LITERATURE CITED
(1) Banks, C. V., Carlson, A. B., Anal. Chim. Acla 7, 291 (1952). (2) Banerjea, D. K., Tripathi, K. K., ANAL.CHEM.32, 1196 (1960). (3) Betteridge, D., Fernando, Q., Freiser, H., Zbid., 35, 294 (1963). (4) Bolton, S., Ellin, R. I., J. Pharm. Sci. 51, 533 (1962). (5) Brady, 0. L., Goldstein, R. F., J. Chem. SOC.1926, 1918.
(6) Charles, R. G., Freiser, H., Anal. Chim. Acta 11, 101 (1954). (7) Corsini, A., Fernando, Q., Freiser, H., Znorg. Chem. 2,224 (1963). (8) Green, R. W., Freer, I. R., J. Phys. Chem. 65,2211 (1961). (9) Guntelberg, E., Z . Phys. Chem. 123, 199 (1926). (10) Job, P., Ann. Chim.9, 113 (1928). (11) Kirson, B., Bull. SOC.Chem. France 1962, 1030. (12) Mathes, W., Sauermilch, W., Klein, T., Chem. Ber. 86, 584 (1953).
(13) Murmann, R. K., J. Am. Chem. SOC. 80, 4174 (1958). (14) Sen, B., ANAL.CHEM.31, 881 (1959). (15) Trussell, F., Diehl, H., Zbid., 31, 1978 (1959). (16) Voter, R. C., Banks, C. V., Diehl, H., Ibid., 20, 458 (1948). (17) Yoe, J. H., Jones, A. L., IND.ENG. CHEM.,ANAL.ED. 16, 111 (1944). for review December 17, 1962. RECEIVED Accepted February 7, 1963. This work
was supported by the National Science Foundation.
Electro-Oxidation of Phenylenediamines and Related Compounds at Platinum Electrodes Effects of Acid Strength HARRY
B. MARK, JR.,
and FRED C. ANSON
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, Calif.
b The electro-oxidation of p-phenylenediamine, o-phenylenediamine, N,Ndimethyl-p-phenylenediamine, Nphenyl-p-phenylenediamine, and diphenylamine a t platinum electrodes has been studied chronopotentiometrically in supporting electrolytes of varying acidity. The €114 values of the waves and plots of ir1/2vs. i for these compounds indicate that over certain p H ranges a rapid single proton dissociation reaction takes place prior to the electron transfer. The rate constants of this dissociation reaction for p-phenylenediamine and N,N-dimethylp-phenylenediamine have been estimated from this data. HE ELECTROCHEMICAL oxidations of T t h e i,Qomers of phenylenediamine and many related compounds have been extensively investigated (5-8, 11-14, Interest in these compounds arises from their widespread applications as aromatic intermediates (8) and antioxidants (7, IS). Adams and coworkers (5, 6, 8,12) have determined the final products of electro-oxidation for a number of these compounds and have established in part the reaction mechanisms with the aid of electron paramagnetic resonance (EPR) techniques. Some of the experimental observations reported in the literature (5, 18, 13) however, have cast considerable doubt on the nature of the reacting species that takes part in the electron transfer reaction at the electrode. For the case of p-phenylenediamine, Adams has postulated that the monoprotonated cation is the reacting species in the p H range 4 to 6 (8). This has not been tested experimentally, however. A chronopotentiometric study of these
722
ANALYTICAL CHEMISTRY
compounds as a function of acid strength has suggested the nature of the reacting species as reported below.
solution was employed, the total salt concentration of the buffer was approximately 1F in all cases.
