Anodic Stripping Voltammetry of Nickel at Solid Electrodes M. M. NICHOLSON Humble Oil & Refining Co., Baytown, lex.
b The anodic stripping voltammetry of nickel has been investigated at platinum and gold electrodes in thiocyanate solutions, with the application of a linear voltage scan during the stripping process. Effects of deposition potential, temperature, and stirring are discussed, and evidence for surface heterogeneity in films approaching monolayer thickness is presented. Two series of measurements on platinum were made in a small cell with a silver-silver thiocyanate reference electrode at nickel ion concentrations from 1 X to 5 X lO-*M.
A
NODIC stripping techniques have been a subject of renewed interest within the last few years. I n their analytical applications, all of these methods involve the electrodeposition of a metal from a dilute solution a t constant potential by some reproducible procedure. The amount so deposited then bears a known relation to the concentration of the metal ion and is measured in terms of an electrical signal produced by its complete or partial dissolution a t more anodic voltages. The stripping has been carried out manually a t various potentials (Y), automatically with a linear voltage scan (3, 6, 9, 18, 14, l b ) , by potentialstep and current-step procedures ( I 1 , 16), and by square wave polarography (1). Platinum (9, 14), mercury-plated platinum (6, 12), and liquid mercury (1, 3, Y, 15, 26) electrodes have been used. Mercury drop or pool electrodes offer the advantages of a renewable surface and a ~7ide cathodic voltage range, while those of platinum or other inert metals provide greater sensitivity through complete recovery of the deposited metal and may be used a t more anodic potentials. This paper reports the results of anodic stripping measurements on nickel in thiocyanate solutions. The highly irreversible nature of the Si-Ni(H20)E++ couple necessitates the use of a complexing agent to bring the plating and dissolution reactions within a suitable potential range. Even in such a relatively favorable medium, the dissolution current of nickel from an amalgam is partially obscured by that due to the
1058
ANALYTICAL CHEMISTRY
oxidation of mercury (16); hence, platinum and gold electrodes were used in the present work. The stripping technique was that employed in a previous study on metallic monolayers (14). The amount of metal dissolving was measured by integration of the current-time curve recorded during a linear anodic voltage scan. In addition to the choice of supporting electrolyte, several other factors were considered in the development of a practical procedure for this metal. These included the nature of the inert electrode and its pretreatment, effects of plating potential, temperature, stirring rate, and contamination from various sources. Preliminary experimental work on each of these points led t o the selection of conditions under which reproducible analytical data could be obtained on cell solutions containing as little as 3 p.p.b., or a total weight of 0.006 pg., of nickel. Contrast in the behavior of mono- and multilayer films of nickel on gold was also noted, and a similar but less pronounced effect was observed on platinum. EXPERIMENTAL
Materials. The water for preparation of the solutions mas purified in a continuous borosilicate glass still which provided steam stripping, followed by distillation from alkaline permanganate and from phosphoric acid solution. The final product had a specific conductance lower than 1 X 10-7 ohm-' cm.-' a t 25' C. Potassium thiocyanate was recrystallized from water, dried over potassium hydroxide pellets, and heated a t 150' C. for 2 hours. Matheson prepurified nitrogen was passed over copper a t 450' C. to remove oxygen and was saturated with water vapor a t the temperature of the cell before entering the nickel solution. The other reagent grade materials were not further purified for the work reported here. It was found earlier that recrystallization of the potassium nitrate did not improve the background current or the reproducibility of the stripping curves. Procedure. Preliminary work a t the level of 10-4M was conducted in a polarographic H cell of conventional size, which was connected to a large saturated calomel electrode through a sintered-glass disk and an agar salt
