Mercury Pool Polarography Ion-Amalgam Reductions Occurring at Surface of a Large Stationary Mercury Pool CARL A. STREULI AND W. DONALD COOKE Baker Laboratory, Cornell University, Zthaca, N. Y . I n order to extend the range of polarographic niethods, possible analytical applications of a large polarized mercury pool electrode were investigated. The polarographic waves obtained have distinct current maxima whose heights vary in a linear fashion with the concentration of the reducible ion. The halfpeak potentials are independent of concentration and correspond, within 0.05 volt, to the half-wave potentials associated with the dropping mercury electrode. Thallous, lead, copper, cadmium, zinc, bismuth, and indium ions have been studied. The polarized pool electrode has been found t o be appli-
T
HE polarographic determination of electroactive substances by the dropping mercury electrode and various solid microelectrodes has been extensively investigated. Although a great variety of electrodes have been developed, the use of a stationary, polarized mercury pool to perform similar determinations has not received attention in application to analytical polarography. Arthur ( 1 ) has recently worked with a stationary mercury drop and stirred solutions, and Lee has developed a rotating mercury electrode ( 5 ) . Zlotowski (9) in connection with work on overvoltage used a mercury pool electrode to investigate polarization phenomena. The main objectives of this paper were to study the character-
h “2
c
cable to the determination of ions a t lower concentrations than the more conventional polarographic methods, while retaining many of the advantages of the dropping electrode. The currents are proportional to concentration, the ions can be identified b y their reduction potentials, and the electrode has a high hydrogen overvoltage. The area of t h e electrode can be varied over wide limits, the charging current density is much smaller, and conventional equipment can be used. The polarograms obtained are theoretically predictable within 5% by the equations applicable to oscillographic polarography.
istics of the mercury pool electrode. to evaluate it as an analytical tool, and to compare it with the more common dropping mercury electrode. The studies were limited to electrode processes involving reduction of an ion to an amalgam. It was believed that the more important factors to consider in such a work should include the magnitude and reproducibility of the residual current, the linearity of the polarograms with concentration, the sensitivity of the electrode as compared to the dropping mercury electrode, the usefulness of the half-wave potentials in the qualitative identification of ions, the influence of the rate of potential scanning, the effect of previously discharged metals, and the effect of surface active agents. From an initial evaluation of the problem it seemed that the mercury pool should have a high hydrogen overvoltage and possess a fluid and reproducible surface. I t would have the advantage of a large area for the electrode surface, and should, therefore, be useful in the range of concentrations lower than those determinable by the dropping mercury electrode. This area could be conveniently varied over a Fide range as dictated by the particular problem at hand. One of the serious limitations to the use of the dropping mercury electrode is the comparatively large charging currents associated with the continuous flow of mercury. By using a stationary pool the magnitude of the charging current density should be greatly decreased. On the other hand, the mercury pool would not have a surface which is renewable during the recording of a polarogram. It would appear that the mercury pool electrode might be uaeful in studies involving derivative and oscillographic polarographv. In both cases the serious difficulties associated with the fluctuations of the dropping mercury might be overcome.
SCE
APPARATUS AND IIIATERIALS
Figure 1. Diagram of Cell
The cell used for this work is illustrated in Figure 1. I t consisted, essentially, of a water-jacketed borosilicate glass tube 2.0 cm. in diameter fused to a borosilicate glass tip 1.0 em. in diameter. The over-all length of the tube and tip was about 13 cm. The area of the mercury surface was calculated as 2.86 sq. cm. on the assumption that the surface of the pool could be represented as the zone of a sphere. The cell could contain from 10 to 20 ml. of the solution to be determined. A platinum wire sealed into the bottom of the tip provided the electrical connection to the mercury pool. The entire inner glass surface of this chamber was treated with Desicote (Beckman Instrument Co.) to prevent the wetting of the glass surface by the aqueous solutions below the surface of the 1691
A N A L Y T I C A L CHEMISTRY
1692
mercu This was done in order to maintain a constant merc y - s % t i o n interface and more reproducible currents. he reference electrode incorporated into the cell was a saturated calomel electrode isolated by means of a sintered-glass disk and agar plug. The nitrogen neressary to remove dissolved oxygen in solutions waa Air Reduction C0.k Seaford nitrogen (99.99% with a hydrogen impurity). It was not further pur5ed before use but, after saturation with water vapor, was introduced into the solution
