return to equilibrium after short anodic or cathodic polarization, and stability was good to +2 to 3 mv. on successive experiments. The Pyrex salt bridge prevents contamination of the melt with the reference electrolyte and no indication was ever seen of a significant asymmetry potential. Solutions of chlorides of the metals studied were prepared in the melt by anodic dissolution of electrodes of the appropriate metals. The cathode was a nickel wire immersed in a nickel chloride-containing melt held in a 7mm. Vycor tube with a Pyrex bottom, analogous in construction to the refere w e electrode. The metal-generating electrodes consisted of wires of the pure metal sealed into 7-mm. borosilicate glass tubing, as shown in Figure 1. About 10 cm. of the wire extended beyond the Pyrex tube, so that the glass-tometal seals were well above the surface of the melt. These seals were mechanically sufficient, but not vacuum-tight. The gas inlet was constructed from
- . - . Manual - - 0.05 v./min. - 0.1 v./min. - - - 0.2 v./min. -- 0.3 v./min. 6-mm. Vycor tubing by sealing the end and blowing three small holes in the side of the tube near the bottom. Also present, but omitted from Figure 1 for simplicity, was a gas inlet tube which was used to flush argon over the surface of t.he melt while polarograms were being recorded and a gas outlet tube, both of 7-mm. glass tubing. All electrodes and gas inlets and outlets were mounted in a No. 11 rubber stopper. The cell itself was made from 57-mm. Vycor tubing by sealing one end to form a test tube-shaped vessel 25 cm. long. The cell was clamped rigidly to reduce vibration and the top was cooled by the two air jets. The furnace was a 700-watt CencoCooly crucible furnace, snd the temperature was maintained at 738" C. by a Wheelco Model 152 Amplitrol controller. A 500-ml. stainless steel beaker was used to provide electrical shielding and more even heating of the cell. A 'lrinch asbestos board, appropriately drilled (Figure l), served as a lid to the furnace and to position the thermocouples. The constant current supply was of the Reilley, Adams, and Furman (19) type, with a d.p.s.t. switch to activate a Standard Electric Time Co. Type S-10 clock when current was being applied to the generating circuit. Polarograms were obtained on an instrument, constructed in this laboratory, which had operating characteristics very similar to those of the Sargent Model XXI. PROCEDURE
The following operations were performed to prepare and purify the melt, to prepare and introduce the electrodes into the melt, and to generate known concentrations of metal ions for polarographic investigation. One half mole each of granulated sodium and potassium chloride was weighed out, mixed well, and poured into the cell without
prior treatment or drying. The cell was then placed in the furnace and the salts fused. The metal ion generating electiode was removed from the rubber stopper and a glass plug inserted. The electrodes were cleaned by polishing with No. 1 emery paper (except the copper electrode which was cleaned in dilute HCl, rinsed with distilled water, and dried in a vacuum oven at 90" C. for a t least 2 hours. All the other electrodes in the assembly were likewise rinsed and dried a t 90" before use. In making a run, the electrode assembly was first transferred to an auxillary cell in a second furnace maintained a t 200" C., then slowly brought up to about 500" C., and finally inserted into the cell contaning the molten salts a t 740" C. This procedure was found to prolong the life of the reference electrodes and generating cathode. The tungsten microelectrode was pulled up the cooler region of the cell, and chlorine gas bubbled through the melt for 20 minutes to remove traces of hydroxide ions (15). A 20-minute argon flush followed to remove the chlorine. Both gases were passed through 30-cm. drying towers charged with magnesium perchlorate before and after passing through the melt. After the removal of water, utmost care was necessary to prevent contamination from atmospheric moisture. The glass plug was removed, and the metal ion generating electrode inserted as rapidly as possible. The desired concentration of the metal ion was generated coulometrically with stirring provided by argon bubbling. The argon flow was then diverted to the inlet which provided constant flushing of the upper regions of the cell, the tungsten microelectrode was lowered, and the desired polarograms recorded. The stability of the metal salt solutions was determined for each metal by recording two polarograms in the most concentrated solution, the second one after bubbling argon through the melt VOL. 33, NO. 9, AUGUST 1 9 6 1
1189
!
