mium(V1) as chromate ion and, in addition, does not suffer from as many interferences. It is more sensitive and less subject to interferences than the spectrophotometric determination of chromium(V1) as dichromate (8). The peroxychromic acid-2,2'-bipyridine method is not as sensitive or as simple as the s-diphenylcarbazide method for chromium (9). All three
methods are subject to some interferences from excess amounts of certain metal ions, but in this method fairly large amounts of vanadium do not interfere as in the s-diphenylcarbazide method. An indication of the precision of this method for the determination of chromium was found by determining 16 samples containing 3.0 ppm of chromium with a standard deviation of 0.016 or a relative standard deviation of 3.5%.
(8) G . Charlot, "Colorimetric Determination of Elements," Elsevier Publishing Co., Amsterdam, 1964, pp 226-30. (9) E. B. Sandell, "Colorimetric Determination of Traces of Metals," 3rd Ed., Interscience, New York, 1959, pp 390-8.
RECIEVED for review August 22, 1967. Accepted November 13, 1967. Presented at the Anachem Conference, Detroit, Mich., October 3-5, 1967.
Oxidation of Some Arylamines by Copper(l1) in Acetonitrile Byron Kratochvill and David A. Zatko2 Department of Chemistry, University of Wisconsin, Madison, Wis. 53706
DIPHENYLAMINE was suggested as an oxidation-reduction indicator for dichromate titrations by Knop in 1929 (1). The more convenient water-soluble sulfonic acid derivative was introduced in 1931 by Sarver and Kolthoff (2). The mechanism of the oxidation of diphenylamine by dichromate had been studied earlier by Kolthoff and Sarver (3), who established that the reaction takes place in two steps, first formation of diphenylbenzidine, then diphenylbenzidine violet.
H
Diphenylbenzidine
0fW-Q / \
/ \
H H Diphenylbenzidine Violet
In the second step, an insoluble, unstable green intermediate was obtained which was postulated to be a 1:l diphenylbenzidine-diphenylbenzidine violet adduct. This green intermediate has been suggested as an indicator because of its small blank correction, but it must be used as a suspension in water ( 4 ) . The exact nature of the intermediates formed in the homogeneous oxidation of diphenylamine to diphenylbenzidine violet has not been completely clarified, although some data have been obtained by chronopotentiometry (5, 6). Additional information can be gotten by using aprotic nonaqueous solvents to reduce solubility difficulties and permit assessment of hydrogen ion effects. Oxidation by copper (11) in acetonitrile has been investigated by 1 Present address, Department of Chemistry, University of Alberta, Edmonton, Alberta. * Present address, Department of Chemistry, University of Alabama, University, Ala., 35486.
(1) J. Knop, J . Am. Chem. SOC.,46,263 (1924). (2) L. A. Sarver and I. M. Kolthoff, Ibid., 53, 2902 (1931). (3) I. M. Kolthoff and L. A. Sarver, Ibid., 52, 4179 (1930). 5, 154, 158 (1933). (4) H. H. Willard and P. Young, ANAL.CHEM., ( 5 ) R . N. Adams, J. H. McClure, and J. B. Morris, Ibid.,30,471 (1958). (6) H. B. Mark, Jr., and F. C. Anson, Ibid., 35,722(1963).
