The spectrofluorimetric determination of orthophosphate as quinine molybdophosphate

Abstract

Between 0·02 and 1·2 µg of phosphorus is determined as orthophosphate via the formation of a complex between a molybdophosphate and the alkaloid quinine. Quinine molybdophosphate is precipitated in 0·5 M sulphuric acid, the excess of quinine reagent is removed by washing the precipitate with 0·5 M sulphuric acid, and the complex is dissolved in the solvent mixture acetone-0·5 M sulphuric acid (9 + 1 v/v). The fluorescence intensity of this solution is measured at 445 nm with excitation at 352 nm. Optimum conditions for the determination have been established, and the effects of 100-fold weight excesses of each of thirty-eight foreign ions have been examined. The determination is selective; large amounts of silicate do not interfere, and arsenic(III) and tungsten(VI) are tolerated at 20 and 50-fold weight excesses, respectively. The nature of the quinine molybdophosphate complex has been examined.