nique are critical and vary with the systems under study. For satisfactory results, the optimum conditions-e.g., rate and direction of polarization, background electrolyte, temperature, electrode pretreatment, pH, etc.-must be found and then maintained throughout the study of the particular redox system involved.
( 4 ) Gaylor, V. F., Conrad, A. L., Landed, J. H., ANAL. CHEM.29, 224 (1957). ( 5 ) Ibid., p. 228. (6) Gaylor, V. F., Elving, P. J., Conrad, A. L., Ibid., 25, 1078 (1953). ( 7 ) Glass, J. R., U. S. Patent 2,732,335 11956). (Sl-Koiihoff, I. ,,M., Lingane, J . J.,
CONCLUSIONS
ACKNOWLEDGMENT
The graphite indicating electrode cannot be used rery profitably for voltanimetry in solutions containing mineral acids, since penetration of acid into the electrode causes poor reproducibility. Investigation of electrode processes in neutral or slightly acidic media can be successfully accomplished ; precision of nieasurernent of E l f 2and i, can be made equal to that obtained n i t h the platinum electrode. However, optimum experimental conditions and tech-
The authors wish to thank the Atomic Energy Commission, which helped support the work described. One of the authors (ilFK) wishes to thank the Standard Oil Co. of Ohio for the grant of a fellowhip.
1953. (10) Lord, S. S., Jr., Rogers, L. B., -43-4L.CHEM. 26, 284 (1954). ( 1 1 ) Lydersen, D., Acta Chem. Scand. 3, 259 (1949). ( 1 2 ) Michaelis, L., Schubert, bl. P., Granick, S., J . Am. Chem. SOC. 61, 1981 (1939). (13) Pa;ker, R. E., Adams, R. N., ANAL. CHEM.28, 828 (1956). (14) Skobets, E. M.,Atamenko, N. N., Zavodskaya Lab. 15, 1291 (1949). (151 Zilbershtein, K. I.. Xakarov, L. P., Ibid., 2 1 , 3 4 2 (1955). ’
true a t the graphite electrode. It seems likely that a small amount of resorcinol is oxidized t o a product which prevents further electrolytic reaction. A free radical mechanism, which is probable in the oxidation of resorcinol, could well result in a polymerization process producing such a film.
LITERATURE CITED
11) direv. L.. dnalust 72. 306 11947). ( Z j Cho$nyl( S. “G,, ‘Doklady Akad. SUU S.S.S.R. ~ 100, 495 (1955). ( 3 ) Elving, P. J , Olson, E. C., J . ;Zm. Chem. SOC. 79, 2797 (1957).
‘Polarography, 2nd ed., Interscience, New York, 1952. (9) Lingane, J. J., “Electroanalytical Chemistry,” Interscience, New York,
~
RECEIVED for review February 3, 1958. Accepted July 15, 1958.
Anodic Chronopotentiometry with a Graphite Electrode Analytica A p plications PHILIP J. ELVING and ALAN F. KRlVlS University of Michigan, Ann Arbor, Mich.
b Electrolysis at constant current (chronopotentiometry) using a graphite indicating electrode was investigated. Poor results were obtained with inorganic reductions; much better results with organic oxidations. Electrolysis of the three dihydroxybenzene isomers a t p H 5.5 results in oxidation of hydroquinone and catechol t o p- and obenzoquinone, respectively, and in destructive oxidation or postelectrolytic polymerization in the case of resorcinol. Binary and ternary mixtures of the dihydroxybenzene isomers can b e analyzed quantitatively; if both hydroquinone and catechol are present, the concentration of one has to b e determined separately. Anodic chronopotentiometry of the three isomeric phenylenediamines indicates the differing nature of the mechanisms involved. At pH 2, 5, and 1 1 , 0 - and p-phenylenediamine undergo a 2e oxidation, presumably to the diimine; at pH 5.5 the ortho isomer undergoes a subsequent l e process. The latter may be due to reaction of the diimine with o-phenylenediamine to produce diaminophenazine, which i s oxidized b y a le step to a free radical which then polymerizes. The meta isomer seems to undergo at pH 5.5 a 3e oxi-
1648
ANALYTICAL CHEMISTRY
dation to a final polymeric product and at pH 11.2 a 2e oxidation which leads either to an insoluble polymer or to destruction of the compound.