422
ANALYTICAL CHEMISTRY
potentiometric titrations and electron spin resonance spectrometry in order to evaluate solvent effects on the reactions of diphenylbenzidine and related compounds and to determine the analytical applicability of copper(II), thus extending the range of this reagent over that reported previously (7). EXPERIMENTAL
Reagents. Acetonitrile (Matheson, Coleman, and Bell, Norwood, Ohio) was purified by the method described previously (7). The water content, determined by Karl Fischer titration, was less than 5 X 10-4M. Anhydrous copper(I1) perchlorate solutions were prepared by dissolving tetrakis(acetonitri1e) copper(I1) perchlorate, prepared from nitrosyl perchlorate and copper metal, in anhydrous acetonitrile (7). The solutions were dispensed from automatic burets and were protected from the atmosphere by drying tubes of magnesium perchlorate. Where exclusion of water was not required, solutions were prepared by dissolving hexaquo copper(I1) perchlorate in commercial acetonitrile, filtering to remove a small amount of white precipitate that formed, and storing the solution in a glass stoppered bottle. Tetramethylbenzidine (TMB) (Eastman Organic Chemicals, Rochester, N. Y.)was recrystallized from acetonitrile before use (m.p. 195-6"; reported 198"). Diphenylbenzidine (DPB) (G. Frederick Smith Co., Columbus, Ohio) was recrystallized from acetonitrile (m.p. 248" ; reported 245 "). Tetramethylbenzidine Orange (TMB Oranges 2), the twoelectron oxidation product of TMB, was prepared by potentiometric titration of a TMB suspension in acetonitrile with copper(I1) to the second equivalence point, filtration of the orange-red needles that formed, and recrystallization from warm acetonitrile. Tests for trace copper in the product were negative. Diphenylamine (DPA) (Eastman Organic Chemicals) was used as received (m.p. 53.0-53.2"; reported values range from 51-55 "). Titrations of diphenylamine recrystallized from an ethanol-water solution (m.p. 53.0-53.2"), and of recrystallized material which was vacuum sublimed (m.p. 53.1-53.4") gave essentially the same results. Apparatus. A 75-ml weighing bottle with a Teflon lid was used as a titration cell. A platinum wire indicating electrode and a silver-0.0100M silver nitrate in acetonitrile reference electrode were used in conjunction with a k e d s & Northrup Model 7401 pH meter, and all potentials are reported relative to this reference couple. The reference electrode was isolated by two ultra fine glass frits and a 0.1M (7) B. Kratochvil, D. A. Zatko, and R. Markuszewski, ANAL. CHEM., 38, 770 (1966).
LiCIO, salt bridge. All titrations were performed under nitrogen. ESR spectra were obtained with a Varian Model V-4503-13, X-band spectrometer equipped with 100-kHz modulation. Measurement of the unpaired spins per ion of the intermediates formed in the oxidation of TMB and DPB was made by comparison of areas of integrated ESR spectra of known concentrations of 2,2’-diphenyl-l-picrylhydrazyl (DPPH) (Eastman Organic Chemicals) and the intermediates.
A
RESULTS AND DISCUSSION
Tetramethylbenzidine (TMB) Oxidation by Copper(I1). Tetramethylbenzidine gives well-defined potential breaks upon titration with copper(I1). The reaction proceeds in two steps in dry acetonitrile (Curve A , Figure 1) corresponding to
+ Cu(11)d TMB Green+ + Cu(1) TMB Green+ + Cu(I1) TMB Orange+2 + Cu(1) TMB
-+
(1) (2)
where TMB Green+ refers to the one-electron oxidation product and TMB Orange+2 to the two-electron oxidation product of TMB. The free radical TMB Green+ has been studied in detail by electrochemical and ESR techniques, and is well characterized (8, 9). A potentiometric titration of TMB with TMB Orange+2in acetonitrile gave formal reduc-
Q
Q H~c/~\cH~
TMB Green+
TM B
Q Q
H,C+CH,
TMB Orange2+
tion potentials within 20 mV of those found for copper(I1) oxidation (0.10 and 0. 30 V, respectively) so copper coordination to any of the reaction species seems minimal. Both of the TMB couples appear reversible, and from the potentials of the two couples the log equilibrium constant for the reaction TMB
+ TMB Orange+2
2TMB Green+
was calculated to be 3.4. If water on the order of 1 is present during the titration of TMB with copper (11), a plot having three breaks is obtained (Curve B, Figure 1). These are attributed to the oxidation of unprotonated TMB in two steps, followed by oxidation of TMBHS to TMB Orange+2 in one step. This interpretation assumes that TMB is sufficiently basic to be partially protonated in the titration medium by water, and is supported by the fact that when TMB is titrated with copper(I1) or TMB Oranges2 in the presence of excess perchloric acid ( [HS]>2 [TMB] ) no inflection point is observed, but when [H+] = [TMB] a single inflection corresponding to one electron per molecule of TMBHS but yielding TMB Oranges2 (Curve C, Figure 1) is seen. Since titrations of TMB with perchloric acid in glacial acetic acid show that the two acid dissociation constants are quite close together, it appears that in the case where [H*] = [TMB], half of the TMBH+ is oxi(8) T. Mizoguchi and R. N. Adams, J. Am. Chem. SOC.,84, 2058 (1962). (9) J. M. Fritsch and R. N. Adams, J . Chern. Phys., 43, 1887 (1965).
