Kinetic method for the determination of traces of thyroxine by its catalytic effect on the Mn(III) metaphosphate-As(III) reaction

A new, highly sensitive and simple kinetic method for the determination of thyroxine was proposed. The method was based on the catalytic effect of thyroxine on the oxidation of As(III) by Mn(III) metaphosphate. The kinetics of the reaction was studied in the presence of orthophosphoric acid. The reaction rate was followed spectrophotometrically at 516 nm. It was established that orthophosphoric acid increased the reaction rate and that the extent of the non-catalytic reaction was extremely small. A kinetic equation was postulated and the apparent rate constant was calculated. The dependence of the reaction rate on temperature was investigated and the energy of activation and other kinetic parameters were determined.

Thyroxine was determined under the optimal experimental conditions in the range 7.0 × 10^?9 to 3.0 × 10^?8 mol L^?1 with a relative standard deviation up to 6.7% and a detection limit of 2.7 × 10^?9 mol L^?1. In the presence of 0.08 mol L^?1 chloride, the detection limit decreased to 6.6 × 10^?10 mol L^?1. The proposed method was applied for the determination of thyroxine in tablets. The accuracy of the method was evaluated by comparison with the HPLC method.