Influence of Visible Light and Malonic Acid Derivatives on the Autocatalytic Oxidation of Ru(II) by Bromate Ions

The influence of visible light on the autocatalytic oxidation of Ru(II) by bromate ions has been investigated. The inflection time (IT) was measured in darkness, at constant illumination, or as a function of the length of a perturbing light pulse. Measurements were performed without organic substrate, as well as in the presence of the malonic acid derivatives HOOC-CHR-COOH (RMA), with R being H, methyl-, ethyl-, n-butyl-, or the benzyl-group. The presence of RMA influences the length of the inflection time in darkness and for light-perturbed conditions. In the presence of benzylmalonic acid (BzMA), small amplitude oscillations are observed in darkness at the end of the autocatalytic oxidation of Ru(II). The most pregnant effects of RMA are observed by malonic acid (MA). At constant illumination, the inflection time is found to increase with increasing MA concentration and small-amplitude oscillations can even be observed in the preexponential phase of the Ru(II) oxidation. Model calculations based on a modified Noyes-Field-Thompson mechanism are in accordance with experimental observations. The calculations suggest that the small-amplitude oscillations observed in the BzMA and MA systems are due to second-order organic radical recombination.