SIMULTANEOUS THERMOMETRIC DETERMINATION OF MANGANESE AND LEAD BASED ON THEIR CATALYTIC EFFECT ON THE OXIDATION OF TIRON BY HYDROGEN-PEROXIDE

A kinetic thermometric method was developed for the simultaneous determination of manganese and lead based on the catalytic action of Mn2+ and the activating effect of Pb2+ on the oxidation of Tiron by hydrogen peroxide. The reaction was monitored by the initial-rate method, which was applied to temperature-time curves. The problem was addressed in two ways. In the first, different concentrations of Mn2+ were added to construct several calibration graphs by plotting the slopes of the thermometric curves as a function of the Mn2+ concentrations. From the slopes of these graphs a new calibration graph was obtained in terms of the Pb2+ concentration. By applying the traditional simple standard addition method to the sample to be assayed a third graph was obtained, the slope and intercept of which allowed the concentrations of Pb2+ and Mn2+, respectively, to be determined. In the second alternative a general mathematical expression was used taking account of the contribution of the non-catalytic effect, the Mn2+-catalysed reaction and the activating effect of Pb2+. By using the optimized values of the different experimental variables involved, Mn2+ and Pb2+ were determined over the linear concentration ranges 0.5-10 and 50-2000 ng ml-1, respectively. The proposed method is subject to few interferences and was successfully applied to the determination of the two metal ions in natural waters.