828
ANALYTICAL CHEMISTRY
Electrode Reaction. It seems probable that the electrode reaction involves reduction of the peroxides to the corresponding alcohols as follows: ROOH
+ 2 H T + 2e
+
ROH
+ H20
A calculation of n, the number of electrons involved in the reduction of 1-butyl hydroperoxide from aqueous solutions was made by means of the IlkoviE equation. This yielded a value of approximately 2. I n this calculation a value of 0.77 X cm.* see.-' for the diffusion coefficient of the peroxide was used. This is the value found in the literature for n-butyl alcohol ( 3 ) and it seems probable that the diffusion coefficient for the corresponding peroxide should be rather close to this. With this approximation n Tvas found to be equal to 1.9. ACKNOW LEDGM EYT
The authors wish to thank Harry S. Mosher and Homer Williams who supplied some of the compounds used in this work. They also wish to thank California Research Corp., whose financial support made part of this work possible.
LITERATURE CITED
(1) Bruschweiler, H., Minkoff, G. J.. -4nal. Chim. Acta 12, 186 (1955). (2) Dobrimskaya, .I.A., Xieman, AI. B., Bcta Physicochim. r.R.S.8. 10, 297 (1939). (3) "International Critical Tables," vol. 5, p. 70, ~\lcGraw-Hill, Xew York, 1929. (4) Lingane, J. J., Laitinen, H. d.,IXD. ESG. CHEM.,ASAL. ED. 11, 504 (1939). ( 5 ) AIacNevin, W.lI.,Urone, P. F.. .ISAL. CHEM.25, 1760 (1953). (6) Xieman, 11. B., Gerber, AI. I., Zhur. Anal. Khim. 1, 211 (1946). (7) Roberts, E. R., Meek, J. S., Analyst 77,43 (1952). (8) Shreve, 0. D., Rlarkham, E. C., J . Am. Chem. SOC.71, 2993 (1949). (9) Shtern, V., Pollyak. S., Acta Physicochim. l'.R.S.S. 11, 797 (1939); J . Gen. Chem., U.S.S.R. 10,21 (1940). (10) Williams, H. R., Masher. H. S., J . Am. Chem. SOC.75, 2984, 2987, 3495 (1954). (11) Willits, C., Ricciuti. C., Knight, H. B., Swern, D., AXAL.C H E Y . 24, 785 (1952). RECEIVED for review October 12, 1955. Accepted January 20, 1956. Division of rinalytical Chemistry, 128th Meeting, ACS, Minneapolis, Rlinn., September 1955.
Voltammetry at Solid Electrodes Anodic Polarography of the Phenylenediamines RONALD E. PARKER' and RALPH N. ADAMS2 Department o f Chemistry, Princeton University, Princeton,
The anodic polarography of the phenylenediamines at a rotating platinum electrode has been examined using a current-scanning technique. 3Ieasurements were made of the variation of Ei/%w-ith pH, and from these data the pK, values of reduced and oxidized species were determined. In all cases examined, linear limiting current 2's. concentration hehaFior was obser, ed. The quantitative determination of 0- and p-phenj lenediamine can be accomplished with a reproducibility of 2 to 394 in the concentration range from to 10-5M, but analysis of mixtures of the two cannot be done except in favorable cases. This study was undertaken to illustrate a potentially valuable adjunct of solid electrode polarography-the measurement of formal potentials of labile organic redox sjstems which cannot be determined by classical potentiometric techniques.