T
use of a graphite indicating eIectrode for voltamnietric (polarographic) studies has been evaluated (6-9, 11). The graphite electrode suffers from extreme sensitivity to experimental conditions; adherence to any set of experimental conditions has to be rigid t o produce useful information. Because this electrode possesses certain advantages, it seemed m-orth while to attempt to find a technique which nould be less critically dependent on a large number of variables. Consequently, its use was investigated for electrolysis at constant current (galvanostatic condition), which has become knovcn as chronopotentiometry XIhen applied to polarographic situations. Delahay (3) has discussed the theoretical and practical applications of chronopotentiometry. The chronopotentiometric technique involves application of a constant current to two polarizing electrodes immersed in the test solution and measurement of the potential of one of them against an auxiliary reference electrode as a function of HE
time. The resulting potential-time curves are then used to evaluate the electrolytic reaction. The transition time, 7 , defined as the time required to reach an equilibrium state in the electrolysis, is related to the diffusion coefficient, D, and concentration, C, of the actil-e species, and the current density, io,as follorrs: ~ 1 =1 ~ ~1 1 n 2 P C D1/2/2t0 (1) \\-here n is the number of electrons transferred per inolecule in the faradaic process and F is the faraday ( 2 ) . The relation betn een potential and time is
where is the potential at 7/4 (IO). Equation 2 has B form similar to that for the polarographic wave; in fact, EIl4is identical with for identical electrode material ( 3 ) . A plot of E 2;s. the log term should give a straight line whose elope is related to n (6). The relation betn-een transition times for consecutive reactions involving the same electroactive species is rz =
71
[Z(nz/ni)
+ (n~/ni)~I
(3)
nhere nl is the number of electrons in-
1l7v A C
4
ON-OFF SWITCH J
POWER SUPPLY
1
CONSTANT CURRENT CONVERTER
REFERENCE OMETER
STRIP CHART
AMPLI-
RECORDER
1
I
PRECISION RES ISTOR
I
--vvfvv
BUCKING POTENTIAL CIRCUIT POTENTIOMETER
LJ
S o electrolytic pretreatment of the graphite electrode surface was carried out; a fresh surface was obtained for each run, unless otherwise specified, by sawing (6). As only one type and size of electrode was used throughout the investigation, the geometric area of the electrolysis surface was essentially constant. Therefore, current densities varied directly with current flow in the electrolysis circuit. This permits simplification of Equation 1; rearranging and substituting i, t h e measured current, for io, the current density, give ~ l / ~ i= / CT
~
n ’F ~D112/2
(6)
For any particular reaction under constant conditions, the right side of Equation 6 will be constant; therefore, Equation 6 simplifies to:
l5v
Figure 1.
T ” ~ ~ /=C constant
Circuit for chronopotentiornetry
(7)
DISCUSSION OF RESULTS
volved in the first step of the reaction and n2 is the number in the second step ( 1 ) . K h e n two different species undergo consecutive reactions, the relation is more complicated ( 2 ) : (71
+
7?)l“
-
Tll”
7 9 ’ 2 FO”2 n*C2/2ic (4) Delahq- and Berzins ( 4 ) have developed the theoretical equations for a totally irreversible reaction; the one for the potential-time relation is RT nFCk RT In 11 E = __ In & +-
an ,F
1.0
an,F
(t/.)’2]
(5)
A plot of E US. log 11 - ( t / T ) ” * ] should give a straight line whose slope can be used to calculate on,. The heterogeneous rate constant, k h , for the electrode reaction can be determined from the potential a t time zero and the value for ana. EXPERIMENTAL
Reagents. Nitrogen used t o deoxygenate t h e test solutions n as purified a n d equilibrated b y passage through two alkaline pyrogallol scrubs a n d a portion of t h e test solution less the electroactire species. The organic coinpounds used a e r e Eastman Kodak white label grade. -411 other chemicals nere of reagent or C . P . grade. Apparatus. Grade U-F4 spectroscopic electrodes (United Carbon Products. Iiic.) were impregnated n i t h ceresin ixay (Fisher Scientific Co.) by immeision in t h e molten wax (SO’ C.) for 1 hour, removed from the molten n a y , allowed t o cool, and then coated with Seal-All cement (Allen Products Corp.) (6). The auxiliary polarizing electrode was a small piece of platinum n-ire sealed in glass. An ex:xnal silver-dver chloride electrode was used as the reference electrode; contact \vas made n i t h the test solution through either a potassium chloride or a potassium nitrate salt bridge. The essential instrumental arrangement is indicated in Figure 1. Thc output of a Hedett-Packard T12B
