Direct and indirect photolysis of water‐soluble azodyes: Kinetic measurements and structure‐activity relationships

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Abstract
Azodyes dissolved in water containing humic acid (total organic carbon = 5 mg/L) and irradiated with a xenon tamp were transformed primarily by an indirect photoprocess involving 1O2 and/or oxyradicals. Dissociation of the phenolic/naphtholic OH groups of the dyes was the dominant factor in increasing indirect photolysis rates. Simplified structure‐activity relationships were developed based on pKa prediction methods and on the observation that the reactivities of the dissociated forms of all the dyes are nearly the same. Products of indirect photolysis could not be identified, but loss of color suggests cleavage of the azo linkage in nearly all cases. Aromatic amines are not likely to be formed.