T
H E solid electrode polarography of a variety of organic compounds has been investigated to date. The majority of these studies have concentrated on quantitative analysis by means of limiting current measurements. The important variation of Ell2with p H that accompanies organic polarographic reactions has received lees emphasis. Hedenburg and Freiser examined the mechanism and the dependence of Eli2 us. p H for the oxidation of phenol at a stationary platinum electrode (9). A comprehensive study of the oxidation of phenolic compounds a t graphite electrodes by Gaylor, Elving, and Conrad includes some data on the behavior of E I 2us. pH for phenol, hydroquinone, and 2,4-dirnethyl-G-tert-butylphenol( 8 ) . The \vork reported herein is mainly concerned with the evaluation of dissociation constants of the electroactive species from E , 2's. p H curves taken a t the rotating platinum electrode Although the dropping mercury electrode has been widely used Present address, Johns Hopkins Medical School, Baltimore. J I d . Present address, Department of Chemistry, University of Kansas, Lawrence, Kan. 2
N. J. for this purpose, similar studies with solid electrodes have apparently not, been made. 911 of the present work was done with the rotating platinum electrode using the current-scanning technique previously described ( I , 6 ) . Similar results can be secured using conventional voltage-scan polarography, but it is believed that more reproducible results were obtained with less effort using t,he new technique. 0- and p-phenylenediamine were chosen for study because the diamine-diimine couple represents a highly unstable organic redox system. Comparatively little work has been reported on the anodic oxidation of phenylenediamines a t solid electrodes. I n a comparison of gold, graphik, and platinum electrodes, Lord and Rogers briefly investigated the oxidation naves of 0-, m-) and p-phenylenediamine (11). More attention has recently been given the substituted p-phenylenediamines which are useful as antioxidants and photographic developers. A summary of half-wave potentials of some 50 p-amino-S-dialkylanilines obtained by automatic recording a t a stationary platinum electrode has been given ( 2 , IO). Gaylor, Conrad, and Lander1 recently described a wax-impregnated graphite electrode for the determination of antioxidants of the p-phenylenediamine class (7). EXPERIMENTAL
Reagents. Samples of 0- and p-phenylenediamine were Eastman White Label, used without further purification because it was desired to work with practical samples. Stock solutions (0.01M) for polarographic work Lwre prepared by dissolving the required weight of compound in the minimal quantity of either 1.11 hydrochloric or 1,M acetic acid (for buffer studies) and diluting to volume with air-free distilled water. These solutions were prepared fresh each day to minimize the effect of air oxidation. Britton and Robinson buffer solutions were used for the p H studies and were prepared according to the directions given by hliiller ( 1 5 ) . The stock buffer solution prepared was 0.0411.1 in phosphoric, boric, and acetic acids, respectively. Varying volumes of this solution were mixed with 0.2M sodium hydroxide to prepare the desired buffers. The exact, p H was measured at the end of each polarographic run using a Leeds I%Xorthrup p H meter, which was standardized prior to each of these p H deter-
829
V O L U M E 2 8 , N O . 5, M A Y 1 9 5 6
minations by checking against a saturated potassium hydrogen tartrate solution (pH 3.56). Citrate-phosphate buffers were examined but found to be unsatisfactory because the limiting current plateaus of the phenylenediamines were poorly defined in this medium. Stock solutions of hydrochloric, sulfuric, and perchloric acids were prepared by proper dilution of the concentrated C.P. acids. Electrodes. An 18-gage platinum xire, sealed in soft glass and extending about 2 cm. vertically downward, was used as the rotating electrode. A Sargent synchronous rotator (600 r.p.m.) was used for all the work. Two other electrodes completed the aseembly. One, a lead amalgam-lead sulfate half-cell, functioned as the shielded cathode ( 1 ) ; the other, a saturated calomel electrode, was used only as the reference electrode. I n all cases, the platinum electrode cleaning procedure consisted merely of rotation for 2 to 5 minutes in 1 5 M nitric acid after each polarographic run, folloived by thorough rinsing with distilled water. The electrode was stored for overnight periods in distilled water and cleaned n-ith nitric acid before the first run.
polarographic current does not pass through the saturated calomel electrode, current-potential curves are obtained directly and do not have to be corrected for the usual iR drop. A 20-pf. electrolytic capacitor may be connected across the p H meter leads to decrease the voltage fluctuations in the vicinity of the limiting current plateau.
rn
1
CURRENT SOURCE
,J CELL
Figure 2.
Variable current circuit
IO0x10.