pon-er supply was controlled by a constant current converter (Electronic Research Associates, Inc., No. CC 60). The constant current was put through a series dropping resistor and then impressed on the polarizing electrode pair (graphite and platinum). The potential of the graphite electrode was measured 1%-itha Leeds & Sorthrup KO. 7664 p H meter, whose output was plotted as a function of time by a Bron-n strip-chart recorder (1-second full-scale response, 2-inch travel per minute). The precision resistor in parallel with the Leeds & Sorthrup Type K potentiometer was a Shallcross Type BX196 4300-ohm resistor; a Rubicon Model 3402” galvanometer was used as a null-detector for the potentiometer. The potentiometer in the bucking potential circuit was a 100-ohm General Radio Type 213. Procedure. T h e stock solution of electroactive species, ea. 0.05 to 0.1M and freshly prepared for each series of runs, was diluted n i t h the proper buffer or other background electrolyte solution to produce the desired concentrations a t final volume; the resulting solution was mixed, transferred to a 100-ml. beaker thermostated a t 25.0’ = 0.1’ C., and deoxygenated. (Although no oxygen ware appeared in the anodic potential range investigated, it seemed best to remove such a source of oxidizing- material from the test solutions.) All three electrodes mere dipped into the solution and the electrical leads attached. The bucking potential was adjusted to give the desired initial potential, a suitable dropping resistor (usually between 0.1 and 15 megohms) put into the circuit, the recorder chart drive brought up to full speed, and the chronopotentiogram started. After the recording was complete, but v hile elettrolysis was still in process, the potential drop across the precision resistor nas measured and used to calculate the current floning in the circuit. Values for 7 and Elil were obtained graphically from the recorded chronopotentiograms.
;Ilthough each redox system examined shon ed different behavior, one factor appears t o be common. The transition time function, ~ ~ /C,*ori ,the analogous i [ ( ~~ ~~) l ’ * ~ ~ 1 ’C, * ]is ~ not constant for a particular reaction and electrode under all conditions, n hen the graphite indicating electrode is used; the function seems to be critically dependent on variables invol\-ed. For example, a change in current level should charge T wfficiently to keep the transition tinie function constant; this is not the case for a graphite electrode. Apparently, there is a certain amount of “sluggishnesb” in the electrochemical balancing mechanism needed to keep the total function constant. Convection or nonlinear diffusion phenomena may possibly have caused the deviations from expected behavior. I n any event, it was found that if the transition time. of the runs to be compared differ by more than a factor of 2 . the product +%/C will probably not be a constant and should not be w e d to compare the runs. The optimum transition time range was from ca. 8 to 20 seconds; shorter times nere difficult to measure accurately and results a t lcnger times seemed to deviate from linear dependence on current and concentration of electroactive qpecieq.
+
INORGANIC CATIONS
Attempts to electroll ze 0.5 n i X silrer(I) in 0.1M potassium nitrate colution a t constant current nere not very succewful. A measurable nave n a s not produced on either anodizing or cathodizing a graphite electrode. A poorly qhaped nonreproducible TT ave n as obtained by anodizing the electrode a t 5.5 pa. for a fen- minutes, rei ersing the leads t o the polarizing electrode pair, and recording the resultant cathodic TI ave. Reduction of 0.5 and 1.0 ni3f 501~1VOL. 30, NO. 10, OCTOBER 1958
1649
tions of mercury(I1) in 0.1.44 potassium nitrate-0.001M nitric acid produced similar results. No wave was found on anodizing or cathodizing a fresh electrode and then recording the cathodic wave. A less poorly shaped anodic wave was obtained b y percathodization, reversing leads, and recording; and T n-ere not reproducible. Evidently, prepolarization, which m:~y form electroactive layers on the electrode surface, is necessary to obtain current-time curves for silver(1) and mercury(I1). The difficulty in reproducing prepolarization conditions and the poor reproducibility and appearance of the curves indicate that the graphite electrode would probably not be useful for chronopotentiometry of inorganic electrode reactions. Silver(I) and mercury(I1) gave satisfactory voltnmmetric curves with the graphite electrode (6). Table I.
