The stoichiometry of formation and the kinetics of reduction of molybdothorophosphoric acid in nitric acid solutions were studied spectrophotometrically. The Mo:Th:P ratio in the complex is 12:1:1, assuming that Mo(VI) exists in acid solutions as a dimer. The ternary heteropoly acid is more stable and is reduced more slowly to the heteropoly blue than 12-molybdophosphoric acid. The kinetics of the fist stages of the reduction were studied with ascorbic acid and p-methylaminophenol as reductants. The experimental rate law is in good agreement with a mechanism involving an equilibrium step in formation of the heteropoly acid followed by the reduction. Under recommended conditions, Beer's law was obeyed up to 40 mg 1-l thorium.
Methods have been reported for determining thorium based on the formation [l] and reduction [ 21 of a thorium molybdophosphoric acid. These __.
-_-~~ methods require strict control of time and pH, and removal of the interference of the simultaneously formed 12-molybdophosphoric acid (12-MPA). The formation of the molybdothorophosphoric acid, MThPA, was investigated [l] , but there are no data for the kinetics of the reduction reaction. These kinetics were studied here, to clarify the nature and composition of MThPA and to provide a theoretical basis for improving the existing analytical methods.
Experimental
Reagents. Stock molybdate solutions were prepared from reagent-grade NazMo04 -2Hz0 and stored in polyethylene bottles. Working solutions were made from the nitrate and used immediately, or prepared in nitric acid to ized nitric acid. Phosphate solutions were prepared from reagent-grade, oven-dried dipotassium hydrogenphosphate. Stock thorium solutions were made from the nitrate and used immediately, or prepared in nitric acid to avoid hydrolysis. Ascorbic acid and p-methylaminophenol sulphate were used as reductants. Ascorbic acid solutions were prepared just prior to use. All other reagents were of analytical grade.
Spectropho tome tric measurements. Spectrophotometric measurements of the formation of MThPA were made with a Beckman DBGT spectrophotometer, equipped with digital readout, at 350 nm, at which it was shown that