20CURRENT lY.1
30-
Figure
1. Apparatus for currentscanning polarography
A. C.
Xlicroammeter, Tripplett hlodel 726, 0 t o 50 pa. 20-pf. electrolytic capacitor
Ref.
Saturated calomel electrode half-cell Shielded cathode, Pb, Hg-PbSOd-lM H2S0,
40,
0 n
P. Polarographic cell Pf. Rotating platinum electrode S.
Polarographic Cell. A 50-ml. open beaker, centered in a 4-inch crystallizing dish, served as the polarographic vessel. Temperature control was achieved by keeping the outer dish filled with water a t 25" C. Provision was made to position the electrode and cell in a reproducible fashion for each polarographic run. Figure 1 indicates the complete apparatus. Current Source. A simple, one-tube current-scanning circuit wa8 used. Such a device is far more convenient than the battery and variable resistance combinations previously described ( 1 ). A single adjustment gives the required current sueep. The cir cuit is shown in Figure 2 ; it is recognized as a modified difference amplifier. The power supply consists of five 45-volt B batteries. T h e current drain is low enough to enable a single set of batteries to be used for over a year. The 6SL7 heaters are supplied from a small filament transformer. A current output from 0 to about 250 pa. can be supplied to the polarographic cell. The 5-K resistor in the plate circuit serves as a zero adjust and the current scanning is accomplished by means of the 50-K potentiometer which provides grid signal to the left half of the 6SL7. In actual practice, the zero current reading was taken m-ith one electrode lead disconnected, because the inexpensive microammeter used did not indicate true readings close to zero. The cell leads indicated in Figure 2 are connected to the rotating platinum electrode and the shielded cathode (Pb/PbSO,). Voltage Measurement. A Leeds Br S o r t h r u p Model 7664 p H meter was used to provide continuous indication of the polarographic voltage. The p H meter leads are attached to the rotating platinum and the saturated calomel electrode. Because the
12
10
I / E
Figure 3.
09
vs
S C E
( V I
0 8
+
0 7
0 6
OS
C
Anodic waves of p-phenylenediamine in 1M hydrochloric acid 1.
2. 3.
2 1 5
x x x
10-4.v1 lO-~.vf 10-6.M
T o measure the voltage, the zero current polarographic voltage -Le., solution redox potential-was recorded, then the current supply leads were connected and the voltage was observed in increasing current increments$usually 1- or 2-pa. steps. The data were plotted directly on standard graph paper. The limiting currents and half-wave potentials were taken from these' plots using conventional polarographic practice. I n general, the polarographic voltage indicated by the p H meter is steady immediatelv after each current increase. Only in the vicinity of the limiting current plateau is there any voltage lag (for oxidations, the voltage drifts to\vard more positive values). In this case it is best to allow 30 to 60 seconds for the equilibrium voltage to be attained. This sluggishness varies with the polarographic system, buffer conditions, and the like. However, the lag is not serious enough to interfere in any way with the reproducibility of results. I n practice, the rate of stepn-ise current increase may be as fast as the operator can read and record the values, eycept as noted above. RESULTS
Anodic Waves in Acid Solution. Figure 3 illustrates the osidation waves obtained for varying concentrations of p-phenylene-
ANALYTICAL CHEMISTRY
830 Table I.
Half-wave Potentials of p-Phenylenediamine in 1'74 Hydrochloric Acid El/*
Concentration,
.M
z x 1 x 5 x
(volt
U8.
