H. Bianchi et Pr. Tremaine, THERMODYNAMICS OF AQUEOUS PHOSPHATE SOLUTIONS - APPARENT MOLAR HEAT-CAPACITIES AND VOLUMES OF THE SODIUM AND TETRAMETHYLAMMONIUM SALTS AT 25-DEGREES-C, Journal of solution chemistry, 24(5), 1995, pp. 439-463
A Picker flow microcalorimeter and a flow densimeter, were used to obt
ain apparent molar heat capacities and apparent molar volumes of aqueo
us solutions of Na3PO4 and mixtures of Na2HPO4 and NaH2PO4. Identical
measurements were also made on solutions of tetramethylammonium salts
to evaluate the importance of anion-cation interactions. The experimen
tal apparent molar properties were analyzed in terms of a simple exten
ded Debye-Huckel model and the Pitzer ion-interaction model, both with
a suitable treatment for the effect of chemical relaxation on heat ca
pacities, to derive the partial molar properties of H2PO4-(aq), HPO42-
(aq) and PO43-(aq) at infinite dilution. The volume and hear capacity
changes for the second and third ionization of H3PO4(aq) have been det
ermined from the experimental data. The importance of ionic complexati
on with sodium is discussed.