The Degradation of Water-soluble Azo Compounds by Dilute Sodium Hypochlorite Solution

The decolorisation of azo dyes by sodium hypochlorite solution has been followed spectrophotometrically and a technique developed for determining the half-life of the process as a convenient measure of the reaction rate. A study of a series of 2-arylazo-1-naphthol-3,6-disulphonic acids suggests that electronic, steric and possibly other effects influence the reaction rate, and that degradation involves initial attack at a nitrogen atom of the hydrazone form, by the electrophilic chloronium ion. Sulphonic and carboxylic acid groups located ortho to the azo linkage are particularly effective in retarding the rate of degradation. The dyes 4-(4-sulphophenylazo)-1-naphthol (I) and 4-(4-sulphophenylazo)-1-(2-chloro-4-amino-s-triazinylamino)-naphthalene-6-sulphonic acid (II) were investigated in more detail to determine (i) the effect of pH on the reaction rate, (ii) the number of molecules of sodium hypochlorite involved in the decolorisation reaction (iii) the nature of the decomposition fragments. This information, together with the fact that the O- and N- methyl homologues of I were extremely resistant to degradation, enabled a mechanism to be postulated for the reaction