Neutral Red has been shown to undergo electrochemical and photoelectrochemical reactions in aqueous solution. At a pH above the pK,, the dye was oxidized with the loss of one proton with each electron. Similarly the protonated dye was oxidized at pH values below the pK,. Over the pH range examined, 1.0-9.2, Neutral Red was reduced with one proton being gained with each electron. The neutral radical which was formed could be further reduced. When the pH was above the pK, the dye was reduced by an H e e H mechanism, and when the pH was below the pK, the reduction proceeded by an H e H e mechanism with the possibility of further protonation.On a platinum electrode, Neutral Red underwent a photoelectrochemical oxidation at pH 7.2. The voltammogram showed a broad peak centred at 0 V (s.c.e.).Neutral Red, viz. 3-amino-7-dimethylamino-2-methylphenazine, has long been flyJbfe used as a pH indicator in biological studies'because its red-yellow colour change at h H, pK, 6.7 falls in the pH range close to that Neurrai Red of biological media. However, problems were encountered in studies of photosynthetic reactions when Neutral Red was used as a spectroscopic probe of the internal pH of thylakoidse3 The observed changes in the probe's absorbance were consistent with acidification of the medium but were much larger than expected. One explanation is that the dye was protonated preferentially in the aqueous phase; this indicated a variation of activity with l~c a t i o n .~ Other explanations for these results are that the unexpected decrease in the absorbance was due to Neutral Red acting as an artificial electron donor or acceptor in the photosynthetic reactions, and that the decrease was due to photoelectrochemical reactions of Neutral Red. As the dye absorbs light, it may undergo a charge-transfer reaction with species generated in the photosynthetic reactions.In concentrated acid solutions Havemann et aL4 examined solutions of sn2 + ions and Neutral Red at two pH values. At pH 0.5, where 75 "/,f the dye was in the
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