The kinetics and mechanism of the silver(I) ion catalysed oxidation of 3,7-bis(dimethylamino)phenothionium chloride (methylene blue) by peroxydisulphate in aqueous nitric acid have been studied. The reaction is acid independent in the range 0.1x10-4 < or = [H+] < or =100x10-4 mol dm-3. The reaction was also first order each in [Ag+], [S2O82-] and [MB+]. The reaction occurs via two successive univalent changes generating the highly reactive sulphate ion radical. Outer -sphere mechanism is proposed for this reaction.

Keywords: acid-independent, methylene blue, outer-sphere, oxidation, rate constants, silver(I)catalysed, sulphate ion radical.