1.00 1.50 2.00 MOLE RATIO Cu(ii) / TMB
0.50
Figure 1. Potentiometric titrations of TMB with copper(I1) in acetonitrile A. Under anhydrous conditions B. -0.1M water present C. In presence of perchloric acid, [H+] = [TMB]tOtal
dized completely to TMB Orange+l while half is converted to TMBH2+2and not oxidized. The overall sequence for TMBHS oxidation then is postulated to be 2TMBH+ Cu(II)+ TMB Green+ Cu(1) TMBH2+Z (3) TMB Green+ Cu(I1) TMB Orange+2 Cu(1) (4)
+
+
+
-+
+
+
+
TMBH2+2 Cu(I1) + No reaction (5) Steps 3 and 4 must take place at about the same potential since only one inflection point is observed. Diphenylbenzidine (DPB) Oxidation by Copper(I1). Titrations of DPB in anhydrous acetonitrile with copper(I1) show inflection points at mole ratios of 1:1 and 2:l (Curve A , Figure 2). The stoichiometry of both breaks is within 1 of the calculated values. As with TMB, equilibrium is reached rapidly. When water (-1x) is present, the first inflection point appears earlier, the second becomes more drawn out (Curve B, Figure 2), and equilibration after each titrant increment is slow before the equivalence points. In both cases the solutions are green up to the first inflection point and violet after it. By analogy with the TMB system, the first break is thought to correspond to the oxidation of DPB and the second to concurrent oxidation of DPB Green+ and protonated DPB. Attempts to isolate the final oxidation product, DPB Violet+2, were not successful. The intermediate in the reaction, DPB Green+, was found by ESR measurement to have 1.2 + 0.3 unpaired spins per ion. Therefore it appears to be a free radical as suggested by Walton (10) rather than a 1 :1 molecular adduct of DPB and DPB Violet+2, although it is not possible to rule out un(10) H. F. Walton, “Principles and Methods of Chemical Analysis”, 2nd Ed., p 367, Prentice-Hall, Englewood Cliffs, N. J., 1964. VOL 40, NO. 2, FEBRUARY 1968
423
800 I -.
I
I
I
I
I
1
I
c 2
2 6001 2
I
2
L
Lu
I-
2 200
400-/ z Lu
a
I
I I I 1 I 1.00 2.00 MOLE RATIO CU(II)/DPB Figure 2. Potentiometric titrations of DPB with copper(I1) in acetonitrile
4
I
I
I
2
Figure 3. Potentiometric titrations of DPA with copper(I1) in acetonitrile
I
Under anhydrous conditions B. -0.1M water present A.