ISOMERIC DIHYDROXYBENZENES
Voltammetry of the three isomeric dihydroxybenzenes at a graphite electrode has been described (6). I n the present study their chronopotentiometric behavior was examined in 0.5N acetate buffer solution of p H 5.5. Illustrative data for oxidative electrolysis of individual isomers are given in Table I. Resorcinol gave a very poor wave, which was not improved b y varying the current or the initial potential. Furthermore, the oxidation contaminated the electrode in that it was impossible to re-use the electrode surface, as was possible following the oxidation of hydroquinone or catechol. The latter two compounds showed almost identical behavior: values were the same and similarly shifted t o more positive values with concentration increase: transition times and, consequently,
Anodic Chronopotentiometry of Isomeric Dihydroxybenzenes at Graphite Electrode”
Compound H>-droquinone
Cat echol
Concn., 0.51 1.01
Applied i, pa. 5.6 16.9 47.9
0.127 If: 0.013 0.166 iz 0.004 0.194 f 0,001
0.20 0.50 0.99
5.6 16.9 47.9
0.123 0.006 0.162 0.006 0.187 If: 0.004
mM 0.20
El,4b>C,
T’.
*
1.oo 16.9 0.099 Resorcinol a I n 0 . 5 M acetate buffer at pH 5.5. b Mean and standard deviation of four values. c Vs. AglAgCl reference electrode. Table II.
r b , See. 31 f 2 . 2 23 f 1 . 0
71’2ilC 153 159 155 156 169 174 172 172 48
11 f 0 . 8
Av. 33 2 . 1 26 zk 2 . 4 13 & 0 . 3 Av. 8
*
Chronopotentiometry of Hydroquinone-ResorcinolMixtures at Graphite Electrode“
o.nn
1.n n b 1 00 1 00 1 00 1 00
o oi
0.099 0 100 0 377 0 438 0 462 0.385 0.377 0.323
KO wave T o wave No wavee
8 r
10 0 150 3 134 16 0 169 19 135 40 0.069 43 191 16 0.10 0.092 45 195 22 0.51 0.069 47 199 30 1.oo 1.01 a Tn 0.5M acetate buffer at pH 5.5. b Constant resorcinol series electrolyzed at 16.9 pa. c Hydroquinone probably failed t o appear because current level was too high for concentration involved. However, enough was present to condition electrode surface and t,o shift resorcinol wave to more positive potentials. d Constant hydroquinone series electrolyzed a t 29.0 pa. 0 11 0 22 0 55 1.OOd 1.00
-
Chronopotentiometry of Hydroquinone-Catechol Mixtures at Graphite Electrode”
Table 111.
Hydroquinone, mM 0.505 0,202 0.501 0.050 8
Catechol, mM
0.49G 0.496 0,496 0.049 0.198 0.495
Applied 2,
pa.
16.9 47.9 16.9 29.0 29.0 29.0
I n 0.5.V ncetntc buffcr at pH 5..5.
1650
ANALYTICAL CHEMISTRY
E114,
v.
0.169 0.210 0.162 0.162 0.105 0.100
7)
See. 137 12 47 15
23 15.5
+%, pa.