I
I
I
I
through a fritted-glass cylinder leading directly into the center of the solution chamber and connected through the head of the cell to the three-way stopcock and bypass. The distance between the surface of the mercury pool and this sintered-glass cylinder was about 0.5 cm. Ten to 20 minutes' passage of nitrogen sufficed to decrease the dissolved oxygen in the solution to a negligible value. During the actual determinations, the nitrogen was bypassed by means of the three-way stopcock over the surface of the solution. All connections in the nitrogen inlet system were of Tygon tubing. The gas leaving the apparatus was passed through a bubbler to prevent back-diffusion of air into the cell. Commercially available triple-distilled mercury was occasionally found to contain traces of oil, and unless this was removed, current transients were obtained during the recording of the polarograms. Although these transients did not affect the height of the waves obtained, they could be eliminated by dispensing the mercury from a dry, grease-free buret in which it had been allowed to stand a t least 24 hours, or alternatively by passing the mercury through a sintered-glass disk before use. The entire electrolysis compartment was water-jacketed to permit the circulation of constant-temperature water (25.0°* 0.1" C.) and it was found necessary to mount the weighted stand supporting the cell on a sponge rubber cushion to protect the pool from shocks and vibrations. A modified commercial model recording polarograph, the Leeds and Northrup Electrochemograph, was used for this work. The polarizer unit in this instrument was found to give troublesome fluctuations in the reeorded polarograms caused by the rather coarse helical winding of the polarizing slide-wire. It was replaced by a ten-turn helipot having roughly five times as many turns and the fluctuations were thus greatly decreased. The polarizing voltage was obtained from dry cells with suitable resistances in series to drop 1 to 3 volts across the helipot. The voltage drop across the slide-wire was checked with a potentiometer. The rate of rotation of the helipot was varied by means of a Georrell and Georrell synchronous motor containing variable-speed gears, so that the mercury pool could be polarized at different rates. It was possible, using this apparatus, to run a polarogram over a I-volt range a t a rate of 50 my. per second to 1 mv. per hour. Appropriate gears were obtained, 80 that the
I
recorder chart could be driven a t a rate corresponding to 100 mv. per inch. AU solutions were prepared by the dilution of 0.01 M stock solutions prepared from reagent grade chemicals and analyzed by conventional procedures. The salts used were zinc chloride, lead acetate, copper nitrate, cadmium chloride, thallium nitrate, bismuth nitrate, and indium chloride. Dilutions were carried out with double-distilled water and enough potassium nitrate solution to make all solutions 0.01 M in respect to potassium
I
I
I
I
nitrate. The bismuth salt was dissolved in sulfuric acid and the solutions were diluted so that the final concentration of the sulfuric acid was 0.5 M. The polarograms were obtained by placing 10 ml. of appropriate solution in the cell, deaerating for 10 minutes, adding 5 mi. of triple-distilled mercury, and deaerating with nitrogen for an additional 10 minutes. After bypassing the nitrogen over the surface of the solution, the electrode was polarized a t the desired rate. Reproducibility of background was determined on 0.01 M solutions of potassium nitrate, the salt used as the supporting electrolyte for most of the determinations. Variation of current with concentration was determined by obtaining polarograms of to M concentrations of solutions containing from 5 X thg reducible ions. Comparison polarograms carried out with the dropping merM solutions with no maximum supcury electrode utilized pressor, since a t the concentrations used, no maximum appeared. RESIDUAL CURRENT
The magnitude of the residual current of an electrode used in polarographic work is determined by the presence of traces of electroreducible substances and the charging current. With the dropping mercury electrode, the charging current is greatly increased by the fact that the mercury is continuously flowing. With a stationary mercury pool, the magnitude of the charging current density should be greatly decreased, as was found to be the case, In Figure 2 the residual current associated with the mercury pool electrode is shown. Two runs made on different solutions at. different times show the reproducibility that can be expected in such work. Although the absolute values of the currents measured with the mercury pool are somewhat greater than the dropping mercury electrode, the area of this electrode is about 75 times greater, so the actual current density is considerably smaller. The contribution to the residual current by traces of reducible material is usually not serious with the dropping mercury electrode be-
1693
V O L U M E 25, NO, 1 1 , N O V E M B E R 1 9 5 3 Be+'+
I -0.2
Figure 3.
