Abstract
The reactions of the reducing species formed on photo‐excitation of d, l‐mandelic acid in aqueous solutions with azo dyes, oxygen, mixtures of azo dyes, and mixtures of azo dyes and oxygen have been investigated. The results indicate that all reactions can be described as reductions of the oxidising agents (azo dyes and/or oxygen) by the photochemically formed reducing species. The azo dyes used can be arranged in a sequence according to their electron affinities. The higher the electron affinity the more susceptible a dye is to reduction in d, l‐mandelic acid on ultraviolet irradiation. The inhibition of the photoreduction of azo dyes in solution by oxygen is mainly caused by a competition of the azo dye present and oxygen for the reducing species. However, the rates of fading of the dyes in aerobic solutions are only partly determined by the electron affinities of dye and oxygen. Other factors, such as the relative rates of intermediate reactions and the occurrence of as yet unidentified side‐reactions, determine to an important extent the sequence of stability of the azo dyes during irradiation in aerobic solutions.
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