199 166 116 112 139 114
Figure 2. Anodic chronopotentiogram for binary mixture of hydroquinone and resorcinol in acetate buffer at pH 5.5
T1/2i/C values were slightly higher for catechol. Behavior of Binary Mixtures. Electrolysis of binary mixtures of t h e isomers showed interesting effects. Electrolysis of a solution containing hydroquinone or catechol as well as resorcinol shifted the resorcinol wave t o much more positive potentials and gave a better shaped wave (Figure 2). The hydroquinone values found in hydroquinone - resorcinol mixtures (Table 11) were as expected on the basis of its behavior when present alone. Because resorcinol gave the second of two consecutive waves, the proper transition time function (Equation 4) was plotted against resorcinol concentration (Figure 3); the result was not a straight line as expected but resembled an exUS. conponential plot. A plot of dt2 centration did, however, produce a straight line, indicating shorter transition times for resorcinol than expected. Unfortunately, lack of comparable data for resorcinol alone makes interpretation of this behavior impossible. The concentration range was limited t o a tenfold variation by the necessity for maintaining a constant current level throughout the series; beyond the concentration range used (0.1 t o 1 mM) transition times were either too short or too long for accurate measurement; results of electrolysis at different current levels were not comparable. The resorcinol oxidation wave is evidently influenced by the hydroquinone oxidation. For example, values of r1’*i/C, calculated for the resorcinol wave, varied linearly with the hydroquinone-resorcinol concentration ratio; the straight line nearly passes through the origin of the plot. Hydroquinone-catechol mixtures gave only one, apparently composite wave on the potential-time curves (Table 111). Values of n. Plots of log [ ( T ~ ‘ ~ ti”J)/t*’*] us. E , based on t h e waves obtained by oxidizing a mixture of hy-
droquinone and resorcinol (Figure 4), were straight lines of equal slope; n, calculated from the slope and Equation 2, was essentially equal to 1. Based on similar plots, n for hydroquinone was 0.59 and for catechol 0.60. These figures only emphasize that mixtures shorn wave slopes differing from those of the individual components. Actually, because the resorcinol wave is due to a destructive oxidation, it might be better described by Equation 5 for irreversible waves. However, plots based on Equation 5 for several resorcinol-hydroquinone mixtures showed distinct curvature for both waves. Electrode Reaction Mechanisms. Both hydroquinone and catechol seem t o undergo t h e same type of anodic reaction, which, in all probability, is a 2e oxidation to the corresponding quinone. Resorcinol differs markedly; it undergoes destructive oxidation, the oxidation products polymerize, or both reactions occur (6). I n the constant current experiments oxidation of resorcinol alone gave very poor waves. Furthermore, coating of the electrode, similar to the effect expected from a polymeric film, occurred whenever resorcinol was in the test solution. Oxidation of resorcinol probably goes through a free radical mechanism with the free radical, the oxidation product of the free radical, or both polymerizing t o form a nonconducting film on the available electrode surface. Addition of hydroquinone or catechol changed the resorcinol wave drastically: the shape was improved and Ey4 shifted to more positive potentials; however, the electrode was still coated, indicating a polymeria end product. The following probably occurs. Because hydroquinone and catechol are more easily oxidized than resorcinol ( 6 ) ,a redox equilibrium is set up a t the electrode surface more easily for them than for resorcinol. At potentials ivhere resorcinol is oxidized, it must compete with the reaction already going on; this might cause the rate of resorcinol oxidation and subsequent polymer formation to be sufficiently decreased t o permit further electrolytic reaction rather than the rapid formation of a nonconducting film. The further reaction might involve oxidation of other resorcinol molecules diffusing to the electrode or further oxidation of the species produced by the resorcinol oxidation itself. The final product, in any case, is polymeric, since the electrode is coated. Quantitative Analysis of Mixtures. Table I1 and the previous discussion illustrate t h e feasibility of analyzing hydroquinone-resorcinol and catechol-resorcinol mixtures b y anodic chronopotentiometry a t a graphite electrode. The appearance of only one wave on