40c
4P
I
I 3 4 1
33-
I
L 690
32 680
m
1
1
1
710
720
730
I
Degrees Centegrode
Figure 4. Ag(l)
Effect of temperature on limiting current of
Table I. Relationship between Limiting Currents and Concentration
ReSpe-
Concn.
cies
(mm) 0.093 0.252 0.374 0.561 0.747
Ag(V
il
(pa.)
irlC
8.4 18.9 28.4 43.0 56.6
89.9 75.0 75.9 76.6 75.8 Av. 7 8 . 6 f 6 . 3 Std. dev. k 8.0%
Ag(I)b 0.312
5.7 10.8 i5.8 25.6 40.4
18.3 17.3 16.8 16.4 16.2 Av. 17.0 & 1 . 0 Std. dev. i 5 . 9 %
Ni(I1) 0.078
7.6 15 0 22 8 32 2 45 0 60 8
97.4 96 2 83 5 82 6 82 6 84 8 Av. 8 1 . 9 Z!Z 7 . 0 Std. dev. f S.O%
0.624 0.936 1.560 2.496
0 0 0 0 0
156 273 390 545 717
Fe(I1) 0.156
9.0 0.311 18.0 0.467 2 7 . 3 0.623 3 6 . 2
Cu(1)
0
*
0.311 0.623 0.934 1.245
57.7 57.9 58.4 58.1 Av. 5 8 . 0 i 0 . 3 Std. dev. f 0 . 5 %
55.3 47.4 50.3 49.2 Av. 50.6 f 3 . 4 Std. dev. & 6.7% Electrode area approx. 7.4 sq. mm. Electrode area approx. 1.2 sq. mm.
1190
-1
2
-1 4
-I 6
I
I
-1 8
-2 0
I2 8
volts
Figure 5.
Analysis of reduction wave of 0.778mm Fe(ll) Pdarogram recorded manually
for 30 minutes. No changes in the limiting currents were observed, showing that neither losses due to volatility nor increases in concentration due to
dueible
.I 0
1 1 740
17.2 29.5 47.0 61.7
ANALYTICAL CHEMISTRY
chemical oxidation of the metal ion generating electrode took place. RESULTS
Polarograms. The polarographic reduction waves of silver(I), nickel(11), iron(II), and copper(1) are shown in Figure 2. Stripping curves of the deposited metals (recorded by scanning from more negative to less negative potentids) are also shown. I n all cases, the limiting current was directly proportional to the concentration within experimental error and over the range of concentration studied. This may be expressed by the well known equation: it = k,C,
(1)
where ir is the limiting current, C, the concentration of the electroactive species in the bulk of the solution, and k, the proportionality constant relating the two for a given electrode area. Table I presents values of i/C obtained over a range of concentrations for each of the metal ions under investigation. The limiting currents were measured a t a convenient potential on the limiting current plateau and the residual current, measured at the same potential in the melt just previous to generation of the metal ion, was subtracted in each case. The data indicate that a precision of 5 to 8% might be expected for an analysis of this type. The “increasing current effect” found by Laitinen and coworkers (IO) for reduction to the solid metal in the lithium chloride-potassium chloride eutectic is much less evident here. Presumably this is due to the larger electrodes and lower concentrations employed in this study. The smallest electrode wed (1.2 sq. mrn.) exhibited an “increasing current effect” to the extent of about 2% per minute. This should not interfere with quantitative applicrttions provided a constant rate
of scan is used. However, the buildup of metal deposits was detrimental in another respect. The limiting current was not entirely constant, but instead fluctuated rapidly around a constant average value. These fluctuations, which were normatly around 10% of the total current, are thought t o be at least partly due to uneven growth of dendritic deposits of metal. Random convective movements of the solution and vibration of the electrode may also contribute. The average value of the current waa used in all cases. Effect of Electrode Area. The areas of the two electrodes used to obtain the two sets of data for silver(1)t (Table I ) were measured using microscope equipped with a stage micrometer. The smaller one w a B 1.2 sq. mm. and the larger one 7 . 4 sq. mm. If the current were purely diffusion-controlled, it would be expected that the k. values would be directly proportional to the area. However, the area ratio was 6.2 while the k , ratio was only 4.6. This discrepancy may be partially due to the difficulty of determining the areas accurately or it may reflect convective complications. In any case, the prediction of k, values from one electrode to another does not appear to be possible. Scan Rate. In contrast to currentvoltage curve8 obtained in unstirred aqueous solutions with solid electrodes, no peaks were observed for the reduction of metal ions in this melt. Evidently steady-state conditions are achieved very rapidly in the melt, possibly as the result of enhanced convection currents due to thermal gradients around the electrode. I n a system with rapidly established steady-shte conditions, variations in scan rate within normal limits would not be expected to affect limiting currents. Black and DeVries (I), however, have reported a large enhancement of the nickel(I1) wave in.
