results. Because the hydrochloric acid released tends to reverse the reaction between hydroxylamine and ketones. it is necessary to neutralize most of the acid with one titration and the rest by a final titration. A 98% yield was obtained consistrntly.
ACKNOWLEDGMENT
The advisory assistance of C. F. Pickett, director of the laboratory, is acknowledged and appreciated. LITERATURE CITED
(1) Jacobs, hI. B., Scheflan, L., “Chemical
Analysis of Industrial Solvents,” x-01. VII, pp. 417, 422, Interscience, Sew York, 1953. (2) hlessinger, J , , J , sot, them. ~ ~ 18, d . 138 (1889). (3) Ripper, AT., Monatsh. 21, 1079 (1900). RECEIVED for review Februray G , 1958. Accepted 3Tay 27, 1958.
Voltammetric Studies with the Graphite Indicating Electrode PHILIP J. ELVING and ALAN F. KRlVlS Department of Chemistry, University o f Michigan, Ann Arbor, Mich.
In preparation for use of the graphite electrode in chronopotentiometry, automatically recorded current-potential curves obtained with it were further evaluated b y studying a variety of inorganic and organic reversible and irreversible systems. In mineral acid solutions penetration of the solution into wax-impregnated graphite resulted in poor reproducibility. Satisfactory results are obtained in slightly acidic and neutral media; alkaline media were not investigated. Experimental conditions must b e carefully controlled for good results. The reductions of silver(1) and mercury(l1) are comparable to those obtained with the platinum electrode; iron(ll1) gave poor results. Results with the quinone hydroquinone system were good, except that Eli2 for the oxidation and reduction processes did not coincide. Oxidation studies of the three isomeric phenylenediamines and the three isomeric dihydroxybenzenes a t p H 5.5 were satisfactory. The oand p-phenylenediamines undergo 2e oxidation, the latter showing semiquinone formation; the meta shows a le wave due apparently to the formation of a free radical which polymerizes. Hydroquinone and catachol show the same 2e oxidation; the meta isomer, resorcinol, apparently oxidizes to a free radical which polymerizes to a film that coats the electrode.
-
T
most Ll-idely used solid indicating microelectrodes in voltammetry have been t h e noble metals, mainly platinum (8). However, a large variety of other materials such as t h e coinage metals (11) and their amalgams ( I ) , tungsten (Z), and some alloys ( 7 ) have been investigated. The major defect of the solid indicating microelectrode is t h e influence of previous use upon t h e results obtained. I n some instances, HE
plating on the electrode surface during electrolysis changes its characteristics; in organic investigations. films may form on the electrode surface. To alleviate some of the difficulties observed with platinum electrodes, the properties of t h e graphite indicating electrode have been investigated in recent years (4-6, 10). A carbon rod has a unique advantage in that the usable electrode surface is readily renewed for each run-e.g., b y simple breaking off of the end of the rod. The graphite electrode was found to be a poor substitute for either platinum or gold in t h e case of some inorganic reactions; it was excellent for some organic systems. The results obtained were critically dependent on experimental technique and electrolysis conditions-e.g., rate and direction of polarization, and pretreatnient of the electrode. The high residual current usually produced mas found to be a function of the binder, or lack of binder. used in formulating the rod. Impregnation of the carbon rod with a waxy material decreased the residual current considerably but did not affect the faradaic current-Le., the current due t o oxidation or reduction of the species under study-to the same extent; the particular wax used controlled the il:ir ratio to a large extent, and also affected Ell2 and il. Pretreatment of the electrode by electrolysis had a large, but not completely consistent effect on the curves obtained. I n view of these difficulties n-ith the graphite electrode, it was decided to investigate the use of the graphite indicating electrode for chronopotentiometric studies and, in particular, for analysis by the latter technique. However, further information on the graphite electrode when used under standard voltammetric conditions seemed desirable. Consequently, a variety of types of electrode reactions which it was desired to investigate chronopo-
tentiometrically, n-ere studied---e.g.. reversible reactions where both oxidized and reduced species were soluble and vihere one species T T ~ Sinsoluble, and irreversible reactions. The availability of a larger store of information about the behavior of the graphite electrode would permit more valid conclusions to be drawn concerning the results obtained with any one electrode reaction. EXPERIMENTAL
Reagents. Cupferron (‘2. F. Smith Chemical Co.) n as purified by recrystallization from ethyl alcohol and Norite and from ethyl alcohol alone. All other organic chemicals were Eastman Kodak Co. white label grade and were used without further purification. Nitrogen used to deoxygenate the test solutions n-as purified by passage through two alkaline pyrogallol scrubs. A11 other inorganic chemicals were of reagent or C.P. grade and were used without further purification. Electrodes. Exploratory espeiinients indicated t h a t exceptionally pure graphite electrodes were needed. because even minute traces of contaminants caused noticeable polarographic waves. Therefore, Kational Carbon Co. Special Spectroscopic Grade electrodes (0.25-inch diameter) mere used. The rods were either coated Kith ceresin a a x (Fisher Scientific Co.) to insulate theni, or were soaked in molten ceresin wax a t 80’ C. for 1 hour, removed from the wax, allowed to cool thoroughly, and then coated with Seal--411 cement (Allen Products Corp.). Electrical contact 11-as made via a small mercury pool held in a short piece of rubber tubing on the upper end of the rod. Three varieties of external reference electrode and salt bridge were used. Reduction of silrer(1) and mercury(I1) was studied with a potassiuni nitrate salt bridge and either a saturated mercurous sulfate or a saturated calomel half cell. The other systems n-ere investigated 11ith a potassium chloride salt bridge and a silver-silver chloride
VOL. 30, NO.
