MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH PHOTOLYSIS—III CONSEQUENCES OF A NEWLY FOUND pKT OF THIONINE ON THE MECHANISM IN BASIC SOLUTIONS
- 1 March 1975
- journal article
- Published by Wiley in Photochemistry and Photobiology
- Vol. 21 (3) , 173-177
- https://doi.org/10.1111/j.1751-1097.1975.tb06648.x
Abstract
Abstract— Evidence for an acid‐base equilibrium between the neutral and monocationic forms of the triplet state of thionine, 3T and 3TH+ (pKT# 8–9) has been found. This equilibrium, which has no counterpart in the methylene blue system can explain the different behaviours of these two similar dyes concerning their photoreduction by ethylene diamine tetracetic acid (EDTA) in basic solutions. With reasonable assumptions about the reactivity of the various species present in solution, it is now possible to calculate over the whole pH range the quantum yield of semireduced dye formation φs, which, according to the previously proposed mechanism, must be proportional to the overall photoreduction quantum yield φR. The very good fit between the calculated curve φs =f(pH) and the experimental one φR =f(pH) for both dyes strongly supports the proposed mechanism.Keywords
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