-0.4
-0.6
VOLTS vs. SCE , Wave Variation with Ionic Change Solutionn 10 -%w
Left. B i + + + Center. Cd'+
cause of the relatively greater contribution of the charging current. In the case of the pool electrode, however, with its increased sensitivity and decreased charging current density, this problem is more severe. Care must be taken in handling the supporting electrolyte solutions to prevent contamination in the analysis of the dilute solutions. In one of the polarograms in Figure 2 a small wave appears a t 1.0 volt in one run but not in the other. This is believed to be a contamination caused by zinc. That the major contribution to the residual current of the mercury pool is due to impurities in the supporting electrolyte can be surmised from the actual shape of the polarogram. The curve has a much smaller slope than that obtained with the dropping mercury electrode, indicating that the relative contribution to the curve by impurities is high as compared to the charging current. This flatter shape of the polarograms is advantageous experimentally. It is easier to compensate these polarograms than those of the dropping mercury electrode with their considerably greater slope. Although the two polarograms shown in Figure 2 are probably not as reproducible as those obtained with the conventional dropping electrode, this is caused by the greater sensitivity of the mercury pool and the greater likelihood of contamination. With the recorder sensitivity used in the analysis of solutions more concentrated than 5 X 10-8 M , the residual currents are reproducible within the accuracy of the recorder. The small peak a t the beginning of the polarogram is similar to that obtained with amalgam electrodes ( 8 ) and is possibly caused by formation of a trace of some insoluble mercury salt a t the more positive potentials. LINEARITY AND REPRODUCIBILITY OF POLAROGRAMS
The shape of the polarograms obtained with the mercury pool electrode differs considerably from conventional polarographic waves, as can be seen in Figure 3. The shape of the wave changes with the number of electrons involved in the reduction of the ion. The pronounced current peaks differ from the maxima obtained with the dropping mercury electrode, and are caused by the fact that the voltage change is more rapid than the rate a t which the diffusion layer is set up. This is a dynamic equilibrium depending on the rate of polarization, and entirely different curves would be obtained by the use of a manual polarograph. I n contrast to the usual polarographic maxima, the peak
Right.
TI+
VOLTS
E. SGE
Figure 4. Reverse Scan of Pool Electrode heights of the current-voltage curves are linear with the concentration of the ions being reduced. These data are shown in Table I. The reproducibility of the current peaks run on the same solution is about 4%. The values for the zinc ion reduction consistently showed the poorest reproducibility of the various ions tried. Two possible procedures can be used in correcting for the contribution of the residual current to the recorded polarogram. One is by recording a background current and subtracting the appropriate current value, and the other is by extrapolation of the base line preceding the polarographic reduction curves. In the concentration range greater than 5 X 10-6 either method can be used with equivalent accuracy. With more dilute solutions, the latter procedure was found to be desirable because of the greater sensitivity of the electrode to tracesof electroreducible substances.
ANALYTICAL CHEMISTRY
1694
Table I. Variation of Current Maximum with Concentration for Various Ions Using Mercury Pool Electrode Supporting electrolyte, 0.01 N KNOa
t = 250
c.
Scanning rate, 200 mv./minute irnsx/C, Ampere/Mole/Liter, When C Is: Av. Ion 5 X 10-6 10-6 5 X 10-8 IO-' WX./C 0 . 2 0 4 0.210 0.210 0.196 0.300 0 . 3 2 0 0,336 0.309 Cd++ 0.334 0.354 0.334 0.300 Cu++ 0.260 0.240 0 . 2 7 2 0.257 Zn++ 0.360 0 . 3 5 0 0 . 3 5 0 0.315 B i + +a 0.660 0.620 0.620 0.610 Supporting electrolyte 0.5 M HzSO4,
FLY+
yo Deviation Max. Av.
If the voltage on the pool is scanned from more negative to less negative values, the polarograms are different, as would be expected. Figure 4 shows a curve of this type. Similar phenomena are obtained in oscillographic polarography. A valley is obtained rather than a peak, caused by anodic dissolution of the metal which has been deposited a t the more negative voltages. The curve is approximately the same shape as that ob'tained from forward polarization but is inverted and reversed. No correlations of these current voltage curves with ion concentrations were attempted. SENSITIVITY OF MERCURY POOL ELECTRODE
As the purpose of this study was to extend the range of polarographic methods to more dilute solutions, a comparison of the sensitivity of the mercury pool electrode and the dropping mercury electrode is presented in Table 11.