EXPERIMENTAL
RESULTS AND DISCUSSION
The chronopotentiometric circuitry followed standard practice (91, and complete descriptions of the circuit components have been previously reported (1, 8 ) . A Moseley Autograph Model 3-S recorder was employed. The p H values were obtained with a Leeds and Northrup direct-reading pH meter calibrated in the low pH range with standard acid solutions. -4standard two-compartment H-cell was used and the working electrode was either a 0.020-inch diameter platinum wire sealed in soft glass (exposed 1 cm.) or a Becksurface area ~ 0 . sq. man Model 39273 platinum button electrode (exposed surface area ~ 0 . 2 3 sq. cm.) which was used for the i ~ 1 ’ 2 us. i studies. For the case of p-phenylenediamine, T mas measured by a “transition potential” method similar t o that described by Lingane (10). For the W,hT-dimethyl-p-phenylenediamine case \There the waves were somewhat poorly defined, two lines tangent to the flat center portion of the oxidation wave and to the approximately linear portion of the wave in the vicinity of the inflection point were drawn, and T was taken as the time elapsed between the initiation of the current and the intersection of the two lines. All organic compounds were recrystallized from either water or ethanol-water mixtures. Because of their susceptibility to air oxidation, the recrystallized compounds were stored under nitrogen and stock solutions were prepared daily. Inorganic chemicals were reagent grade and were used without further purification. -411 solutions were prepared with triply distilled water, and all potentials are reported with respect to the saturated calomel electrode. When a buffered
Figure 1 shows the anodic chronopotentiograms of 5 X 10-3F pphenylenediamine solutions of various pH values between 2 and 0. The applied current was 2.5 ma. per sq. cm. The supporting electrolyte for the pH 0 to 1 solutions was HzSOl and for the pH 2 solution a NazSO,-NaHSO, buffer was used (approximately 1.OF in total sulfate). As the p H was changed from 1 to 0 the quarter wave potentia1 increased by 310 mv. instead of the 59 mv. predicted by the Nernst equation. The T values obtained for the waves at a pH of 0 and 1 indicated a single two electron oxidation by comparison with T values obtained for hydroquinone under the same conditions. This behavior was similar to that found by Adams and coworkers in the pH range of 2 to 6. S o t e , however, that two distinct waves, A and B, are obtained a t pH values between 1.0 and 0.20; the less positive wave, A , corresponding to the single wave obtained a t pH 1.2 or greater and the more positive wave, B, corresponding to the single Tvave obtained a t pH 0.0. The transition times for wave, A , T A , decrease as the pH is decreased between 1.0 and 0.20, while those for wave B, T B , increase. The sum of T A and T B remains constant. The value of ~ T A I ‘ ~ was studied as a function of i for a 5milliformal p-phenylenediamine solution over a pH range of 3.4 to 0.70 (only one wave was observed above p H 1.20). It was found that ~ T A ~ / was independent of i for pH values greater than about 1.2 but decreased
~
U
~~
TIME
Figure 1. Chronopotentiograms of 5 X 1 O-3F p-phenylenediamine a t various p H values. The applied current was 2.50 Fa. per sq. crn Curve 1. Curve 2. Curve 3. Curve 4. Curve 5. Curve 6.
pH = 0.0 (H2SO4) pH = pH = pH = pH = pH =
0.2 (H2SOJ 0.5 (H~SOI) 0.7 (H~SOI) 1.0 (H2SOJ 2.0 (HS04--SOr-2 buffer)
linearly with increasing current density at pH values less than 1.2 as shown in Figure 2. Chronopotentiograms of pphenylenediamine in HC1O4 and HCl supporting electrolytes were identical to those obtained in H Z S 0 4 in the p H range of 1to 0. The Ella values of N,N-dimethyl-pphsnylenediamine (3 X 10-3F) chronopotentiograms became about 130 mv. more positive when the p H was changed from 1.0 to 0.0 instead of the 59 mv. predicted by ihe Nernst equation. The chronopctentiograms obtained for pH values intermediate to 1.0 and 0.0 did not exhibit a distinct second (more positive) wave. The wave form became Inore irreversible in appearance as the pH decreased. The i & 2 values of ;he one distinct wave were independent of i for p H values greater than 1.0, but decreased with increasing values of i a t a p H of 0.20. The oxidations of 5 >( lO-3F solutions of o-phenylenediamine, diphenylamine, and N-phenyl-p-phcnylenediamine were alqo studied in I to 12F solutions. The value of T a t a coqstant current of 2.5 ma. per sq. em. and a reactant concentration of 5 X LO-3F was found to decrease markedly with increasing HzSOl concentration until the wave practically disappeared in 12F HzSOI. The values of for N-phenyl-pphenylenediamine in t1ii.i concentration range were found to b’? independent of current density ( i + 2 values obtained for o-phenylenediamine were less reliable because of the poor sqaration of the wave from background; for diphenylamine the T values were not reproducible). Chronopotentiopams for hydroquinone (a well behaved species which can be considered analogous to the amines being studied) obtained in 1 to