bridge, 36 cm. X 0.8 sq. cm., prepared from saturated potassium nitrate. I n this cell, the platinum electrodes were vertical wires of 0.051-cm. diameter and about 0.8-cm. exposed length, sealed into soft glass tubing. A smooth gold plate approximately 4 microns thick was applied to some of the platinum wires by electrodeposition from a cyanide bath. Solid gold wires gave similar results. For most of the measurements in this cell, the supporting electrolyte was 0.1M potassium thiocyanate. Several types of smaller cells were constructed and tested for use with submicrogram quantities of nickel. The most satisfactory design is shown in Figure 1. After assembly of the cell, the polarized and reference electrodes were separated by two 5mm. diameter Corning Grade F sinteredglass disks. The need for an agar salt bridge was eliminated by using the same electrolyte solution throughout the cell, with a silver-silver thiocyanate reference electrode in the inner section. Severe contamination of the platinum electrodes from the agar bridge was encountered with other experimental designs, in which there was direct contact betneen the gel and the sample solution. This problem \%asmore important in the very dilute solutions, which required plating over relatively long times, with stirring. Silver-silver thiocyanate electrodes were prepared on platinum spirals by the electrolytic method of Vanderzee and Smith (17). The supporting electrolyte in the small cell was 0 . M in potassium nitrate and O.OL!4 in potassium thiocyanate. This low concentration of thiocyanate yielded a relatively low background current and prevented the excessive dissolution of silver as complex anions ( 2 ) . I n this medium, the silver-silver thiocyanate electrode had a measured potential of - 0.020 volt with respect to the saturated calomel electrode. The value calculated from thermodynamic data (17 ) , neglecting the liquid junction potential, was -0.029 volt. Although the concentration of thiocyanate vias lower than is usual in such reference systems,jit was enough t o stabilize the potential against the small currents passed by the nickel electrode. The platinum electrodes in the small cell were polished cross sections of No. 16 wire, 0.013 sq. cm. in area, which had been found convenient in other voltammetric work (13). Transfer of solution between the compartments was minimized by filling the three sections
i/
COPPER DEPOSITION POTENTIAL, VOLTS vs. S.C.E. -0.2 -0.4 0.6 -0.8
0
'.el+
I
,
I
,
,
-
-1.0
4
PLATINUM AG-AGSCN
POLISHED
0 013 CM2
Figure 1.
Figure 2.
Small cell and electrodes
Effect of deposition potential
Wire electrodes, plated 2 min. without stirring
to predetermined levels, which required approximately 2 ml. in each side of the main cell. When the nickel solution was placed only in the platinum electrode section, some of it was drawn into the adjacent frit by suction, as a further precaution against concentration changes. To avoid unnecessary risk of contamination, no lubricant was used on the ground joints. The cells were immersed in a water bath a t 25.00' =t 0.03" C., except for a few measurements a t higher temperatures. The electrodepoqition and dissolution steps were carried out on the high speed pen recording polarograph described previously (14). After the application of a constant plating potential for a suitable time, the voltage was changed linearly in the anodic direction at the rate of 50.0 niv. per second. The areas under the resulting currenttime (or current-voltage) curves were measured with a planimeter. The automatic cell resistance compensator was applied in work with the small cell, which had a resistance of 4500 ohms. Oxygen was removed from the large cell by the passage of nitrogen through the sample compartment for several minutes. Most of this 11-ork was at relatively high nickel concentrations, and