SOME EFFECTS OF METAL-IONS ON THE PHOTOGENERATION OF HYDROXYL RADICALS

The effect of common transition metal ions on the photochemical degradation of a model compound, dequalinium chloride in aqueous solution, irradiated with simulated sunlight at 38 +/- 1 degrees C has been investigated. The reactions follow first order kinetics; the addition of a standard volume of hydrogen peroxide markedly enhances the reaction rate constants of these reactions, suggesting that the reactions are hydroxyl radical-mediated in a similar manner to that of the Fenton process. In order to evaluate the contribution of the hydroperoxy radical to photochemical degradation of pharmaceutical formulations as aqueous solutions, the effect of light-generated hydroperoxy radicals was investigated with the same group of metals. The reactions in this case followed second order kinetics and were very much slower than those mediated by hydroxyl radicals; only zinc ions showed a notable increase in the reaction rate constant as compared to that of the control system.