Studies on the Formation of 12-Tungstophosphoric acid: Application to the Determination of Phosphate

Abstract

<p>A new extraction-spectrophotometric method has been developed for the determination of phosphorus as orthophosphate. The method is based on the chemistry of the 12-tungstophosphoric acid, H3PW12)40 (TPA). In the method, the TPA is formed in aqueous solution from tungstate, phosphate and acid and is subsequently extracted into n-amyl alcohol. The formation of TPA was studied as a function of reagent concentration (e.g., tungstate and phosphate), pH, time and temperature. For particular molar ratios of tungstate and phosphate, quantitative formation of TPA occurred instantly when the pH was 1.3-1.9 and the solution heated to 60-100ºC. Under the conditions, Beer's law was found to be valid over the entire range of phosphate concentrations studied (0-25 ppm P). The method was successfully applied to a wide variety of phosphorus-containing materials and was found to be less prone to solution conditions and interfering species [especially As(V) and Si(IV)] than the widely used molybdenum-blue and molybdovanado-phosphate methods. The accuracy, precision and sensitivity of the TPA method is at least competitive with these methods.</p> <p>Under similar conditions, TPA was found to form from paratungstate Z and freshly-precipitated white tungstic oxide. From the metatungstate, TPA could be formed only in very acidic solution. It was not formed at all from yellow tungstic oxide.</p> <p>Since the formation of isopolytungstates is very relevant to the TPA method for the determination of phosphorus, appropriate studies of these species were made. These studies resulted in a re-interpretation of the point-titration curve of tungstate solutions, and in the discovery that a poorly-defined species called polytungstate Y is stable in n-amyl alcohol, from which it can perhaps be characterized. The extractability of isopoly species into n-amyl alcohol was studied as a function of pH. The results of this study were important with regard to the analytical application of TPA.</p>