Phosphoric acid dissociation equilibria in aqueous solutions to 300°C

Abstract

The dissociation equilibria of phosphoric acid were studied potentiometrically using a hydrogen-electrode concentration cell to 300°C at KCl concentrations up to 1.0m. Least-squares analysis of data atP(V) concentration of 0.008 and 0.045m indicate the absence of significant amounts of polyphosphates up to 200°C.P(V) data at concentrations less than 0.01m extending to 300°C were analyzed in terms of the species H _n PO ⁽ⁿ⁻³⁾₄ . The equilibrium quotients expressed for neutralization reactions have been fitted with a nine-parameter expression for the first stepwise neutralization and 11 parameters for the second. Fewer data were obtained for the third neutralization since the HPO₂₋ becomes a strong base above about 150°C in water. Thermodyanamic parameters have been derived for these equilibria, and comparisons have been drawn with other similar data recently reported. There is evidence from the salt effects that the extent of association of HPO₄ ²⁻ or H₂PO ⁻₄ with K⁺ is not great in 1m KCl.