equivocally the presence of a paramagnetic molecular adduct having electron interaction. No fine structure was observed in the ESR spectrum of DPB Green+, though it is well defined with TMB Green+. ESR studies of the intermediates formed in the oxidation of DPB with lead dioxide in glacial acetic acid indicate that the green intermediate is a tetraphenylhydrazide free radical (ZZ), but the copper(I1) oxidation intermediate in acetonitrile does not give the same ESR spectrum as lead dioxide even if acetic acid is present, and in acetonitrile lead dioxide will give the same spectrum only if acetic acid is available to provide hydrogen ions for the reaction. It appears that the mechanisms of DPB oxidation by copper (11) and lead dioxide are completely different. Diphenylamine (DPA) Oxidation by Copper(I1). Potentiometric titrations of DPA with copper(I1) in anhydrous acetonitrile show only one break at a copper to DPA ratio of unity (Curve A , Figure 3). The color of the solution changes during the titration from yellow to green to blueviolet, indicating the formation of some DPB Violets z. When water is present, two breaks are found at copper to DPA ratios of 0.5-0.7 and 1.9-2.4 (Curve B, Figure 3). The reactions are both slow. A reaction sequence fitting the oxidation ratio observed in titrations of DPA with copper (11) in anhydrous acetonitrile is:
+ 4 DPA 2 Cu(1) + DPB + 2 DPAH+ Cu(I1) + DPB Cu(1) + DPB Green+ Cu(I1) + DPB Green+ Cu(1) + DPB Violet+2 Cu(I1) + DPAH+ No Reaction
2 Cu(I1)
-+
-+
+
-+
(6) (7)
(8) (9)
A . Under anhydrous conditions B. -0.1M water present
+ 4 DPA
-+
4 Cu(1)
+ DPB Violet+2+ 2 DPAH+
(IO)
The hydrogen ion is produced in the formation of DPB from DPA. Only one break is observed because DPA oxidation ( 1 1 ) M. R . Das, A. V. Patankar, and B. Venkatararnan, Proc. Itzdiun Acud. Sci., 53A, 273 (1961); C.A. 56, 3044f(1962). ANALYTICAL CHEMISTRY
occurs in the potential range of the oxidation of DPB and DPB Green. In aqueous acetonitrile, a possible sequence for the first break, considering only DPB formation and a copper-DPA ratio of 0.5, is 2 Cu(I1)
+ 4 DPA
+2
Cu(1)
+ DPB + 2 DPAH+
(11)
and for the second, using a copper-DPA ratio of two, concomitant oxidation of DPB and DPAHs to give
+ 2 DPAH+ + 4 H20 4 Cu(1) + DPB Violet+2+ 4 H 3 0 + 2 Cu(I1) + DPB 2 Cu(1) + DPB Violet+2
4 Cu(I1)
-+
-+
(12) (13)
This sequence requires that water act as a base to accept the protons released in the DPAH+ oxidation. However, the solution is green before the first inflection point, so partial conversion of DPB to DPB Green+ must be occurring in the first step. Complete oxidation of DPB to DPB Green+ is not observed at this potential apparently because of partial protonation of DPB by water. Oxidation of Other Amines. A number of other amines were surveyed to outline the scope of copper(I1) oxidation in hydrated acetonitrile. Some of the compounds which showed well-defined titration breaks, along with copper(I1)amine ratios, were: dithizone, two breaks at 1.0 and 2.5; triphenylamine, one break at 2.0; tetraphenylpyrrole, two weak breaks at 1.0 and 1.5; and diphenylcarbazone, one break at 6.0. The following compounds underwent reaction, but did not show a definite stoichiometry and were accompanied by severely drifting potentials ; benzidine, aniline, ophenylenediamine, p-nitrophenol, p-nitrophenylhydrazine, and crystal violet. Carbazole and N-phenylcarbazole gave no indication of reaction. ACKNOWLEDGMENT
The authors thank Peter Quirk for assistance with portions of the experimental work and the G. Frederick Smith Chemical Co. for supplying the copper(I1) perchlorate.
with the overall reaction being
424
I
MOLE RATIO C U ( I I ) / D P A
0
4 Cu(I1)
-
I-
RECEIVED for review August 18, 1967. Accepted November 8, 1967. Presented in part, Division of Analytical Chemistry, 152nd Meeting, ACS, New York, N.Y. September 1966. Work supported in part by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation.