plating for 2 minutes in unstirred solution usually produced sufficient metallic deposit. Reproducible stirring for deposition from the lower concentrations was achieved in both cells by bubbling nitrogen through the solutions a t controlled rates, measured on a rotameter. I n each case, the stirring gas entered through a single ring-sealed inlet tube. The usual practice in the small cell was to pass nitrogen through both sides for several minutes, stopper the section serving as a salt bridge, and continue the flow of gas in the platinum electrode compartment only during the electrodeposition step. The stirring w s discontinued shortly before the beginning of the anodic voltage scan. Several methods of electrode pretreatment were investigated: chemical treatments with 1 to 1 nitric acid, concentrated hydrochloric acid, and acidified ferrous sulfate (8); polishing of the platinum with metallographic alumina; and application of a 1000-cycle alternating current signal of 1- to 2-volt amplitude ( I S ) . I n general, the results on
0 3 X 1 O-'M Ni(ll) in 0.1 M KSCN on platinum 0 3 X 1 O-'M Ni(ll) in 0.1 M KSCN on gold A 1 X lO-'M Cu(ll) in 0.1 M KNOa on platinum Eeq. = potential of pure metal in equilibrium with bulk concentration of its ion
platinum showed little variation with the method of pretreatment, while those on gold tended to be somewhat more erratic. In subsequent work, the wires were cleaned several minutes in 1 to 1 nitric acid before each run, and the circular cross-section electrodes were given a light polishing preceding the nitric acid treatment. Tn some instances, a preliminary anodic scan without plating appeared to be beneficial. To provide a basis for quantitative comparisons of results on the various circular electrodes, cathodic currentvoltage curves of 0.001M potassium ferricyanide in 0.1M potassium chloride were recorded in unstirred solution a t a scanning rate of 33.3 mv. per second. For electrodes A and B employed here, the peak currents were 1.80 f. 0.00 and 1.83 f. 0.01 pa., respectively. The areas thus were equal within 2%. On the basis of rather extensive preliminary work, of which only selected features are reported here, conditions were chosen for the determination of nickel in the concentration range of 1 X 10-5 to 5 X lO-*M. Two series of anodic stripping measurements were made in the small celi under the conditions indicated in Table I.
Table I.
RESULTS AND DISCUSSION
Plating Conditions. A lowered overvoltage for nickel deposition does not require a highly stable complex but, rather, one which is easily discharged and readily available kinetically. At 20' C., t h e distribution of nickel in thiocyanate solutions is approximately the following, when the total nickel concentration is much less than the total thiocyanate (4): NiNiNiSCN- Xi++,(SCN)+, (SCN)*,(SCX)$-, M % % % % 15 2 0.1 33 50 0.01
87
13
0.4
0.006
An important variable in the anodic stripping procedure is the plating potential. The relative amounts of nickel deposited at various potentials from 0.1M thiocyanate solution on platinum and gold electrodes are plotted in Figure 2, where q represents the area measured under the anodic stripping curve. The form q/qm,,. was plotted to make the shapes of the curves directly comparable. A similar curve for copper
Conditions for Analytical Measurements
Temperature 25.0" C.; nitrogen flow rate 74 cc.jmin.; plating potential -0.90 v. us. Ag-AgSCN; scanning rate 50.0 mv./sec.; cell resistance compensated Series A Series €3 Platinum electrode B Electrode pretreatment Polishing, 1: 1 HXOI Polishing, 1 : 1 HNO,, preliminary scan made after 2 min. at -0.90 v. Source of nickel Si(YH,),(S0,),.6H,O ?;i(NOs), 6H20 Solutions in cell compartments Supp. elec. only Ag-AgSCN Supp. elec. only Sample Intermediate Supp. elec. only Platinum Sample Sample Data reported Initial run First run following preliminary scan
VOL. 32, NO. 9, AUGUST 1960
1059
(11) is also shown. The q,,. values corresponded t o 2300 microcoulombs per sq. em. for nickel on platinum, 2200 for nickel on gold, and 770 for copper on platinum.