200
I
I
1
I
100
200
300
400
CURRENT, pa
Figure 2. The irt v5. i of 5 X 10-3F p-phenylenediamine (solid lines) and 3 1 O - V N,N-dimethyl-p-phenylenediamine (dashed lines) a t various pH values. A platinum button electrode with a surface area of about 0.23 cmS2 was employed
x
0 pH = 3.4 (HzPOA--H~PO~ buffer) 0 p H = 1.9 (HS04--S04’2 buffer)
10F HS04 solutions showed a decrease in T approximately equal to that observed for the three amines. The decrease in T for the amines is the result of decreases in the effective diffusion coefficients resulting from increases in the viscosity of the solutions as the concentration of Hi304 is increased. Chronopotentiograms of solutions containing larger amine concentrations (0.01 to 0.05F) showed that for o-phenylenediamine exhibited a very large potential shift (approximately 250 mv.) between 5F and 10F H2S04 concentrations while that for N-phenylp-phenylenediamine increased only slightly and that for diphenylamine actually decreased slightly in this concentration range. I n the p H range where the large shift of the quarter-wave potentials were observed for p-phenylenediamine and N,N-dimethyl-p-phenylenediamine, the values of i + j 2 for the chronopotentiometric waves become current-dependent which suggests control by a chemical kinetic process that precedes the electron transfer. A similar kinetic process probably precedes the electron transfer when o-phenylenediamine is oxidized in 5 to 10F HzS04 solutions as indicated by the large potential shift observed. I n all these cases, the regions of kinetic coiitrol fall a t pH (or acidity) values considerably less than the pK for ionization of the completely protonated ion: p-phenylenediamine, pK,, = 2.8 (13,16); pK,, for N,N-dimethyl-p-phenylenediamine, would be expected to be similar to that of p-phenylenediamine (16) and a value of 2.8 was used in the calculations: and o-
phenylenediamine, pK,, = 0.6 (15). If it isassumed that the electrons are removed more readily from these compounds when a free pair is present on a nitrogen atom, a mechanism for their oxidation can be postulated t h a t is consistent with the experimental observations. The more positive waves (wave B, Figure 1) found for these compounds a t the lower pH values probably correspond t o oxidation of the totally protonated ion of each compound in which no free pair of electrons is present on the nitrogen. The less positive waves (wave A , Figure 1) found a t the higher p H values corresponds to the oxidation of the dissociated form in which an unshared pair of electrons is present on the nitrogen atom. The finite rate of the dissociation reaction is responsible for the decrease in the values of i ~ I ’ 2 with increasing current density. Typical plots of ?i us. i for p-phenylenediamine and N , N dimethyl-p-phenylenediamine in the p H ranges where kinetic control is observed are shown in Figure 2. The rate constants for the ionization of the first proton by the reaction:
can be calculated by means of the following equation for the straight-line plot of iT1’2 us. i (3, Q) :
where k j is the ionization rate constant, k b is the pseudo-first order recombinaVOL. 35, NO. 6, MAY 1963
723
Table I.
(4) Delahay, P., Berzins, T., J . A m Chem. SOC.75, 2486 (1953). (5) Galus, Z., Adams, R. N., Ibid., 84, 2061 (1962). (6) Galus, Z., JThite, R. M., Rowland, F. S., Adams, R. N., I b i d . , 84, 2065
Ionization Rate Constants Calculated by Means of Equation 2
Compound p-Phenylenediamine
PH 1 . 0 and 0.7
kf1 eec.
K
1.59 x 10-3
ca.
N ,~?’-Dimethyl-p0.2 (1.59 x 10-3)b phenylenediamine Average of values obtained at the two pH’s. * Assumed equal to K for p-phenylenediamine; see text.
tion rate constant of the reaction given by Equation 1 (kb = ksb[H+])where k ” b is the second-order recombination rate constant), and K is the equilibrium constant, (kf/k”b),of the reaction given by Equation 1. I n Equation 2, C” is the total concentration of H2A++ and HA+ in solution; and n, F , and D have their usual electrochemical significance. The first term on the right hand side of Equation 2, n”W’CoD112/2 is the intercept of the straight.line plots and is equal to the diffusion controlled value of i+. The rate constants are calculated directly from slope of the straight-line plots which is equal to n112/2K(kr ka)1’2. The rate conqtants
+
k”b, litermole-’eec.-l
lo4”
ca. lo4
f 19621. \----I
ca. ca. lo7
calculated for the ionization reactions of the species studied are given in Table I. The fact that even in 12F H2S04N phenyl-p-phenylenediamine and diphenylamine do not exhibit the large behavior that potential shift or were observed for the other three compounds is no doubt because of their much weaker basicity. LITERATURE CITED
(1) Anson, F.
