LASER RAMAN-SPECTROSCOPIC STUDY OF AQUEOUS ORTHO-PHOSPHATE SALTS

Raman spectroscopy has been used to study aqueous solutions of orthophosphate salts from 0.005 M to saturation levels. Both H2PO4 and HPO42- salts show changes in band half-widths and frequencies with changing concentration and temperature. In particular, the growth of a low-frequency shoulder of the P-O stretching bands of H2PO4- (1074 cm-1) and HPO42- (987 cm-1) with increasing concentration appears to be characteristic of hydrogen-bonded phosphate dimers or higher polymers in solution. From resolution of the bands, the association constants for H2PO4- + H2PO4- ⇌ H4P2O82- and HPO42- + HPO42- ⇌ H2P2O84- were estimated to be 0.6 ±0.3 and 0.3 ± 0.1 L mol-1, respectively. Na3PO4 and K3PO4 solutions undergo hydrolysis to form HPO42-; it was possible to measure the solution concentration of HPO42- and obtain a concentration quotient, Qh, for the hydrolysis. Qh was approximately 0.0084 at the lowest concentrations, and then decreased more rapidly with concentration for the sodium than for the potassium salt. ΔH for the third dissociation of phosphoric acid was estimated to be +5.8 kcal mol-1 for K3PO4 and +4.8 kcal mol-1 for Na3PO4 (measured from temperature dependence of the hydrolysis of 0.1 M solutions).