ciable and the current peak is much greater. This allows a longer diffusion path to be set up, resulting in lower current density. By increasing the voltage scanning rate, the diffusion layer can be decreased and the peak currents increased. USEFULNESS AND CONSTANCY OF HALF-PEAK POTENTIALS
The fact that the half-wave potentials obtained with the dropping mercury electrode are independent of concentration is a valuable asset in identifying the substances being reduced. With the mercury pool electrode, it was found that the voltage a t which the current peak was obtained varied slightly, and in a roughly linear fashion, with concentration. However, with all metals except copper the half-peak potential (the potential of the wave a t one half the current maximum) was found to be independent of concentration over the range low4to 5 X M. The half-peak potential of copper varied from +0.03 to -0.01 volt vcrsus the saturated calomel electrode for a concentration change from 5 x 10-6 to 10-4 M. The half-peak potential agreed well with the half-wave potential obtained with the dropping mercury electrode in the same solution, as can be seen in Table 11. EFFECT OF PREVIOUSLY DISCHARGED METALS ON POLAROGRAMS
The effect of the previous discharge of lead ion upon the height of the zinc wave is illustrated in Table 111. Solutions of 8.3 X 10-6 M zinc ion in a supporting electrolyte of approximately 0.2 M potassium acetate containing varying amounts of lead,
tc
Table 11. Current Values and Half-Wave Potentials Obtained at Mercury Pool and Dropping Mercury Electrodes'
3 xI0"M
LEAD ION
Concentration, 10-4 .M
t = 25'
Ion T1+ Pb++ Cd++
cui:
E:.+
.+ tb
C.
Supporting electrolyte, 10-2 Af KSOa Scanning rate, 200 mv./minute Dropping Mercury Mercury Pool Cathode Electrode imax, pa. I/P peak, volts id, Ma. I/$ wave 0.82 -0.43 19.4 f 0.1 -0.46 -0.42 1.05 -0.38 31.0i0.1 -0.56 1.00 -0.61 30.0 f 0.0 -0.01 +0.03 0.92 25.7 f 1 -1.03 -0.99 0.87 31.5 f 0 . 5 -0.02 -0.01 1.60 61.0 i 2
ntz/st'/o = 2.63 a t zero applied potential. b Supporting electrolyte 0.5 M HzSO4.
These data list only a comparison of the magnitude of the currents obtained by both methods. What is probably more significant is the ratio of the measured currents to the residual current. In the case of M zinc solution, this ratio for the mercury pool is 24.0, while for the dropping mercury electrode it is only 2.3. M. The range of concentrations studied was 10-4 to 5 X However, more dilute solutions can be determined if the accuracy desired is less critical. Peaks can be identified to about 5 X lo-'
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-0 3
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i
-05 VOLTS
VS. SCE
Figure 5 . Effect of Gelatin on Lead Wave
Table 111. Determination of Zinc in Presence of Lead by Mercury Pool Electrode
M. Although the area of the mercury pool is about 75 times as large as the dropping mercury electrode used in these comparimns, the current values obtained are only 25 to 40 times greater, depending on the specific ion involved. This is probably caused by a thicker diffusion layer a t the mercury pool. With a dropping mercury electrode of 3 seconds' drop time, the diffusion layer is still comparatively thin. With the pool electrode, the time lag between the point a t which reduction becomes appre-
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0
Scan ing rate, 100 mv./minute
t
Concn. of Zn++, M x 10' 8.3 8.3 8.3 8.3 8.3
9
%o
c.
Supporting electrolyte, 0.2 M KOA. Concn. of Wave Height, pa. Pb++, M x 106 Zn++ Pb++ 0.00 0.00 1.90+ 0.05 1.82i 0.06 0.82f 0.02 4.17 1.80 0.05 1.72 3~ 0.02 8.33 4.46 + 0.02 16.6 2.00 f 0.05 166 1.60 0.02 54.5
*
+
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3
1695 the similarity of both processes, an attempt was made to apply the equations developed for oscillographic polarography to the mercury pool cathode. Randles (6) has derived an expression for the height of an oscillographic recorded polarogram for an ion-amalgam reversible reduction. This derivation was based on the diffusion of ions to a plane surface and adapted to the dropping mercury electrode, where the surface area is expressed as 7 4 3 t 2 ' 3 . If area is resubstituted for the functions involving the dropping mercury electrode, the espression is:
1
in which F is the faraday, A is the aiea in square centimeters, n is the number of electrons involved in the reduction of the ion, V is the voltage scanning rate in volts per wcond, D is the diffusion coefficient, and C is the concentration in moles per liter. Combining constants, the equation becomes:
,i = 1922/2 A ~ L ~ ' Z V ~ / ~ D ~ / ~ C ( 2 ) Gevcik (8) has also derived an expression for the wave height in oscillographic polarography: imax = 0.361nFACD1l2 -
0
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I
-0 I
-0 3
I
VOLTS YS.
