kinetics and mechanism of reaction of acidic chlorite with phenoxazine dyes, Nile blue and Meldola’s blue

The kinetics and mechanism of the oxidation of two phenoxazine dyes namely Nile blue (7-amino-3-diethylamino-8,9-benzo phenoxazine chloride, NB+) and Meldola’s blue (3- dimethylamino-8,9-benzo phenoxazine chloride, MB+) with acidic chlorite and hypochlorous acid have been investigated using a UV-visible and a stopped flow equipment. For both Nile blue and Meldola’s blue reactions the rates have first-order dependence on each substrate, chlorite and acid. Both reactions showed negative salt effect indicating the reaction is between the oppositely charged species, likely the substrate cation and chlorite anion. The acidic chlorite reaction with MB+ was very slow compared with NB+ and was studied at higher temperature of 40 oC. The overall third order rate constants for the reaction of acidic chlorite with Nile blue and Meldola’s blue were (0.363 plus or minus 0.005) M-2 s-1 at 25 oC and (3.09 plus or minus 0.08) x 10-3 M-2 s-1 at 40 oC, respectively. The energy parameters for NB+ reaction were Ea = 47.8 kJ mol-1, H = 40.4 kJ mol-1 and S = -233 J K-1 mol-1, while the corresponding values for MB+ reaction were 62.4 kJ mol-1, 54.6 kJ mol-1 and -248 J K-1 mol-1, respectively. The second-order rate coefficients for HOCl reaction with Nile blue and Meldola’s blue at 25 oC were (5.14 plus or minus 0.01) x 103 M-1 s-1 and (1.25 plus or minus 0.03) x 102 M-1 s-1, respectively.