1st wave
0.57 0.58
10-4 10-4 10-5
0.5s
6.C.E.) 2nd wave 0.72 0.73 0.74
10 -
OXlD 20
-
CURRENT U b )
30-
Essentially identical wives for 0- and p-phenylenediamine were obtained in lJ1 sulfuric and perchloric acids. However, the split m v e s are best defined in the hydrochloric acid medium. Reproducible results have been obtained over a long period using several different platinum electrodes. Considerable work was also done on the meta isomer. I n I M hydrochloric acid two waves are usually obtained. The El/* of the first wave is ahvays a t about +0.95 volt us. S.C.E., and appears to shift only slightly with increasing pH. However, much difficulty was experienced in reproducing the second wave. The reasons for this behavior are not known. N o further data for the meta isomei ale reported here. Anodic Waves of 0- and fl-Phenylenediamine in Buffer Media. Typical oiidation waves for the ortho and para isomers in Britton and Robinson buffers are shown in Figure 5 . I n general, the limiting current plateau for the para wave was better defined (flatter) than the ortho. Polarograms were taken over the p H range from l . i to 10 for each isomer. Figure 6 shows the variation of E, 2 with p H compiled from the polarographic data. The slopes of the individual portions of the curves are given.
40-
1.0
1.1 E
VS
1.0 0.0 S C E. I V . )
0.7
0.0
+
0.6
0.5
Figure 4. Anodic waves of o-phenylenediamine in 1.74 hydrochloric acid 1. 2
x
I O - 4 ~
2.
I
x
io-4,~
3.
X 2 X 16' Y
C
5
x ~o-~M
20 CURRENT
I#.)
30
diamine in 1M hydrochloric acid. The background oxidation 2 e ) is indicated as wave B. The para wave (2C1- + Cle isomer gives two waves of about equal height, although the separation is not complete. Table I indicates that the values of E112 for these waves are independent of concentration within the estimated reproducibility of 1 0 . 0 1 volt. Based on data discussed later in connection with the buffer studies, these split waves can be correlated Fvith single-electron oxidation stagesLe., semiquinone formation. The relative limiting currents for the individual waves are about one half of that obtained for the two-electron process in buffer mpdia. Such an interpretation is in accord with the stability of the semiquinone of p-phenylenediamine in acid solution found by Alichaelis, Schubert, and Granick (14). Further support of this view is found in the color of the solutions after the polarographic runs. These solutions were pale yellow, stable for several days in some cases. Indeed, oxidized solutions up to about p H 4 showed the same yellow color, while above this pH, the color after the polarographic run was pink. According to Xchaelis, Schubert, and Granick, the color of the semiquinone is yellow, while the fully oxidized diimine is pink ( 1 4 ) . Although only a small fraction of the sample is oxidized during a polarographic run, the intense absorption of these organic free radicals probably accounts for the visible color. S o quantitative data-were gathered on this point, but the existence of the yellow color only within the p H range of maximum semiquinone stability complements the polarographic evidence for the oneelectron oxidation wave. A factor \? hich contributes to the stability of the semiquinone is the presence of a large excess of the unoxidized diamine ( 1 4 ) . Such conditions are inherent in a polarographic reaction where only a small fraction of the bulk concentration is reacted. Figure 4 indicates that the ortho isomer is similar to the para in 131 hydrochloric acid, but the split into two waves is far less pronounced in this case. The color of the oxidized solutions indicates a change in the reaction a t about the same p H as for the para isomer. While much might be speculated on the poorer definition of the split waves for ortho, no real conclusions can be reached from the data of Figure 4.
