Effect of Structure of Certain Amine Indicators on Oxidation Potential and Color Intensity on Oxidation F. T. EGGERTSEN and F. T. WEISS
Shell
Development Co., Emeryville, Calif.
The polarographic half-ware oxidation potentials of a number of amine-ty pe redox indicators habe been measured at pH 9.4 in 1 to 1 water-acetone solution using a stationary platinum electrode. The indicators were derhatives of benzidine, 4-aminodiphenylamine, o r p-pheny lenediamine. The relative susceptibilities of these indicators to oxidation were also measured in the same medium, using hy drogen peroxide or heminhydrogen peroxide as oxidants in the test. In general, the amount of color produced by a fixed amount of oxidizing agent parallels oxidation potential, the indicators with the lowest potentials giving the greatest color intensit?. The effect of molecular structure on oxidation potential is discussed briefl?
.
THE
suitability of any redox indicator for a particular application is determined to a large extent by the magnitude of its oxidntion-reduction potential. I t is true, of course, that what may appear to be a suitably low potential does not necessarily ensure a sufficiently rapid rate of reaction to produce the oxidized form: however, a correlation has heen found in some instances between reaction rate and oxidation potential, at least n-ithin a given type cf compound, so that susceptibility of the indicator to oxidation parallels potential. For cxample, in :i study of the behavior of phenylenediamine-type color developers in the photographic process, Rent and others ( 2 ) found a logarithmic relationship between reaction rate and polarographic halfwave oxidation potential of these compounds. .\ theoretical basis for this relationship exists when conditions are such that the total fi,ee-energy change of the reaction, A F , is proportional to the free-energy change in the formation of the transition state, a F * , because the rate of reaction has been s h o r n to be ii Eunctioii of the latter (6).I n another study of several amine c o ~ n p o i ~ n d a , Lord and Rogers ( 9 ) shon-ed that the polarographic half-tvnve oxidation potentials of these compounds are in good :tgree iient with critical oxidation potentials obtained hy chemical measiirernent of their relative stabilities to oxidation. In connection with a study requiring use of a redox indicator, the polarographic half-n-ave potentials of a numher of amirietype indicators, dissolved in alkaline 30°C acetone. were memired using a stationary platinum electrode, with the object of estahlishing correlations x-hich might be iised as a guide for the selection or synthesis of more effective indicators. The indicators selected for study were derivatives of benzidine, 4-aminodiphenylamine, or p-phenylenediamine, and included redox indicators in common use as well as new compounds obtained from other soi~rces. The mechanism of the Oxidation of amines such as those eniployed in the present study has been the subject of ~)revious investigations ( 4 , 10, 12-15). I t has been found that the coniplesity of products is great, esr)ecially in alkaline solution, but in general it appears that the reaction proceeds in steps, following the initial removal of an amino hydrogen atom, to leave a colored radical (JT-iirster salt). This radical can undergo several alternative reactions, including dimerization (12-14), dismutation to higher and lower oxidation states, forming the unstable quinonediimine (10, 1 2 ) , or autoxidation to an anilide ( 4 ) . For the purposes of this investigation, oxidation potential measurements were correlated \vith susceptibility to oxidation by hydrogen peroxide, alone and also in the presence of a small
amount of hemin as a catalyst. The hemin-peroxide test employed is similar to the xell-known benzidine test for occult blood ( 7 , 11), but is applied here in alkaline rather than in acid medium. -1s will be seen, this test proved to be a satisfactory basis for comparison of the indicators under study, because of the apparently intermediate position of hemin-peroxide as an oxidizing system with respect to the reactivity of the indicators. I t would be desirable to express color intensity obtained in the test as a percent,age of the maximum obtainable if the oxidation were complete. This would be a means of correcting for the limitations in color development for each particular indicator, and would permit a true comparison of extent of oxidation by hemin-peroxide. The maximum intensities are not known and would in fact be difficult to obtain, because, as already mentioned, the oxidation is a complicated process. However, if comparisons are made among closely related compounds, variations in maximum possible intensities per mole would be small, and a direct comparison of color intensities obtained in the peroxide test might then be a valid measure of extent of oxidation. EXPERI3IENT.A L
Polarographic Half-Wave Potentials. These measurements were made essentially b>- the proredure described by Julian and Ruby ( 6 . ) . Hoxever, the more common polarographic sign convention \vas employed-i.e., the more difficultly oxidized indicators have the higher expressed (positive) values. . ~ P P A R A T L - S . Sargent .\lode1 IS I'olarograph and a Leeds & Sorthrup potentiometer (smallest scale division = 1 mv.) for making accurate potential measurements. H-type cell, having a capacity in each section of about 30 ml., wit,h a large calomel electrode on one side and the sample solution on the other, separated by a sintered-glass and saturated potassium chloride-agar partition. Cover assembly for the sample side of the H-cell fitted {vith the follon-irig: two platinum microelertrodes consisting of a length of platinum wire 0.020 inch in diameter sealed in soft glass tubing and extending ahout 'I1inch from, and a t right angles to, the tubing: nitrogen inlet and outlet for flushing oxygen from the test solution: small saturated calomel electrode (Beckman p H meter type with glass sleevr).