Figure 3. Monolayer dissolution on platinum and gold 1 X 10-4M Ni(ll1 in 0.1M KSCN, plated 2 min. - 0.80 volt vs. S.C.E., without stirring
at
- Platinum wire electrode --- Same electrode, gold plated
____
Approximate monolayer, based an proiected area
For an easily discharged metal having a small deposition overvoltage, a curve of this type will be nearly flat over a wide voltage range. Such was the case for copper in Figure 2. With nickel, however, complicating factors prevented the full development of a diffusion plateau. Even with such a favorable complexing agent as thiocyanate ion, irreversibility was apparent in the small slope on the ascending part of the curves. At more cathodic potentials, a decrease in PIPrnax. accompanied the onset of hydrogen evolution. This difference in gold and platinum was also reflected in blanks for the two electrodes in the same medium. It is possible that the decrease in nickel deposition was associated with the increased alkalinity in the vicinity of the surface, which precipitated the nickel in a form that was not further reduced. Results in an acetate buffer containing thiocyanate ion seemed to confirm this suggestion. For anodic stripping purposes, a practical result from the deposition experiments is that one must work near the maximum q and control the plating potential more closely for nickel than for easily discharged metals. Increasing the temperature from 25' to 55' C. effected only minor changes in the shape of the nickel deposition curve in the q/qmaX.form, although the value of q,,. increased by about a factor of 3. Some additional sensitivity could be gained in this way, primarily because of more rapid diffusion. I n addition, the sensitivity was greatly increased by stirring the solution during the plating step. The need for stirring
1060
ANALYTICAL CHEMISTRY
was indicated also by certain characteristics of the dissolution curves which are discussed in the next section. Surface Heterogeneity. Anodic stripping curves have been calculated, under the assumption of a reversible electrode couple, for uniform fractional monolayers and for multilayer films thick enough to approach the behavior of bulk materials (14). Each of the calculated curves has a single peak, the shape of that for the fractional monolayer depending on the initial surface coverage and other variables. For nickel, this simple model is inadequate; not only is the electrode reaction known to be irreversible, but deposits in the monolayer range showed evidence of surface heterogeneity] as illustrated by Figure 3. The estimated monolayer is based on the assumption that a nickel atom occupies 6 sq. A., which corresponds to 500 microcoulombs per sq. em. Two nickel dissolution stages were visible on platinum and pronounced on gold. The average thickness of the deposit appeared to be the critical factor in determining the shape of the dissolution curve, because two peaks of about the same prominence could be obtained from many different combinations of concentration, plating potential, and plating time, as long as the deposit was equivalent to about 500 microcoulombs per sq. em. Any increase in thickness shifted the distribution of current toward the first peak, near -0.4 volt us. S.C.E. on gold; any decrease emphasized the second, in the vicinity of -0.1 volt. For thicknesses of several atomic layers, the stripping curves on gold and platinum were almost identical, as shown in Figure 4. These results strongly suggested that the double pattern was a surface phenomenon, that the first peak corresponded to the removal of nickel atoms from what was substantially a nickel surface, while the second
Figure 4. Multilayer dissolution on platinum and gold 3 X 1 O-'M Ni(ll)in 0.1 M KSCN, plated 2 min. at
- 0.95 volt vs. S.C.E., without stirring
- Platinum wire --- Gold-plated platinum wire
stage involved atoms attached directly to the gold or platinum with a binding energy favoring the deposition process. Further support of this interpretation is found in Figure 2. With deposition on the gold surface, an abrupt change in slope occurred a t -0.6 volt, a t a surface coverage equivalent in this case to 73% of the estimated minimum monolayer. -4similar predeposition effect was observed by Funkhouser (6) in the deposition of nickel from thiocyanate solutions onto a mercury pool. Measurements at Very Low Nickel Concentrations. From Figures 3 and 4, it is evident t h a t the stripping curves from deposits exceeding 500 microcouloinbs per sq. cm. were more amenable to accurate integration and more desirable] therefore, for analytical purposes. T o attain this condition in extremely dilute solutions, it !vas necessary to carry out the deposition n-ith stirring. This was accomplished by the passage of nitrogen through the cell, as described in the Experimental section. This method gave surprisingly reproducible results, and it also eliminated oxygen throughout the long plating times. Figure 5 shows that the nitrogen flow rate was not critical when the plating time was as great as 5 minutes. I