+
ORTHO
-
omlo I O
09
08
E
07 "I S C E
OS
( " 1
+
06
04
03
02
c
Figure 5. Tj-pica1 oxidization waves in BrittonRobinson buffers
The shape of the curves for Ell2 us. p H can be interpreted in terms of the equations developed by Clark and others in their classical potentiometric studies of organic oxidation-reduction systems ( 3 ) . These equations, tvhich involve an expression for the sum of all the oxidant and reductant species in terms of hydrogen ion concentration, can be applied to polarographic practice with slight modifications (16). The following information can be derived from such a curve. Each inflection represents an acidic dissociation constant (pK:) of either an oxidized or reduced electroactive species. ,4n inflection followed by a steepening of the curve is due to the dissociation of an oxidized form. Similarly) if the curve is flattened with increasing pH, it is due to the dissociation of a reductant. For a two-electron oxidation process, each dissociation constant alters the slope by a factor of 0.03 volt per p H unit. If the p K of an oxidant form is equivalent or very close to that of the reductant, then no change or only a very slight bend is seen a t this point (3, 13). Only those dissociation constants which fall within the evperimental p H range and are modified by the oxidation-reduction process are detected. A complete interpretation of the curve usually involves correlation with the structural properties of the molecule and some p K data obtained from independent methods. T o evaluate the curves of Figure 6, it is first necessary to establish the number of electrons involved in the oxidation process a t the rotating platinum electrode. The slopes of the individual ortho and para waves (log i us. E plots) indicate a two-electron process. However, the irreversibility of the phenylenediamine oxidation leaves much to be desired in such a calculation. -4 further check, which is felt to be more reliable in this case,
V O L U M E 28, NO. 5, M A Y 1 9 5 6
831 Kr2 = second acidic dissociation constant of reductant (diamine)
K O = acidic dissociation constant of oxidant (diimine) 0 501
i+: 030
0 10
' 0 0 0
0 03.
Figure 6. Curves of E l / * cs. pH for p-phenylenediamine
0-
I n the curve for the para isomer (Figure 6), the first break indicates a pK: for the reductant of 3.0. This corresponds to the first dissociation exponent of p-phenylenediamine given as 2.7 and 2.8 (1'7). S o clean break is observed for the second dissociation of the reductant, where pK: = 6.2 (17), and therefore the effect must be counterbalanced by a dissociation of the oxidant. Thus, a pK: value of 6 can be assigned as the second dissociation exponent of the oxidant. The first dissociation of the oxidant is indicated by the 0.081 slope in the p H range from 1 to 3. Hence, an estimated value for this first oxidant dissociation exponent is < 1. This latter value could be more precisely established b y extending the curi'e to lower p H values, when the 0.081 slope should shift back to 0.06. However, the split waves noted in the earlier section interfere v ith half-wave potential measurement in this region.
and
Slopes of individual portions of curves: .4. 0 050 D. 0 . 0 8 1 B . 0 017 E . 0 060 C. 0 054
is indicated by the data of Figure 7. Here the oxidation waves for equimolar concentrations of the ortho and para isomers are compared with a n equivalent concentration of hydroquinone, all in the same buffer medium. The oxidation of hydroquinone at the platinum electrode is a well-established tiTo-electron process. The limiting currents for all three compounds are equal within 3 to 4%. These particular data were taken at p H 6.8; checks a t other p H values gave similar agreement. There appears t o be little doubt that the oxidation of 0- and p-phenylenediamine a t the rotating platinum electrode is a two-electron process in buffer media, corresponding to the diamine-diimine redox system. The two-stage univalent process (semiquinone formation) occurs only in a fairly acidic solution, as discussed earlier. Returning t o Figure 6, the interpretation of the curve of El,* LIS. p H for the ortho isomer can be given as follows. The first break occurs a t a p K i of about 4.5 and is due to the dissociation constant of a reductant form. This value checks well m-ith the recent values of 4.6 and 4.74 for the pK62 of o-phenylenediamine given by Vanderbelt, Henrich, and Vandern Berg, using electrometric titration and ultraviolet absorption methods, respectively ( 1 7 ) . The pKL1 of the ortho is about 0.6 ( 1 7 ) , and is outside the range of detection in the present method. The next break in the curve appears a t a p K i of 7.8 and is due to the dissociation constant of the oxidant (diimine). K O other dissociation constants are discernible over the p H range from 1 to 10. The slopes of the end portions of the curve (0.056 and 0.054 volt per p H unit) are as close to the theoretical value of 0 059 as can be expected. The middle portion should have a slope of 0.030 volt, whereas the experimental value is only 0.017. This is understandable from an examination of the individual oxidation waves in this p H region. Starting a t p H 6, the lyaves for the ortho isomer become more drawn out; this probably gives rise to a positive error in the E1/2 value. Hence, the slope of the curve of E1122s. p H in this region is less than the theoretical value. Oddly enough, beyond p H 8, the n v e s tend to become more reversible in shape (less d r a n n out). It appears that this particular p H range represents an unstable region for one or more of the electroactive species. The over-all curve for E, z's. p H is represented by the general equation:
where K,,
=
first acidic dissociation constant of reductant (diamine)
40
OXID
08
07
E
Figure 7.