Table IV.
Chronopotentiometric Analysis of Hydroquinone-Catechol Mixtures by Oxidation a t Graphite Electrode Hydroneviquinone, Catechol, ationd, mM mM 9 Bcb B H ~ Bc+Bx 70 0.505 0.496 166 85e 791 164 1.2 0.202 0.496 116 858 31, 116 0.0 0.501 0.049 112 110 98h 109 2.7 0.501 0.198 139 420 98h 140 0.7 0.050 0.495 114 106g lo* 116 1.8 Av. 1.3 * r 1 4 values for composite catechol-hydroquinone wave. * r1'2i/C values for catechol alone (at proper current level) multiplied by catechol concentration in mixture. c r1i2i/C values for hydroquinone alone (at proper current level) multiplied by hydroquinone concentration in mixture. Deviation = 100 [(Bc B f i ) - -4]/-4. r l i 2 i / C = 172. f r1i2i/C = 156. r 1 / 2 i / C= 213. T1%/C = 195. 5
+
05
IO
15
RESORCINOL CONCENTRATION,
mM
+
Figure 3. Variation of ( a ) [(TI ~2)''~ - T11/2]and (b) r2'l2with resorcinol concentration Anodic chronopotentiornetry of solutions containing 1.00mM hydroquinone and varying concentrations of resorcinol
electrolysis of hydroquinone-catechol mixtures results because Ell4 for the two species is almost identical. However, the transition time for the composite wave can be used to determine the concentration of one of the species if the concentration of the other is known or can be determined by another means-e.g., catechol in the presence of hydroquinone may be determined colorimetrically with ferrous sulfate (12, IS). When transition times are in the optimum range-e.g., 8 to 20 seconds+%/C is a constant for a particular reaction, current level, and electrode area. As it is readily possible to maintain the latter two uniform, such a constant may be obtained for the particular reacting species, permitting ready calculation of the concentration in subsequent electrolyses. Table IV shows the accuracy with which the concentration of either catechol or hydroquinone may be determined in mixtures of the two, where the relative concentration ratio varies over a hundred-fold range. I n this
POTENTIAL,
Y
Figure 4. Plots of log [ ( T ' / ~ - t'/?)/ t'/'] vs. E for dihydroxybenzenes a. Oxidation of hydroquinone n presence of resorcinol b. Oxidation of hydroquinone alone c. Oxidation of resorcinol in presence of hydroquinone
case, the individual constants for hydroquinone and catechol were multiplied by the known concentrations of each isomer; the sum of the resulting +i values was conipared to the esperimentally determined 7% for the composite rvave; the average deviation was 1.370. Because resorcinol, when catechol or hydroquinone is present, gives a second and separately measurable wave, whose 71'2 varies linearly with concentration, it should be possible to analyze a mixture containing all three isomers if the concentration of either hydroquinone or catechol is separately determined, ISOMERIC PHENYLENEDIAMINES
The oxidation of the three phenylenediamines, whose voltammetric behavior a t a graphite electrode has been described ( 6 ) , was investigated in three buffered solutions: moderately acidic (hydrochloric acid-potassium chloride, VOL. 30, N O . 10, OCTOBER
1958
1651
Table V.
Anodic Chronopotentiometry of Phenylenediamines a t Graphite Electrode
Applied Isomer Ortho Para
Ortho
Para
Neta
Concn., mM
Applied Eitc,
8,
v.
See. rl‘ai/C 0 01111 HC1 - 0 2 N KC1 pH 2 0 2.16 55.3 0.354 77 225 0.40 71 214 1.36 55.3 0.115 12 141 0.123 12 141 0.123 13 146 0,110 13 146 pa.
0 . 2 5 X KpHP04 1.002 55.3 0 501
29 0 29 0
0.100
6.0
0.488
29.0 14.3
0,097
6.0
0.488
P/Cl where P =
(rl
+
~
~
6
0 185 0 131 0 146
54 8 8
0.154 208 200 208 146 .O 062 0 054 0.085 0 0 0 0
0 085 0 100
29.0 14.3
0.099 0 146 )
ANALYTICAL CHEMISTRY
Isomer
Concn., mM
Ortho
0.976
0.488 0.098
0 092
; .35 9 I
c
35 4 4 4 4 15
136 ~ -
213 164 164 174 160 160 160 168 174 174 125 125 126 119 114
.