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- 1 500
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- 1 volts 700
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-8
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800
- 1 900
Figure 7. E:/, a s function of log C
VdtS
Figure 6.
Analysis of reduction wave of 0.545mm Ni(ll) Polarogram recorded manually
the lithium-potassium chloride eutectic when the scan rate at a rotating platinum electrode was increased. A study was, therefore, made of the effect of scan rate on the reduction of silver(1). As seen in Figure 3, the wave shape was unaffected and only a slight decrease in limiting current with increasing scan rate could be detected. Since this is just the opposite of what would be expected if nonsteady-state conditions were being approached, it is felt that steady-state conditions do exist even at the highest scan rate used. These curves were recorded on a small (1.2 sq. mm.) electrode, and the increase in limiting current with decreasing scan rate is, therefore, probably due to the “increasing current” effect which is more pronounced a t lower scan rates. With the larger electrodes it can be expected that the limiting current is independent of scan rate. In none of the metals studied was there any sign& cant difference between limiting currents obtained by manual and automatic scanning. Effect of Temperature. The effect of temperature on the limiting current of a 2.50 mm solution of silver(1) is shown in Figure 4. The temperature coefficient was 0.23% per degree a t 738” C. Therefore, control to within *4O should be sufficient to keep the variahility due to temperature fluctuations below rtl%. Analysis of Wave Shapes. Assuming that the current is directly proportional to the difference between the concentration of the oxidized form at the surface of the electrode and in the bulk of the solution, that the metal is deposited a t unit activity of the electrode surface, and that the reaction is reversible, the equation describing the rising portion of the polarographic wave is given below (8):
where the symbols have their usual significance. If k, and f. are constant, a plot of log ( i ~- i) us. E should be a straight line with a slope of 0 201 at 740’ C. Such a plot for iron(I1) is shown in Figure 5. The results obtained for manually recorded polarograms of silver(1) and copper(1) were similar, and the observed slopes are summarized in Table 11. Agreement with Equation 2 is reasonable in light of the uncertain nature of the mass transfer process. This indicates that Equation 2 and the assumptions made in its derivation are at least approximately correct. It also demonstrates that such an analysis can be used, with some caution, to estimate the n value of the reduction in question. This can be of considerable value because the oxidation states of many metal ions are not known with certainty in high temperature melts. The stripping curves shown in Figure 2 reveal no hesitations as they pass through the zero current axis, which also supports the supposition that the reactions are reversible, In all instances the deposited metal stripped off rapidly, suggesting that no appreciable alloy formation was taking place. The anomalous linear segment of the nickel reduction wave is as yet unexplained. To check on the oxidation state of anodically generated nickel, a plot of potential of the nickel electrode us. logarithm of the concentration of nickel was made, the concentration being varied by generation of successive increments. The slope of the straight line, which should be equal to 2.303 RTIn, led to a value of 1.8 for n. The stripping curve is characteristic of that of metal dissolution; hence there seems to be no doubt that nickel(I1) is the species generated and nickel metal is the final reduction product. The potential of a nickel electrode in the melt corresponds (-1.16
+--
-
Table 11;
Slopes Measured from Plots
of Log (it - i ) vs. E
Metal Ion Silver(I )
Observed Calculated 0.235 0.091 0.181
Iron(I1) Copper(1)
0.201 0.101 0.201