05
06 VI
SCE
04
Iv)
03 02 +c
01
00
-4nodic waves of 0- and p-phenylenediamine and hydroquinone
The general shape of the curve for the para isomer checks well with potentiometric data of Michaelis and Hill on a variety of alkyl-substituted p-phenylenediamines whose p K values (reductant) correspond closely with the unsubstituted compound (13). The generalized expression for the curve of Eliz US. p H or the para isomer is:
An equivalent expression for potentiometric studies was first developed by Clark and others (4). Regarding the reliability of the p K data obtained by the present method, it can be seen that the values for the reductants compare 1% ell 1% ith the independent measurements. The instability of the unsubstituted diimines makes it impossible to obtain corresponding independent data for the oxidants. Attempts to apply the classical potentiometric methods to the unsubstituted phenylenediamines were unsuccessful because of this instability ( 4 , 1 2 ) . I n such a situation the polarographic method is of decided advantage. The value for the pK0 of the ortho isomer may be considered quite reliable. The values for the oxidant species of p-phenylenediamine was approximated as indicated. The entire curve of EL,*us. p H for each compound \%asrechecked three times over a 2-year period using a t least five different platinum electrodes. Consistent values for the apparent dissociation constants (within 0.5 p K unit) n ere obtained. Constant ionic strength was not maintained in the buffer solutions used. Elving and others have shown that ionic strength differences may considerably influence half-n ave potential measurements ( 5 ) . Such an effect might very well account for the scatter of points a t the high p H region in the curve for the para isomer (Figure 6 ) . An indication of the degree of correlation between the present
ANALYTICAL CHEMISTRY
832 methodand corresponding potentiometric studies is afforded by the data of Figure 8, which is a curve of El,t us. p H for o-tolidine. The potentiometric data were taken from the study of Clark, Cohen, and others (4),and recalculated t's. S.C.E. The more positive potentials in the l o a p H range can be attributed t o the irreversibility of the o-tolidine oxidation wave. I n this case, the polarographic method cannot be extended beyond a p H of about 6, because the waves are too d r a a n out for accurate E l / ?measurement. Even in this unfavorable case, the slope changes and hence the pK values check well with the potentiometric data.
Table 11.
Limiting Current cs. Concentration for p-Phen ylenediamine (Background, p H 7.7 buffer)
Concentration. Jf x 10-4 0.40 0.80 1.19 1.60 2.40
ihlll.,
pa.
semiquantitative results are obtainable. This is due in part t o the irreversibility of both oxidation waves in this pH region. CONCLUSIONS
X
POTENTIOMETRIC
0
POLAROQRAPHIC
The application of solid electrode polarographic techniques to the measurements of the formal potentials of a labile organic redox system such as the diamine-diimine syst,emhas been shown to give valuable information which cannot be obtained by potentiometric techniques. This technique has already yielded very interesting information on the oxidation of sulfa drugs (18). I n addition it has been shown that o- and p-phenylenediamine can be quantitatively determined a t the to 10-5Alf level from limiting current measurements a t the rotated platinum electrode. The analysis of mixtures is difficult and only semiquantitative results can be obtained.