n the small cell, the amount plated n-as constant within =t3yO for flow rates between 52 and 96 cc. per minute. I n the large cell, the variation was = t l % from 100 to 260 cc. per minute, but because of an interest in minimizing the total size of the nickel sample, further data were obtained in the smaller cell. Table I summarizes the conditions of two series of measurements, covering the conto centration range from 1 X 5 x 10-831. Detailed data are presented in Table 11. To approach the film thickness equivalent to 500 microcoulombs per sq. cm., plating times up to 100 minutes were used. This presented the possibilities of losing nickel by diffusion through the fritted disk and by the deposition process itself. The first of these difficulties was circumvented in the Series B measurements by placing the nickel solution in both compartments of the main cell, although no evidence of loss from this cause had been observed. The greatest concentration change due t o deposition occurred with the plating time of 100 minutes in the 5 x 1O-*M solution. At the end of this time, the concentration had decreased 13% on the basis of the dissolution area. At 30 minutes, the decrease was about 3%. Adsorption on the walls of the container is sometimes a problem in the storage of extremely dilute solutions. The nickel solutions were stored in glass, and those below l O - 4 K were prepared within an hour or so before use. The
adsorption losses apparently were not serious under these conditions. The effect of varying the deposition potential a t extreme dilution was not investigated in detail, but a t 10-6M the amounts deposited were 13.1, 14.7, and 15.3 microcoulombs at -0.80, -0.90, and - 1.00 volt, respectively, us. Ag-AgSCN in the 0.01M thiocyanate solution. Because of the limitations in plating a t highly negative potentials, the value of -0.90 volt ts. Ag-AgSCK was selected, somewhat arbitrarily, for the work reported in Tables I and 11. Typical dissolution curves on platinum in the more dilute thiocyanate solution are shown in Figure 6. The extraneous anodic peak a t +0.35 volt was also piesent in the nitrate-thiocyanate blank after the same exposure t o the plating potential, but not in the potassium nitrate alone. On a rapid scan without plating, this peak was absent. Although the current apparently originated from the thiocyanate or from one of its decomposition products or impurities, the detailed nature of the process has not been established. I n both seiies, a decrease in dissolution area with successive scans was noted when the electrode was not cleaned between measurements. The rate of decrease varied with the cell solution but was of the order of 5% per scan. Because of this effect, more confidence was placed in the initial runs after cleaning the electrode (Series -4)or in those preceded by a preliminary anodic scan made after only 2 minutes of plating (Series Bj. The background currents under the dissolution peaks were dranm by inspection. This introduced some uncertainty in evaluating the currenttime integrals, but it appeared necessary, because a recorded blank would not be applicable, in general, to the changing surface present during the actual stripping process. I n view of the difficulties to which these measurements were subject, the constancy of the term p/cot in Table I1 is as good as one might expect, and the ohserved precision would be considered satisfactory for many trace analytical purposes. It is of interest to compare the sensitivity of the anodic stripping measurements with those of other methods for traces of nickel. In the most dilute cell solution reported here, the nickel concentration was 3 p.p.b., and the platinum electrode compartment contained 0.006 kg. of the metal. McDowell and coworkers, measuring the absorption of the nickel complex with 4-isopropyl-l,2-cyclohexanediovime in xylene solution, reported the determination of 5 p.p.b. of the metal ion in water (10) Interference with the anodic stripping method may be expected from substances which form deposits on the electrode, electrolytically or by ad-
b
50
0
30 I
cc
I
LARGE CE,L
MIN.' NITROGEY IN 100 150__ -
~
2 0 0
250
= 800
Figure 5. Electrodeposition with nitrogen stirring Plated at -0.90 volt vs. AgAgSCN in 0.01M KSCN
0.1 M KNO3
0
+
1 X 1 O-SM Nilll) in large cell, 5 min. 5 X lO-6M Ni(ll) in small cell, 6 min.
--z 1 ~-
0
~
25
cc
MIN:'
50 75 NITROGEN I N SMALL CELL
100
-4
Figure 6. Dissolution curves at low nickel concentrations
+
Supporting electrolyte 0.01M KSCN 0.1M KNOa, plated at -0.90 volt vs. Ag-AgSCN with 74 cc./min. nitrogen Upper. Lower.
1 X 10-6M Nilll), plated 30 min. 1 X 10-7M Ni(ll), plated 60 min.
sorption, Rithin the operating potential range, or which oxidize the nickel by direct chemical reaction during the anodic scan. As an example in the latter category, ferric ion above 1 x l O - 6 N caused serious interference in the measurement of 10-7Jf nickel. Materials which codeposit with the nickel tend to interfere a t relatively lower levels because their effects in-
Table II.
