THE REDUCTION OF SOME ADSORBED OXIDATION-REDUCTION INDICATORS HAROLD A. ABRAMSON AND IVON R. TAYLOR The Biological Laboratory, Cold Spring Harbor, New York
Received December 6, 1086
Although many oxidation processes in biological systems apparently take place at surfaces or in their presence, the study of oxidation-reduction reactions at phase boundaries in liquids does not seem to have been examined in simple systems. Furthermore, the analysis of the effects of an adsorbed substance capable of undergoing a reversible oxidation and reduction on the r-potential (electrokinetic potential) of inert and ionogenic surfaces may provide a new method of approach to the surface chemistry of oxidations and reductions. For these reasons we have investigated the effect of reductants on adsorbed methylene blue, litmus, and phenosafranine. It has been found that these dyestuffs can be reduced and reoxidized reversibly in the adsorbed state. METHYLENE BLUE
Filter paper1 was dipped into methylene blue solutions (about 0.5 per cent or more dilute) and the excess methylene blue washed off in running tap water, leaving the paper stained blue. If a piece of this blue paper is suspended in a stream of hydrogen in water or in phosphate buffer in the presence of platinized asbestos, reduction of the adsorbed methylene blue by the gaseous hydrogen does not occur to any appreciable extent.2 The presence simultaneously of relatively large amounts of methylene white in solution complicates this result, because of the equilibrium set up between dissolved methylene white and adsorbed methylene blue. This was first observed by Doctor I. Korr in a preliminary experiment. Although hydrogen gas in the presence of large quantities of catalyst does not reduce adsorbed methylene blue, this can be readily brought about by addition of sodium hydrosulfite, cysteine, or thiourea. The thiourea, however, is efficacious only in acid solution. On addition of sodium hydrosulfite, 1 Schleicher and Schull No. 1. Also dye which has been adsorbed by blotting paper, adsorbent cotton, cotton toweling, porcelain, and hairs of a cleaning brush can be reversibly reduced. a This confirms a personal communication t o one of us (H. A. A.) from Professor L. Michaelis.
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the slightly tinted solution of methylene blue becomes colorless, the filter paper itself then bleaching more slowly and in a spotty fashion but finally becoming white. If the white filter paper is now removed from the solution and the excess of reductant washed off, the paper remains white for some time unless the washing is prolonged. Most of the adsorbed methylene blue still remains on the filter paper in the form of methylene white, for addition of quinone or potassium ferricyanide restores the blur color to almost its original intensity. The adsorbed blue dye can now be reduced again and reoxidized several times. Cysteine does not reduce the adsorbed dye as quickly as hydrosulfite. Even with a 5 per cent solution of cysteine (Eastman Kodak) slight heating is needed for the reduction reaction to go to completion nithin a reasonably short time. With dilute solutions a t room temperature the reduction may lake hours. The autooxidation of adsorbed methylene white seems to take place readily in thP air but rather slowly. As mentioned previously, traces of hydrosulfite or of cysteine adsorbed onto the filter paper inhibit the autooxidation of methylene white for a period quite sufficient to test the effects of other oxidants. Methylene white is known to be adsorbed by various surfaces. This can be demonstrated by dipping filter paper into a solution of methylene blue reduced by hydrosulfite or cysteine. After moderate washing in running water, little or no color appears. Addition of an oxidant however, reveals at once that a large quantity of methylene white has been adsorbed. LITMUS
Litmus paper is rendered colorless by hydrosulfite in both acid and neutral solutions. Autooxidation is slow. The white paper can be washed in running water and then reoxidized to the red or blue dye depending on the pH. PHENOSAFRANINE
This dye is readily adsorbed by filter paper and can be reversibly oxidized and reduced in the adsorbed state. DISCUSSION
The bonds responsible for the adsorption of the dyestuffs here investigated do not appear to affect appreciably those groups involved in the oxidation-reduction process. A parallel instance is that observed for the ionization of adsorbed protein. Since the electric mobilities of proteincovered quartz particles do not differ very much from the dissolved protein, the free amino and carboxyl groups are not primarily involved in the adsorption reaction. It is of some interest to see a similar phenomenon occur with smaller molecules. A surface having selective adsorption for one form of the constituents of
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a reversible oxidation-reduction system could shift the oxidation-reduction potential of the system. Adsorption reactions of this type have apparently been investigated only with pH indicators. Thus Deutsch has shown that indicators like bromothymol blue, malachite green, and many others have the undissociated form of the dyestuff selectively adsorbed if adsorption occurs at a pH near the value of pk of the indicator. In this instance the pH of the solution does not change appreciably, for the color change occurs in well-buffered solutions. Since adsorbed methylene blue is not readily reduced by hydrogen gas-platinized asbestos, and since an electrode would indicate that the solution was very near the potential of the hydrogen electrode, it is evident that the electrometric measurement gives no indication of the potential at the surface (where adsorption occurs), even though the electrode potential has reached “equilibrium.” In heterogeneous systems the attainment of “equilibrium” in solution does not necessarily indicate the reduction intensity at the surface. This should be borne in mind in connection with discussions involving the reduction intensity in living systems.