volt) with the start of the normal steeply rising portion of the curve, confirming the deposition of metallic nickel as the electrode process taking place. The slope of the log (i, - i) os. E plot for this segment (0.147) was higher than the theoretical value of 0.101 (Figure 6). Explanations of the anomalous linear branch in terms of formation of a monolayer, deposition of material not in the standard state, formation of nickeltungsten alloys (do), solubility of nickel metal in the melt, and reduction to univalent nickel could not be made to correspond with the experimental facts. Fortunately, the unexplained behavior of the linear branch does not affect the quantitative applicability of the normal branch, as shown by the data in Table I. The limiting current is still directly proportional to the concentration. Half Wave Potentials. If the same assumptions are made as were made for Equation 2, the equation for E l l B as a function of concentration is (8): Ellz =
E
+RT In!, + RT -In nF
nF
2 C
(3)
2
where Ell2 is the potential a t one half the wave height, and all other terms have their previously ascribed meanings. Again a plot of log C, os. Ell* should yield a straight line with a slope equal 0 201 to -. n Straight lines were obtained as shown in Figure 7 ; however, the observed slopes were not in good agreement with the calculated slopes as shown in Table 111. Nickel(I1) was not included because of uncertainty as to what to consider the half wave point. VOL 33, NO. 9, AUGUST 1961
1191
ACKNOWLEDGMENT
Table 111.
Slope of Plot of Concentration
Metal Ion Silver(I ) Iron(11) Copper( 1)
Eli2
vs. Log
The authors express their appreciation of the award of an American Cyanamide Fellowship to D. L. Maricle for the 1957-58 academic year, and thank Earl W. Balis of the General Electric Co. for his advice and assistance.
Observed Calciilated 0.143 0.049 0.156
0.201 0.101 0.501
LITERATURE CITED
( 1 ) Black, E. D., DeVries, T., ANAL. CHEM.27, 907 (1955). (2) Bockris, J. 0. M., Hills, G. J., Menzies, I. A., Young, L., Nature 178, 654 (1956). (3) Colichman, E. L., ANAL. CHEM.2 7 , 1559 (1955). (4) Delimarskii, Yu. K., Kuz’movich,
CONCLUSIONS
Polarography with tungsten-in-Vycor electrodes can be used as a method of analysis, and to obtain information about the nature of electroactive species in a chloride melt at temperatures as high as 740’ C. The same techniques can presumably be used in other melts which are compatible with a glass system. No attempt was made t o determine the upper temperature serviceability limit of the tungsten microelectrode, and the technique should be applicable t o higher temperatures if a suitable reference electrode is employed.
V. V., Dopmidi Akad. Nauk. Ukr. R.S.R. 1959, No. 1,55. (5) Ferguson, W. S., Ph.D. thesis, Uni-
versity of Illinois (1954). (6) Flengas, S. N., J . Chem. SOC. 78, 534 (1956). ( 7 ) Hills, G. J., Inman, D., Young, L., Proc. Intern. Comm. Electrochem. Thermodynam. and Kinet. 8th Meeting 90 (1958). ( 8 ) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Vol. 1, Interscience, New York, 1952.
(9) Laitinen, H. A,, Ferguson, JJr. F., ANAL.CHEM.29. 4 (1957). (10) Laitinen, H. A., Liu, C. H., Ferguson, W.S., Ibid., 30, 1226 (1958). ( 1 1 ) Laitinen, H. A,, Pankey, J. W.,J . Am. Chem. SOC.81, 1053 (1959). (12) Lvalikov, Yu. S., Karmazin, V. I., Zavodskaya Lab. 14, 144 (1948). (13) Lyalikov, Yu. S., Karmazin, V. I., Zhur. Anal. Khim. 8 , 38 (1953). (14) Maricle, D. L., Ph.D. thesis, Massachusetts Institute of Technology (1959). (15) Maricle, D. L., Hume, D. N., J. Electrochem. SOC.107,354 (1960). (16) Nachtrieb, N. H., Steinberg, M., J . Am. Chem. SOC.70, 2613 (1948). (17) Osteryoung, R. A., Ph.D. thesis, University of Illinois (1954). (18) Randles, J. E. B., White, W.,Z . Electrochem. 59, 666 (1955). (19) Reilley, C. N., Adams, R. N., Furman, N. €I., ANAL.CHEM.24, 1044 (1952). (20) Smithell;,, C. J., “Metals Reference Handbook, 2nd ed., Val. 2, p. 558, Butterworths, London (1955). (21) Stein, R. B., J. Electrochem. SOC. 106, 528 (1959). (22) Steinberg, M., Nachtreib, N. H., J. Am. ,Chem. SOC.72, 3558 (1950). RECEIVED for review October 19, 1960. Accepted May 4, 1961. Work supported
in part by the U. S. Atomic Energy Commission under Contract AT(30-1)905.