Analytical Data on Nickel
Mole/Liter
t,
Min.
Erns,., Volt US. AgAgSCN
5 x 10-7 7 . 5 x 10-7 1 x 10-6 2 x 10-6 4 x 10-6
30 30 30 30 30 15.5 15 15 15 15 10
-0.18 -0.22 -0.24 -0.25 -0.22 -0.26 -0.26 -0.24 -0.23 -0.22 -0.24
cop.
9,
Microcoulombs
( q /10-8 col) X
Series A
6 X 10-6 8 1
x x
1010-5
8.35 12.9 15.4 30.6 64.5 37.4 35.2 46.9 50.2 69.2 59.2
0.56 0.57 0.51 0.51 0.54 0.60 0.59 0.52 0.56 0.58 0.60 Av. 0.56
0.00 $0.01 -0.05 -0.05 -0.02 $0.04 +0.03 -0.04 0.00 +0.02 + O , 04 50.03
2.76 3.49 5.54 6.80 6.49 5.09 8.46 8.19 8.74 8.94 5.58 18.2 66.1
0.55 0.58 0.48 0.55 0.47 0.48 0.47 0.46 0.62 0.60 0.53 0.61 0.66 Av. 0.54
+0.01 +0. 04 -0.06 +0.01 -0.07 -0.06 -0.07 -0.07 t o , 08 +0.06 -0.01 +0.07 +o. 12 k0.06
Series B 5 x 10-8 1 x 10-7 2 x 10-7
x
10-7
x 7 x 1x 1x
10-7
3 5
10-7 1010-5
100 60 58 62 70 35 60 60 20 30 15 30 10
-0.12 -0.14 -0.18 -0.16 -0.15 -0.19 -0.23 -0.23 -0.21 -0.21 -0.20 -0.29 -0.23
VOL. 32, N O . 9, AUGUST 1960
1061
crease with the plating time. Appropriate precautions must, therefore, be taken in the application of the anodic stripping method to complex materials. ACKNOWLEDGMENT
( 2 ) Cave, G. C. B., Hume, I>. X., J . Am. Chem. Soc. 75, 2893 (1953). ( 3 ) DeMars, R. D., Shain, I., ASAL. CHEM.29,1825 (1957). ( 4 ) Fronaeus, S.,Acta Chem. Scand. 7 , 21 (1953). (5) Funkhouser, J. T., Ph. D. dissertation,
Massachusetts Institute of Technology,
The writer acknowledges with thanks the experimental work by Carl P. Tyler. LITERATURE CITED
( 1 ) Barker, G. C.,
“Modern Electroanalytical Methods,” G. Charlot, ed., p. 118, Elsevier Publishing Co., Amsterdam, 1958.
1954. (6) Gardiner, K. W., Rogers, L. B., ANAL.CHEM.25, 1393 (1953). (7) Hickling, A., Maxwell, J., Shennon, J. V., Anal. Chim. Acta 14,287 (1956). ( 8 ) Kolthoff, I. M., Tanaka, N., ANAL. CHEM. 26,632 (1956). (9) Lord, S.S., Jr., O’Neill, R. C., Rogers, L. B., Ibid., 24, 209 (1952). (10) McDowell, B. L., Meyer, A. S.,
Feathers, R. E., Jr., White, J. C., Ibid., 31.931 -~~ (1959). (11) Mamantov, G., Papoff, P., Delahay, P., J . Am. Chem. SOC.79,4034 (1957). (12) Marple, T. L., Rogers, L. B., Anal. Chim. Acta 1 1 , 574 (1954): (13) Nicholson, M. M., ANAL.CHEM.31, 128 (1959). (14) Nicholson. M. M.. J. Am. Chem. SOC. 79. 7 (1957). ., ANAL. I
\ - - - - I
J . Am. W. E..
RECEIVEDfor review March 14, 1960. Accepted May 5, 1960.