Figure 6.
1
-05
t
l'*
(3)
where R and T have their usual significance and C is expressed in moles per milliliter. iigain on combining constants and dividing through by 1000 so that concentration may be expressed in moleq per liter, the equation becomes:
-07
i,,
SCE
Influence of Scanning Rate on Shape of Polamgrams
= 217
A
(4)
where
A were analyzed for both lead and zinc content. The scanning rate was 100 mv. per minute. .4s can be seen, a lead content of one half to twice as much zinc had little effect on the wave height of the zinc. As the lead concentration increased to 20 times the zinc concentration, the accuracy involved in measuring the small greatly compensated zinc wave decreased. INFLUENCE O F SURFACE ACTIVE AGENTS
The effect of gelatin upon the lead wave was also studied. The supporting electrolyte was 0.2 Af potassium acetate and the polarization rate 100 mv. per minute. Maxima are usually observed in the automatic recording of polarograms using stationary solid electrodes. This type of maxima is not removed by the addition of surface active agents ( 7 ) . With the polarized pool electrode the lead maximum could be greatly reduced by the addition of gelatin, but was not entirely eliminated by the amount of gelatin used. The half-peak potential was not affected. An illustration of the behavior of the lead wave with and without gelatin is Phown in Figure 5 . THEORETICAL ASPECTS
The polarograms obtained a t the mercury pool cathode are similar to the type which are found by oscillographic procedures employing the dropping mercury electrode. The electrolytic process involved in both cases is fundamentally the same. Using oscillographic techniques the polarizing voltage is scanned across the drop so rapidly that the electrode can be considered t o be stationary. The only difference seems to be the rate a t which the potential change is applied. Current peaks are obtained in both cases as the result of the reduction of ions near the surface of the electrode, the establishment of a diffusion layer, and the subsequent thickening of this layer with time. The increase in the length of the diffusion path accounts for the decrease in the reduction current, resulting in a current maximum. Because of
= electrode area, square centimeters n = number of electrons involved in reduction v = rate of voltage scanning, volts per second D = diffusion coefficient of ion, square centimeters per second C = concentration, moles per liter
.4-Delahay ( 3 )has pointed out, the equations are then identical except for the value of the numerical constant. Both Randles and Delahay have tested their data from oscillographic methods against the Randles equation. Randles obtained excellent agreement for thallous, lead, and cadmium ion, poorer agreement for manganous and zinc ion. Delahay obtained good agreement for thallous ion, poorer agreement for lead and cadmium. However, he used much faster voltage scanning rates in his work. He concluded from these results and from the departure from linearity of inaxmith square root of the scanning rate, that the cadmium reduction is not truly reversible, a fact apparent only through oscillographic techniques. The effert of each of the factors in the above equation on the magnitude of the current peak has been evaluated. The contribution of the diffusion roefficient and the number of electrons transferred cannot be checked separately. However, the value of i m s x / D 1 should /2 he in the ratio 1:2.83:5.20 for ions having electron changes of 1, 2, and 3, respectively. If the value for imar/D1lzis divided by the 3/2 power of the electron change, a constant value should be obtained for all ions. provided that the concentration, electrode area, and scanning rate are not changed. The values of the diffusion coefficients recorded for ions a t infinite dilution are not particularly applicable to this work. Since the diffusion coefficient is related to the ionic species present, its value was determined, under the same conditions in which they were used, by polarographic procedures. The corrected form of the Ilkovii: equation ( 4 ) was used to calculate these coefficients. The study of the effect of rate of polarization upon wave height and curve shape is illustrated in Figure 6. The data are shown in Table IV for a solution 10-6 M in lead ion. The proportionality
ANALYTICAL CHEMISTRY
1696 Table IV. Variation of Wave Height with Scanning Rate Ion 10-8 M P b + +
1 -'25" C.
Supporting electrolyte, 0.01 M KNOi imu
Saanning rate, Volts/Min.
imar, @.
