Formation of monofluorophosphate from fluoride in phosphoric acid – water and phosphoric acid – sulfuric acid – water mixtures

When dissolved in concentrated phosphoric acid, sodium fluoride reacts rapidly to form monofluorophosphate. In less concentrated acid, the reaction does not proceed to completion, and the reaction kinetics become very much slower. The equilibrium and rate constants for the reaction have been determined. In ternary mixtures of phosphoric acid, sulfuric acid, and water, the formation of monofluorophosphate is considerably enhanced, and the kinetics are rapid. The results are interpreted in terms of the very low water activity coefficients in strong-acid solutions.Key words: monofluorophosphate, monofluorophosphoric acid, 19F NMR, 31P NMR, phosphoric acid, sulufric acid, equilibria, kinetics.