10, OCTOBER 1958
1645
electrode. The potentials of these electrodes a t 25" C. us. S.H.E. are 0.242 volt for S.C.E., 0.64 volt for the sulfate electrode, and 0.197 volt for the silver-silver chloride electrode (9). Procedure. A test solution of the substance under study was prepared in the desired background electrolyte, a portion transferred to a 100-ml. beaker (thermostated at 25.0' =t0.1" C.) and purged with purified and equilibrated nitrogen, the graphite electrode and salt bridge (from the reference electrode) were dipped in the solution, and the polarogram mas recorded on a Leeds& Sorthrup Type E Electro-Chemograph. An auxiliary polarizer (Leeds & Northrup No. 7739) was switched into the circuit for rates of polarization slower than the standard 200 mv. per minute. ,4 fresh graphite surface was prepared before each run by sawing off the tip of the carbon rod with a fine-toothed saw. Poorer looking and less reproducible waves resulted from breaking off or grinding down the tip. The peak current, i,, was measured from the base line (obtained by polarographing the background solution) to the peak of the current maximum on the polarogram. Eii2 was taken as the potential corresponding to ip/2 and is reported us. the particular reference electrode used. The direction of polarization was toward either more positive potentials (polarization in the positive direction) or more negative values (polarization in the negative direction). Pretreatment, when used, consisted of a &minute application of a potential ivhich was approximately 75% of the final potential to be applied in that run, followed by application of the initial potential for 3 minutes to equilibrate the system. I n some instances a simplified cathodic pretreatment was used; this involved a 5-minute application of a potential of -0.5 volt and then immediate polarographing. The total resistance of the polarographic cell and electrodes was always 1000 ohms or less (measured with a General Radio Model 65OA impedance bridge). At the usual current levelse.g., 10 to 20 ba.-the iR drop is of the same order of magnitude as the measurement error; therefore, no correction for iR drop was made. INORGANIC SYSTEMS
Reduction of iron(II1) in 0.1M sulfuric acid solution, using either impregnated or unimpregnated graphite electrodes, did not produce results comparable in reproducibility to those possible n-ith a platinum electrode, at which the reduction is reported t o be irreversible (8). The results in 1.OM hydrochloric acid solution were somewhat better; precision was slightly improved, but was still too poor for analytical purposes. Each variety of electrode-i.e., impregnated and unimpregnated-not only produced different Ellz and i, values, but did not reproduce its own values 1646
ANALYTICAL CHEMISTRY
20
-
A
-
-
-
-
-
10-
rn 02
04
-
08
06
POTENTIAL, v
Figure 1.
