Raman spectroscopy of aqueous phosphate solutions at temperatures up to 450 °C. Two liquid phases, supercritical fluids, and pyro- to ortho-phosphate conversions

Abstract

A new high-temperature Raman furnace-cell assembly has been built and used to obtain Raman spectra of aqueous phosphate solutions at temperatures of 25–450°C. The solutions studied were those of mono- and di-sodium orthophosphates and pyrophosphates, and of phosphoric acid. Rates of hydrolysis of the pyrophosphates to orthophosphates at temperatures between 100 and 250°C were observed. The Raman spectra of both the highly concentrated and dilute immiscible solutions, formed at temperatures above 275°C at particular compositions, and the Raman spectra of supercritical fluids containing the dissolved phosphate species were recorded. These observations confirm the strong stability of the ortho-phosphate species at the high temperatures although weak pyrophosphate peaks appear in several cases. Concetrated second liquid phases show essentially the same orthophosphate species as those in the dilute liquid phases in equilibrium. The Raman spectra of water in these solutions show characteristics at 300–450°C very similar to those observed in the Raman study by Lindner of pure water.