(19611.
c., ANAL. CHEM.3 3 ,
939
199, Interscience, Yew York,
(7) Gaylor, V. F., Conrad, A. L., Landed, J. H., ANAL. CHEM.27, 310 (1955). (8) Lee, H. Y., Adams, R. N., I b t d . , 3 4 , 1587 (1962). (9) Lingane, J. J., “Electroanalytical Chemistrl-,” 2nd ed., Chap. S X I I , Interscience, New York, 1958. (10) Lingane, J. J., J . Electroanal. Chem. 4 , 379 (1960). (11) Lord, S. S., Rogers, L. B., S s . 4 ~ . CHEM.26, 284 (1954). (12) Mizoquchi, T., Adams, R. S . , J . Am. Chem. SOC.84, 2058 (1962). (13) Parker, R. E., Adams, R. S., h s . 4 ~ . CHEM.28, 828 (1956). (14) Piette, L. H., Ludwig, P., Adams, R. N., Zbid., 34, 916 (1962). (15) Vanderbelt, J. M,, Henrich, C., Vandenberg, S.G., I b i d , 26,726 (1954). (16) Whitmore. F. C.. “Ornanic Chemistry,” 2nd ed., p. 618, Vin Sostrand Co., Kew York, 1951. RECEIVED for review December 31, 1962. ilccepted Xarch 1, 1963. Supported in part by the National Science Foundation, Contribution No. 2925 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology. Division of Analytical Chemistry, 144th Meeting, ilCS, Los Angeles, Calif., April 1963.
A Partial Condensation Variable Reflux Stillhead for the DistiIIation of Trace Components NORMAN ADLER Chemical Division, Merck & Co., Inc., Rahway,
b The design of a partial condensation variable reflux (PCVR) stillhead in comparison to various total condensation heads is discussed in terms of start-up behavior and of internal and external holdup. External holdup is divided into regular and random mixing effects, and the former is treated mathematically. The tendency of a PCVR still to approach a constant distillate composition still in behavior is described. The advantages of a PCVR head in the distillation of trace components-i.e., SOX-from macro mixtures are discussed. Performance is evaluated by use of the azeotropic, minimum boiling, acetone-isooctane system.
I
IS OFTEN DESIRABLE in many laboratory problems, and particulady in gas chromatographic analysis, to distill quantitatively trace components-i.e., 50X-from large quantities of less volatile liquids. The T
724
ANALYTICAL CHEMISTRY
N. J.
objective is usually to achieve a highly concentrated solution of the volatile impurities. The degree of fractionation required to achieve this goal for a particular system may be estimated from established distillation theory (11). I n practice, however, once a given fractionation system is selected, it is often the stillhead that is the limiting factor in the sharpness of separation attainable (10). Particularly for micro components, the holdup phenomena implicit in the design of many laboratory variable reflux stillheads may significantly reduce the over-all efficiency of fractionation. Commercially available stillheads that minimize these defects are often complex and costly. I n this work, the properties of the basic laboratory scale partial condensation variable reflux (PCVR) stillhead are compared to those of the more conventional total condensation variable reflux (TCVR) stillhead type, to show its advantages for the distillation of more volatile micro components
from macro mixtures. =Ilthough dills using a partial condensation head have been described previously ( I , 3 , 4, 8, 9 ) and have been recoinmended for selected special applications ( 2 , E ) , the intrinsic properties of this type of head do not appear to have been documented fully. DESIGN CONSIDERATIONS
The chief functions of a distillation column stillhead are to condense the vapors to the liquid phase, to direct the flow of vapor and condensate, and usually to regulate the reflux ratio. The TCVR type is considerably more popular than the PCVR type, presumably because a constant reflux ratio may be set and then maintained without subsequent attention ( 5 ) . Control of the reflux ratio is usually obtained by movement of some element to divert the flow of condensate in a proportional manner, either in terms of time or of volume. The movable element may be