Anodic polarograms
A. Isomeric phenylenediamines 6. Isomeric dihydroxybenzener (catechol, resorcinol, and hydroquinone) Letters on curves indicate positional isomers producing the wave. Experimental conditions are indicated in Table II
even on successive runs on the same stock solution; neither electrode was distinctly superior to the other. Pretreatment (cathodic) in the presence of iron(II1) had a very small ameliorative effect with respect to increasing reproducibility. Uranyl ion in sulfuric acid did not produce a noticeable wave on polarization in either positive or negative direction within the range of +1.0 to -0.5 volt. Reduction of mercury(I1) t o mercury(I) in 0.1M potassium nitrate-0.001M nitric acid solution was observed with an impregnated graphite electrode; the consequent reduction of mercury(1) to mercury(0) did not seem to occur within the potential range used (1.0 to -0.5 volt). Pretreatment in the presence of mercury(I1) produced the best results; Eliz (-0.13 volt us. saturated mercurous sulfate) had an average deviation of =tO.Ol volt; i, was essentially constant. The polarization rate had a profound effect, as the wave seemed to disappear at rates below 200 mv. per minute. The experiments reported and preliminary examination of the cerium(1V)cerium(II1) system indicate that voltammetry or polarography with a graphite electrode cannot be carried out with any accuracy in mineral acid solution, because of the poor reproducibility. Apparently, acids penetrate the graphite even when the latter is impregnated with a wax. A recent study of the effect of mineral acid solutions on spectroscopic electrodes (16) disclosed that ragged penetration into the electrode pores occurs, which is not prevented by impregnation of the graphite with waxes or polystyrene; the depth of penetration varied randomly with acid concentration, time, etc. Essentially neutral media can be used successfully, as indicated by the experiments involving silver(1) described
in the next subsection; the precision approaches that obtainable with a platinum electrode. No runs were made n-ith inorganic systems in alkaline medium. Reduction of Silver(1). Electiolpsis of 1mM silver(1) in 0.1M potassium nitrate solution showed satisfactory reproducibility. At 200 mv. per minute, Elizvalues had a n average deviation of 10.006 volt; i, was reproduced to *3%. At 50 my. per minute, Elizn-as reproducible t o 3=0.004 volt and i, to *2%. At 20 mv. per minute, the wave became too rounded for accurate measurement. Runs were made a t different silver(1) concentrations with pretreatment in the presence and absence of silver ion. Pretreatment in the absence of silrer(1) shifted Eliz to more negative potentials and lowered i,; over a tenfold increase in silver concentration Eliz increased by 0.06i volt (theoretical shift is 0.059 volt) and i, deviated from i, = ICC by 5%. The curves obtained following pretreatment with silver(1) in solution had a better shape; a tenfold increase in silver(1) concentration increased 0.052 volt; i, showed an average deviation of 2.7% from a constant i,/C ratio. The latter deviation is within the range encountered lvith the platinum electrode. I n order t o increase i,, stirring of the solution during polarization \vas attempted, This increased the current two- or threefold. but the reproducibility mas poor, ORGANIC SYSTEMS
The major potential field of application of the graphite indicating electrode in voltammetry would seem t o be in the oxidation of organic compounds where the advantage of a reproducible, renewable surface would distinctly favor graphite over platinum.
I n the present investigation the wellknown quinhydrone system, the organic reagent, cupferron, n hose polarography has been thoroughly investigated ( 3 ) , and two sets of isomeric benzene derivatives were studied. The potentials reported for these compounds are us. the Ag/AgCl reference electrode. Pretreatment a t the initial potential alone gave nonreproducible results. Pretreatment near or a t the plateau of the wave gave better results; consequently, the latter technique m s used in all runs involving organic compounds. Quinone
- Hydroquinone
System.
This system was studied in 0.5M acet a t e buffer at p H 5.45 using a n inipregnated graphite electrode a n d pretreatment in t h e presence of t h e particular electroactive species concerned. Reduction of quinone by polarization in a negative direction from 1.0 to -0.5 volt produced a reversible-looking cathodic w v e (E1,*= 0.03 volt) ; polarization in the positive direction (-0.5 to 1.0 volt gave a poor-looking wave (E1/*= -0.02 volt; i, reduced by about 20%). Hydroquinone produced a good-looking anodic wave (EIIP= 0.302 volt) on polarization in the positive direction from -0.5 to 1.0 volt and a n irrevers= 0.26 volt) ible-looking wave (Ei/* on negative polarization; i, was essentially constant for both directions of polarization. Polarization in either direction produced two waves for quinhydrone, one of n hich corresponded to the reduction of quinone (EllP = -0.02 volt) and the other t o oxidation of hydroquinone (EllP = 0.30 volt); attempts t o fuse the tn-o waves by changing the geometric characteristics of the electrode surface (sandpapering) or by varying the initial potentials n-ere unsuccessful. Equal heights in both rvaves Ivere observed on positive polarization; the hydroquinone oxidation wave was one half the height of the quinone reduction wave when polarization in the negative direction was used. Cupferron. Electrolysis of cupferron in 1.8X sulfuric acid solution produced no anodic wares a n d one cathodic wave Tyhich was considerably more positive in potential than that obtained with the dropping mercury electrode (S). The extremely poor reproducibility encountered can be partially attributed to the oxidation of cupferron by residual oxygen in the solution (it is difficult to remove the last traces of oxygen from the 10% sulfuric solution). Three anodic waves were obtained in acetate buffer at pII 5.5 (Table I). Honever, only two of the waves appeared on polarization in any one direction. The central ware (E1 = 0.41 volt), which appeared on polariz-