Simultaneous Polarographic Determination of Indium and Tin SIDNEY L. PHILLIPS’ and EVAN MORGAN2 Research laboratory, international Business Machines Corp., Poughkeepsie, N. Y.
b A procedure for the simultaneous determination of milligram quantities of tin and indium is described. In a supporting electrolyte containing 0.1 M ammonium thiocyanate and 0.2y0 pyrogallol, tin and indium yield welldefined waves, suitable for analytical purposes.
T
work was prompted by a necessity for determining the tin-indium ratio of thin fdms of a tin-indium alloy. The advantages of the polarographic method for simultaneous determinations have been discussed (6),and a polarographic method was developed for milligram quantities of samples, provided the indium content is no more than 100 times that of tin. Because of the possible air oxidation of tin(II), it is preferable to determine tin polarographically as tin(1V). Tin (IV) and indium(II1) have been deHIS
‘
l Present address, Chemistry Department, University of Wisconsin, Madison, Wis. * Present address, Olin Mathieson Chemical Corp., New Haven, Conn.
1192
ANALYTICAL CHEMISTRY
termined separately in halide solutions ( I , 2: 4, but in these supporting electrolytes the second tin wave was found to interfere with the indium wave. Satisfactory results were obtained by chelating tin(1V) with pyrogallol and obtaining the polarogram in ammonium thiocyanate solution. Both indium(II1) thiocyanate ion and the tin(1V)-pyrogallol chelate show welldeveloped waves with the respective diffusion currents proportional to the concentrations of each (Table I). Tin(1V) pyrogallate is also reduced in acidic perchlorate solution, but in this medium the indium(II1)-aquo ion gives a poor wave ( I ) .
Mackay and Co. The pyrogallol was a n Eastman Kodak Co. material, used as received. All other materials were reagent grade. Doubly distilled water was used throughout. Gelatin was used as a maxima suppressor. Procedure. Dilute a n acidic solution containing 1 to 10 mg. each of tin(1V) and indium(II1) to about 30 ml. Add 0.1 gram of pyrogallol and 0.38 gram of ammonium thiocyanate t o the beaker and stir until dissolved. Adjust to a p H of 1 t o 1.2 with 1 t o 1 ammonium hydroxide; then, quantitatively transfer the solution to a 50-ml. volumetric flask, dilute t o volume, and mix thoroughly. Rinse the cell with portions of the test solution, add another portion, and deaerate for 10 minutes.
EXPERIMENTAL
The half-wave potentials for tin(IV), tin(II), and indium(II1) are -0.16, -0.42, and -0.56 volt vs. a saturated calomel electrode, respectively. The indium(II1)-thiocyanate has a dip in the limiting current starting a t -0.9 volt vs. S.C.E. similar to the one described for the indium(II1)-chloride ion ( I ) . Data obtained by the above procedure on known concentrations of tin and indium are shown in Table I.
The polarograms were recorded on a Sargent Model XXI Polarograph using the water-jacketed H-cell previously described (4). Reagents. Standard tin(1V) solutions were prepared from Mallinckrodt analytical reagent tin metal according t o the procedure of Lingane (3). Standard indium solutions were prepared by dissolving, in nitric acid, 99.9% pure metal obtained from A. D. Apparatus.