0.1 0.2
2.50 3.60
0.4
4.90 9.00
1.2
0.0408 0.0577 0.0816
61.2
60.6
60.0
0.141 Av.
83.6 61.3 f 1.1
one-electron reductions. The height of the maxima is, of course, also dependent upon the diffusion coefficient if other factors are the same. The cadmium polarogram is typical of the two-electron reductions but zinc and copper show somewhat broadened peaks. Indium failed to give a wave similar to that of bismuth, but instead showed a wave flattened at the top and without a sharp peak. The failure of this ion to follow normal behavior may be caused by the slow dissociation of complex chloride ions limiting the rate of electrode reaction. For this reason the data for indium have not been included in this paper, as its behavior does not appear to be at all predictable by the equations under consideration. A summary of id/cm*~'t"' values, diffusion coefficients, and the quotient
me are given in Table V.
the values for
Lax
s2
It can be seen that
are approximately constant.
The average
value is 3.56 X 103 with a mean deviation of about 5%. The dif,fusion coefficients given here can be considered to apply only to the cases under study. From the above data a proportionality constant was calculated corresponding to the value of 19243 of the -CLLCULATE D Randles equation (6) and 217 for 0 EXFLRIYENTAL the Sevcik equation (8). The values for each of the variables was aut+ stituted in the equation t
1
1
.04
1
1
1
08
.06
1
1
/
.IO
1
.I2
1
1
.I4
(VOLTS/SECOND)~
Figure 7. Relation of Current to Square Root of Rate of Polarization
The calculated constant, k, derived for each ion, is shown in Table V. The average value of 218 is in good agreement with that derived by Sevcik. In the cases of zinc and copper, which show the largest deviations, the reaction does not appear to be reversible from the s h a p e of t h e polarograms. A comparison of peak current heights, predicted by the Sevcik equation, and those obtained experimentally, can be seen in Figure 8.
of peak height us. scanning rate for other ions is shown in Figure 7. I n agreement with the work of Rogers et al. (7) on polarography using solid platinum electrodes, the height of the maxima -5 -5 -4 was found to increase with increasing rate of 10 51 10 10 polarization. The half-peak potential of the Figure 8. Comparison of Experilead reduction was independent of scanning mental Values and Those Calcu0.01 volt. This fact rate, remaining a t -0.42 lated from Sevcik Equation also agrees with the work of Rogers, who found the half-peak potential to be independent of ACKROWLEDGMENT scanning rate for platinum electrodes. If the relative magnitude of the residual current can be kept small, the relative increase in The authors are grateful for the support of the United States peak height with more rapid scanning rates will obviously Air Force under Contract No. AF18 (600)-485 monitored by the heighten the sensitivity. Office of Scientific Research, Air Research and Development Ions which undergo three-electron reductions such as bismuth Command. show higher, sharper peaks than those which undergo two- or LITERATURE CITED
Table V.
Effect of Diffusion Coefficient and Electron Change on Peak Heights
~
Ion
3.12 2.57 X 10-6 1.06 3.99 0.96 3.80 0.80 3.50 0.71 3.31 1.09 6.08 Average value of k = 218 i 16.
TI + Pb++ Cd + + cu++ Zn++ Bi+++ a
id
Cd/atl/O, DME
Canon., 10-4 M t = 25' C. Diffusion Coe5cient Calcd. from DME
ilMr
Dl/¶+/,
3.82 x 10-1 3.38 3.43 3.31 3.88 3.55
(1) Arthur P., Maness, R. F., Komyathy, J., and Vaughn, H., ANAL. CHEM.,23,1891 (1951). (2) Cooke, W.D.,Ibid., 25,215 (1953). (3) Delahay, P.J., J. Phus. and Colloid C h a . , 53, 1279 (1949);54.. 630 (1950). (4) Kolthoff, I. M., and Lingane, J. J., "Polarography," 2nd ed., p. 70,New York, Interscience Publishers, 1952. (5) Lee, T.S., J. Am. C h a . Soc., 74, 5001 (1952). (6) Randles, J. E.B., Analyst, 72,301 (1947). (7) Rogers, L. B., Miller, H. H., Goodrich, R. B., and Stehney, A. F., ANAL.CEEM.,21,777 (1949). (8) Sevcik, A., Collection Czechoslov. Chem. Commun., 13,349 (1948). (9) Zlotowski, I., Rocmiki Chem., 14, 640 (1934).
RECEIVED for review April 10, 1953. Aocepted August 13, 1953. Presented a t the Pittsburgh Conference on Analytical Chemietry and Applied Spectroscopy, Pittsburgh, Pa., March 2 to 6,1953.