ation in either direction, seemed to be approximately one half the height of = -0.12 volt) present the wave only on polarization in the negative direction and slightly less than one half the height of the most positive wave ( E l l z = 0.79 volt), which appeared only on positive polarization. The height of each wave for a given direction of polarization varied fairly linearly with concentration. S o cathodic wave byas observed in the acetate buffered solution. Phenylenediamines. The oxidation of t h e three phenylenediamine isomers 17-as investigated in pH 4.5 acetate buffer (Table 11, Figure 1,A). Preliminary experiments demonstrated that polarization in the negative direction Ivould not produce a wave; therefore, only positive polarization was used. El,z values follow the trends expected in such a series ( I S ) : the para isomer is most easily oxidized and the meta most difficultly. Both ortho and meta forms show only one wave, with the meta wave height being one half that of the ortho isomer; the para isomer produces two waves, each of which is approximately half the height of the ortho n-ave. The wave split observed in the oxidation of p-phenylenediamine is evidence for stepwise oxidation and semiquinone formation. The semiquinone of pphenylenediamine has a reported stability ( l a ) ,which would account for the separate l e waves. Parker and Adams ( I S ) found for the polarographic oxidation of the phenylenediamines a t a platinum electrode an over-all 2e
oxidation for the ortho and para forms; semiquinone formation and consequent wave splits occurred in acidic solution. I n the present investigation, waves for the ortho and para isomers have approximately equal total heights, indicating that a 2e oxidation to the diimine probably orcurs. As the meta compound wave is approximately one half of this height, it seems safe to assume that m-phenylenediamine undergoes a l e oxidation at the graphite electrode in acetate buffer. I n all probability, a free radical is formed 11-hich subsequently polymerizes; difficulties previously reported (13) for the oxidation of nz-phenylenediamine ma! be due to such polymeric reactions. Dihydroxybenzenes. The thiee dihydroxybenzene isomers n ere studied in t h e same manner as t h e phenylenediamines (Table 11, Figure 1,B). values shoir- the same trend, hvdroquinone being most easilj oxidized and resorcinol most difficultly. For coniparable concentrations of the different isomers, the resorcinol n ave i q only about one tenth the height of the other two waves, n-hich are appro\imatelgequal. Anodic voltaminetry of the dihydroxybenzenes consequently indicates that both hydroquinone and catechol undergo the same type of oxidation (probably a 2e processj. Resorcinol, on the other hand, seems to undergo a different type of oxidation; the current is very much less than expected. Resorcinol has been reported to be destructively oxidized a t the platinum electrode (I4); the same is probably
Table 1.
Voltammetric Oxidation of Cupferron a t an Impregnated Graphite Electrode" Kave C Wave -1 Kave B CupDirection ~. ferron, of Polar-El?,* i, Eli?, b. Eiii, ZP, V. Pa. v. Pa. v. w. mM ization 0 1 Negative 011 0 9 0 42 0 5 S o wave 0 3 Negative 013 0 39 1 2 S o xave 2 1 0 9 Segative 012 0 40 3 6 KO wave 8 9 0 1 Positive No wave 0 42 1 3 080 27 0 3 Positive N o wave 0 12 3 3 081 8 1 0 9 Positive No wave 0 39 9 8 0 7i 21 0 a Polarization at 200 mv./min. in 0.5M acetate buffer (pH 5.5). b Vs. Ag/AgCl. ~
Table II.
Voltammetric Oxidation of Isomeric Phenylenediamines and Isomeric Dihydroxybenzenes a t an Impregnated Graphite Electrode"
Compound o-Phenylenediamine m-Pheny lenediamine p-Phenylenediamine
C, m31
0.95 1.01
1.02
Eii2,'
V.
0.354 0.723 10.197 I1 0.462 0 349 0 613
i,, pa. 13.6 7.4 7r . -5
Literature E , 2 Vn1ues.b 1'. 0 . (i39,c0 38d O.85Gc 0 . 5 4 0 , ~0 .27,d 0.257'
1 . 1
Catechol 0 90 16 1 0 320e Resorcinol 1 00 1 8 0 69ie Hydroquinone 1 20 0 234 22 7 0 415,c 0 223e Polarization in positive direction at 200 mv./min. in acetate buffer at pH 5.5. us. Ag/AgCI. Literature values, 2s. S.C.E., adjusted to dglilgC1 electrode. At a graphite electrode ( I O ) . 4 t rotating platinum electrode ( 1 3 ) . .4t impregnated graphite electrode ( 4 ) .
VOL. 30, NO. 10, OCTOBER 1958
1647
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.
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.,
ACKNOWLEDGMENT
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). ’
CONCLUSIONS
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. 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-