DATA STUDY
0 .o
Figure 8. Potentiometric and polarographic data for o-tolidine
ACKNOWLEDGMENT
Quantitative Determination of Phenylenediamines at Rotated Platinum Electrode. Table I1 illustrates the linear relation between limiting current and concentration for p-phenylenediamine. The average reproducibility of these five measurements is 3.6%, however, 2 to 37, appears to be a more reliable estimate based on a larger number of determinations. Similar relations for both the ortho and para isomers can be obtained over the entire buffer range and in acidic solution. KO concentration study of the meta compound was undertaken. I n general, best results are obtained with the para isomer because the limiting current plateau is flatter than that of the ortho. Limiting currents for ortho and para were found t o be independent of pH within i 5 % (exclusive of the acidic region where two waves were obtained).
pH 7 7
40
I
0.8
0.7
0.6
I
0.5 0.4 0.3 E v. S C E I " . )
1
0.P
Figure 9. Polarograms of mixtures of p-phenylenediamine at pH 7.7
0-
01
0.0
and
From the data of Figure 6, it vias hoped that the analysis of mixtures of o- and p-phenylenediamine might be possible a t a p H of 7 to 8. The greatest separation of half-wave potentials occurs in this region. However, such a determination v a s only partially successful. Figure 9 shows the most favorable results obtained from a 1 to 1 mixture of 2 x 10-4M ortho and para isomers. K i t h other ratios, the waves tend to merge and only
The authors wish to express their appreciation t o R. Mansfield Clark for valuable discussions during the course of this work. Thanks are also due to John A. Strother for his aid in the design of the current-scanning circuitry, and t o Laura 11. 11eyer for her invaluable aid in preparing the manuscript. LITERATURE CITED
(1) ..idams, R. S . , Reilley, C. S . . Furman, S . H., AXAL.CHEM.25, 1160 (1953). (2) Bent, R . L., Dessloch. J. C., Duennebier, R. C., Fassett, D. W.. Glass, D. B., James, T. H., Juilan. D. B., Ruby, W.R., Snell. J. AI., Sterner, J. H.. Thirtle, J. R.. Yittum, P. W., U'eissJ . A m . Chem. SOC.73. 2100 (1951). bereer. -4.. (3) Clarkyi?'. hi., Cohen. B., others, Oxidation Reduction Studies I-X, Hyg. Lab., Bull. 151, 1-363. 1928. Ibid., IX. Elving, P. J., Komyathy, J. C., Van Atta, R. E., Tang, C., Rosenthal, I., XN.AI.CHEM.23, 1218 (1951). Furman, S . H., J . Electrochem. SOC.101, 19c (1954). Gavlor. V. F.. Conrad. .i.L.. Landerl. J. H.. Abstracts. Pittsburgh Conference on dnalytical Chemistry, IIarch 1955; A4N.AL. CHEM. 27, 310 (1955). Gaylor, V. F.. Elving, P. J , Conrad, .1. L., Ibid., 25, 1078 (1953). Hedenburg, J. F..Freiser. H.. Ibid.,25, 1356 (1953). R., J . Am. Chem. Soc. 72, 4719 (1950). Julian. D . R., Ruby, \I7. Lord, S. S.. Rogers, L. B., -&SAL. CHEW26, 284 (1954). Michaelis, L., Hill, E. S., J . A m . Chem. SOC.55, 1481 (1933). Michaelis, L., "Oxidation Reduction Potentials." J. B. Lippincott, Philadelphia, 1930. Michaelis, L., Schubert, AI. P., Granick, S., J . A m . Chem. SOC., 61, 1981 (1939). 3Iiiller, 0. H., "Polarographic Method of Analysis," Chemical Education Publishing Co., Easton, Pa., 1941 lIuller, 0. H., "Polarography." chap. 28 in Weissberger's "Physical llethods of Organic Chemistry," Part 11, Interscience, New York, 1949. Vanderbelt, J. RI., Henrich, C., Vanden Berg, S. G., ;Is.AL. CHEM. 26, 726 (1954). Voorhies. J. D., Adams, R. S . .private communication. RECEIVED for review Augnst 11, 1955, Accepted February 15, 1956. Presented in part a t the Pittsburgh Conference on .Analytical Chemistry, Pittsburgh, Pa., March 1955. Parts of t h e material contained herein were taken from t h e thesis submitted b y Ronald W. Parker t o the Chemistry Department, Princeton Unirersity, i n partial fulfillment of the requirements f o r the degree of bachelor of arts.