Polarographic Determination of Tin in Stannic Oxide SIDNEY L. PHILLIPS’
IBM Research laboratory, Poughkeepsie, N. Y. ,The
tin content in milligram amounts
of stannic oxide is determined by dissolving the oxide in hot concentrated hydriodic acid, followed by polarographing a portion of the resulting solution in acidic iodide or chloride media.
T
was prompted by a necessity for determining tin in milligram amounts of stannic oxide. The usual methods of decomposing stannic oxide involved fusion or reduction ( 2 ) and were unattractive as further treatment of the reaction product (melt or metal) was required before analysis. A rapid method was developed by dissolving the oxide in 57% hydriodic acid, then polarographing an aliquot in a supporting electrolyte of either ammonium iodide or ammonium chloride. Caley (1) observed that stannic oxide obtained from varied sources was soluble in hot concentrated hydriodic acid according to the metathesis SnOz 4HI = SnIc 2H20. Refluxing the oxide in a n all-glass apparatus with 48% hydriodic acid was satisfactory, but a Carius combustion tube method using 57% hydriodic acid was more rapid. The latter method is described here. Tin is most conveniently determined in its highest valence state because of the danger of air oxidation of stannous ion. Unfortunately, tin(1V) does not HIS INVESTIGATION
+
+
1 Present address, Department of Chemistry, University of Wisconsin, Madison, Wis.
1062
ANALYTICAL CHEMISTRY
give a favorable polarographic wave in most of the common supporting electrolytes, and halogen-rich solutions must be used t o obtain analytically acceptable results (4). I n the method presented here, the reduction wave of tin(1V) in acidic ammonium iodide solutions is shown to be diffusion-controlled and suitable for analytical purposes. Fluorostannate (6, 9) does not appear suitable for analytical purposes, and the only useful media have been those containing a large supply of chloride or bromide in 1M hydrochloric acid ( 7 ) . The reduction of tin(1V) in ammonium iodide solutions has not been previously reported. Only one tin wave is observed a t the suggested iodide concentration (2M) instead of the expected doublet. Lingane ( 7 ) found that in 4M ammonium bromide the anodic reaction H g 4Br- = H g Br4+ 2e obscures the start of the first wave and the apparent half-wave potential for reduction of tin(1V) to tin(I1) is too negative. Iodide is a stronger complexing agent with respect to mercury, and the analogous anodic reaction masks the first tin wave in 2M ammonium iodide because of the shift of the mercury wave to a more negative potential. At low iodide concentrations, two waves are clearly visible (Figure 1). The useful potential range in ammonium iodide is fairly narrow. Hydrogen production limits the most negative in values to -0.8 or 1.0 volt vs. S.C.E. 0.5M perchloric acid and 0.lM hydrochloric acid, respectively, while the wave starts at about -0.4 volt in solutions containing either acid when the
+
+
iodide concentration is greater than about 0.5M. Nevertheless, the analysis of certain binary tin-metal mixtures such as tin and lead is feasible by taking advantage of the nonreducibility of tin(1V) in weakly complexing media. This may be done by first obtaining a polarogram from a perchloric acid solution where only lead is reduced, then running a second polarogram after adding solid ammonium iodide to the solution (lead plus tin). Lead and tin mixtures have been analyzed polarographically by either Lingane’s (5) or Miura’s (8) method. EXPERIMENTAL
Reagents. Reagent grade materials were used throughout. Standard tin(1V) solutions were prepared according to Lingane (7). The 57% hydriodic acid contained about 1% hypophosphorus acid preservative, and was not further purified (11). The 0.5% gelatin solutions were prepared by dissolving 0.5 gram of gelatin in 100 ml. of water. These solutions were usable for at least a week when stored a t refrigerator temperatures. The water used mas twice distilled, once from an alkaline permanganate solution in a n all-glass still. Apparatus. All polarograms were obtained with a Sargent Model XXI recording Polarograph. A temperature of 25.0’ i 0.2’ C. was maintained by circulating water at this temperature through a water-jacketed H-cell available from Corning Glass Works. Linde HP dry nitrogen, after being passed through hot copper turnings, was used t o deaerate the test solutions. Borosilicate glass test
