Studies on Azo Compounds: Part VII-Protonation Constants and Kinetics of Some Thiazole Azo and Azomethine Dyes

~I The UV, visible and IR spectra of some thiazole azo dyes and their cationic isomers were reported earlier1• Thiazole and its derivatives act as proton acceptors and hence they should possess basic character. The active centre of protonation is the imino-nitrogen of the thiazole ring2 -6. Danek et al.3 determined the pka values of some 2-amino-5-nitrothiazoles spectropho­ to metrically, while Joshi et al.4 determined the pka values of a number of thiazoles and benzothiazoles using a pH-meter in 50% aq. dioxane (vol./vol.) at an ionic strength (11) of 0.1. Chanon et al. 5 argued that the protonation reaction on N was less sensitive to steric effects than acylation. Thus, the spectra of some thiazole azo dyes (0 and azomethine dyes (II) were studied in universal buffer solutions (BR buffers) of varying pH values.