Photo-reduction of Methylene Blue and Thionine by Ferrous Sulphate using Flash Irradiation

Abstract

WHEN dilute solutions of methylene blue in 0.1 N sulphuric acid are subjected to flash photolysis with orange-red light in the presence of ferrous sulphate (∼ 0.1 M), the kinetic curves obtained by monitoring at various wave-lengths can all be interpreted in terms of the following reaction scheme: By direct measurement, the rate constant (k _d) for the dismutation of the semiquinone radical (B ⁻) was found to be of the order 1.5 × 10⁹ l. mole⁻¹ sec.⁻¹. By observing the degree to which the formation of leuco methylene blue (B ²⁻) was suppressed by the addition of ferric iron, the value of k ₁/k _d and hence k ₁ (∼ 2.4 × 10⁵ l. mole⁻¹ sec.⁻¹) was obtained. The latter is so much smaller than k _d that the quantity of ferric iron produced by the photochemical reaction has no appreciable effect on the dismutation of the semiquinone radical.