0.049
Para
0 493 0 099
0.049 Meta
1.010 0.508 0,101
-1 r11/2. / ~
p H 2.0), weakly acidic (acetate, p H 5.5), and moderately basic (phosphate, p H 11.2) (Table V). p H affects the oxidation of 0- and m-phenylenediamine to a greater extent than it does the para form. At p H 2 and 5 t h e para isomer is more readily oxidized than the ortho, except that at p H 5 the ortho has another wave of EII4comparable to those of the para and meta forms. At p H 11 El/*and ~1/*i/Cvalues for the three compounds are similar within experimental error; only the meta isomer caused contamination of the electrode surface. The one p-phenylenediamine wave which has essentially constant Eu4and T ~ % / C over the p H range, probably represents a 2e oxidation to the diimine (6). Although semiquinone formation was expected in the acidic solutions, no wave split appeared; therefore, it seems likely that under the conditions of the investigation either the electrode reaction involves a multiple electron step or the transfer of the second electron is fast enough to avoid a break in the current-time curve. At pH 2, a-phenylenediamine produces one wave; T ~ % / C values are distinctly higher than those for the para isomer, but valid conclusions cannot be drawn because the transition times were far beyond the optimum range. Coating of the electrode, evidenced by inability t o re-use the electrode] is indicative of a postelectrolytic reaction which did not occur with the para isomer. At p H 5 two separate waves
1652
7,
were found; application of Equation 3 shows that wave I1 involves one half the number of electrons that wave I d o e s i . e . , +i/C values for wave I were twice the size of the analogous - T11’2]i/cl for function, [ ( T ~ wave 11. Since r1%/C for wave I approximates that produced by the para form, which probably involves a 2e oxidation, the first ortho wave is probably also a 2e oxidation and the second ortho n a v e a l e process. A likely explanation is that a 2e oxidation t o o-phenylenediimine is follon-ed by reaction of this diimine with unoxidized o-phenylenediamine t o form diaminophenazine, which would be easily produced in acetate solution (14). Diaminophenazine could then be oxidized in a l e step to a free radical n.hich would polymerize to form the film coating the electrode. I t is difficult to formulate R le oxidation of the diimine itself. The ortho isomer gave only one wave at p H 11, whose chronopotentiometric values are very similar to those of the para isomer; therefore, o-phenylenediimine is probably formed. The electrode is not fouled, indicating that no polymerization occurs. At p H 5 m-phenylenediamine gave one poorly shaped wave, whose rl/*i/‘C values indicate a 3e oxidation, as they equal the total for both ortho isomer waves; the wave shapes and the electrode fouling indicate a polymeric end product. At pH 11 the wave seems to be a 2e step similar to the p-phenylenediamine oxidation; hon ever, be-
+
Ei14, 7, V. Sec. 0 . 5 M NaOAc - HOAc KIH5 . 5 I 0.162 7 55.3 I1 0.400 8 I O . 154 55.3 7 I1 0.392 8 I 0.154 29.0 8 I1 0.369 10 29.0 I O . 154 8 I1 0.377 10 I O . 115 7 6.0 110.292 9 6.0 IO. 123 8 I1 0.300 10 6.0 10.09 2 I1 0 . 2 6 2 6.0 2 10.10 2 I1 0.25 29.0 8 0.138 29.0 0.154 9 6.0 0.110 8 6.0 8 0.123 0,077 2 6.0 2 6.0 0.085 29.0 45 0.162 0.115 29.0 53 21 0.092 29.0 19.5 0.115 29.0 0.100 11.8 6 rn 0.123 11.8 2,
w.
TI/
2i/C
150
ioa
150
7Oa
168 84a 168 84= 164 80a
175 88.
174 74a 174 74= 166 166 174 174 173 173 192 210 262 252 286 310
cause coating of the electrode occurs, a destructive oxidation is likely. The compound was not studied at p H 2 because of lo^ solubility and the poor results in other media. ACKNOWLEDGMENT
The authors wish to thank the Atomic Energy Commission, which helped support the work described, and the Standard Oil Co. of Ohio for a fellowship given one of the authors
(AFK). LITERATURE CITED
( I ) Berzins, T., Delahay, P., J . -4m. Chem. SOC.75, 4205 (1953). (2) Butler, J. A. V., Armstrong, G., Trans. Faraday SOC.30, 1173 (1934).
(3) Delahay, P., “New Instrumental Methods in Electrochemistry,” Interscience, New York, 1954. (4) Delahay, P., Berzins, T., J . S m . Chem. SOC.75, 2486 (1953). (5) Delahay, P., Mattax, C. C., Ibid., 7 6 , 874 (1954). (6) Elving, P. J., Krivis, A . F., AXAL CHEM. 30, 1645 (1958). ( 7 ) Gaylor, V. F., Conrad, 9.L., Landerl, J. H.. Ibid.. 29. 224 (1957). (8) Zbid., p. 228. (9) Gavlor, V. F., Elving, P. J., Conrad, A. L:, Ibid., 25, 1078 (1953). (10) Karaoglanoff, Z., 2. E/ektrochem. 12, 5 (1906). (11) Lord, S. S.,Rogers, L. B., ANAL. CHEY. 26, 284 (1954). (12) Mitchell, C. A , , ilnalyst 48, 2 (1923). (13) Price, P. H., Ibid., 49, 361 (1924). (14) Sjdgwick,,?. V., “Organic Chemistry of 1itrogen, Oxford University Press, London, 1937. RECEIVEDfor review March 31, 1958. Accepted July 15, 1958.
