Thermal lens determination of vandium(V) and its activators by oxidation of aniline by bromate ions

Abstract

A dual-beam thermal lens spectrometer was used to study the possibilities of utilizing thermal lens spectrometry in kinetic methods as exemplified by the well known oxidation of aniline by bromate ions catalysed by vanadium(V) in the presence of 8-hydroxyquinoline or pyrocatechol as catalyst activators. This approach allowed a 100-fold decrease in the limits of detection of vanadium (3 × 10^–10 mol l^–1) and a 10-fold decrease in the limits of detection of both activators (1 × 10^–5 mol l^–1 for 8-hydroxyquinoline and 2 × 10^–5 mol l^–1 for pyrocatechol). It was found that the thermal lens kinetic curve, after an induction period, becomes nearly exponential and rapidly grows to the maximum thermal lens signal. This phenomenon can be accounted for by the accelerated formation of a colloidal solution of the reaction products, which are poorly soluble in water, in the volume irradiated by the laser beam. The parameters of the exponential curve depend on the concentrations of the reductant and the activator. The sensitivity of the determination of 8-hydroxyquinoline and pyrocatechol by the use of the exponential curve is 5 × 10^–6 and 1 × 10^–5 mol l^–1, respectively. New parameters of the curve can be used to develop procedures for the simultaneous determination of the participants of the reaction.