The Oxidation of Azo Dyes and its relation to Light Fading

Abstract

It has been shown that oxidation of azo dyes in aqueous media, with several common reagents, including ceric sulphate, potassium dichromate, and hydrogen peroxide, disrupts the azo group and leads to the formation of a diazo compound and a quinone. Subsequently these further decompose to give, respectively, a phenol and nitrogen (in acid media) and phthalic acid. This reaction thus appears to be common to many types of oxidising agent, since previous workers have observed like effects with e. g. hypochlorite, lead dioxide, and nitric acid. An acceptor has been used to characterise the quinone formed. By boiling the acid solution after oxidising the dye a quantitative yield of nitrogen from the azo group is obtained, except when free amino groups are also present.A hypothesis is put forward suggesting that the initial step in such chemical oxidations is one of hydrolytic attack on the ‐C = N– bond of the hydrazone tautomer of the dye. This is shown to fit the facts reasonably well by a study of the relative ease of oxidation of several phenylazo dyes of both soluble and insoluble types, with different substituents in the benzene ring.An examination of available light fading data shows evidenoe of agreement with this theory in the case of insoluble azo dyes, but soluble dyes in many cases behave in an opposite sense.It seems unlikely that a chemical reagent can be found to fade dyes in the same manner as light exposure and thus be suitable for a rapid sorting test.