4 - A m i n o -4'
-
MethoxyDiphenylamine
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B e n z i d i n e , 2, 2'-
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3 0 0 400 5 0 0 600 700 A p p l i e d P o t e n t i a l vs. N. H. E.
200
.
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800
900
mv.
Figure 1. Polarograms of amine indicators Phosphate buffer, pH 9.4; 50% acetone
I
1 1 1000
V O L U M E 28, N O . 6, J U N E 1 9 5 6 Table I.
1009
Polarographic Half-Wave Potential cs. Hemin-Peroxide Test (Measurements a t p H 9.4)
Indicatoi
Benzidine c l a s ~ .
Source"
Benzidine, ",2',6,6'-tetrafluoroBenzidine, 2,2'-difluoroBenzidine, 2,2',6,6'-tetrametIiylBenzidine, 2,2'-disulfonic acid Benzidine, 3,3'-dimeth~l-G,O'-disulfonic acid Benzidine, 3-methylBenzidine Benzidine, 3,3'-dimethyiBenzidine, 3,3'-dimethyl-'o-tolidine) Naphthidine, repurified Benzidine. 3.3'-dinietl1os)--(o-dianijidine), repurified
13.
H9N-L
ACC ACC ACC E a s t man Eastman Belcher Eastman Belcher Eastman Bios Labs. Eastman
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(750) (780)
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7 11
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41 48 IF7 170 143 162 74
4
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400 400 430 430 480 430 440 440
45:
420
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395
420
26
206
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Eastnian
499
440
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Eastman
387
4.70
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bhell Drvelopment
371
440
19
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363
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288
460
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N-H
s Et),-I.,Ibid., 75, 5312 (1953). Clark, W. AI., Cohen, B., Gibbs, H. D., C. S. Public Health Repts. 1926, Suppl. 54. (5) Eyring, H., Marker, L., Kwoh, T. C., J . Phys. & Colloid Chem.
(1) (2) (3) (4)
5 3 , 1463 (1949).
Gershinowitx, H., J . Chem. Phys. 4, 363 (1936). (7) Hepler, 0. E., Wong, P., Pihl, H. D., Am. J . Clin. Pathol. 23,
(6)
i263 (1953).
(8) Julian, D. B., Rubs. IT. R., J . Am. Chem. SOC.72, 4719 (1950). (9j Lord, S . S., Rogers, L. B., ANAL.CHEM.26, 284 (1954). (10) Luvalle, J . E., Glass, D. B., Weissberger, A., J . Am. Chem. SOC. 70, 2223 (1948). (11) Merck Index, 5th ed., p. 624, llerck &- Co., Inc., Rahway. S-. J., 1940. (12) Michaelis, L., Chem. Revs. 16, 243 (1935). (13) hfichaelis, L., Granick, S., J. Am. Chem. SOC.6 5 , 1747 (1943). (14) Rlichaelis, L., Schubert, hl. P., Chem. Revs. 22, 437 (1938). (15) Michaelis, L., Schubert, &I. P., Granick, S.. J . Am. Chem. SOC. 61, 1981 (1939).
RECEIVED for review November 18, 1955. Accepted March 8,1956. Work done under contract with U. S. Army Chemical Corps, Washington 25, D . C.
