Raman spectra of a number of azo dye structures have been analyzed. In azonaphthol and azopyrazol dyes, hydroxyazo to ketohydrazone tautomerizations, —N=N—C=C—OH →← —HN—N=C—C=O were observed in aqueous basic and acidic media. Observations of azo and ring vibrations, as well as hydrazone (C=N plus C=O) bands, were made. It was noted that the azo-anions (examined at pH 12) convert to the protonated neutral hydrazone (keto) form in aqueous acid (run at pH 2). Infrared spectra were used to confirm the presence of the C=O group in keto-tautomeric forms.

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