Apparent Molar Volumes and Standard Partial Molar Volumes of Aqueous Sodium Phosphate Salts at Elevated Temperatures

Abstract

Densities and apparent molar volumes of aqueous NaH2PO4 (0.1 to 2.1) mol·kg−1, Na2HPO4 (0.1 to 0.5) mol·kg−1 and Na3PO4 (0.4 to 0.6) mol·kg−1 have been determined using a platinum vibrating tube densimeter at temperatures from 473 K to 598 K, 473 K to 570 K, and 373 K to 497 K, respectively, and a pressure of 15 MPa. For the monosodium and disodium phosphate salts, the Pitzer ion-interaction model was used to extrapolate the apparent molar volumes to infinite dilution and to obtain standard partial molar volumes, V°, of NaH2PO4(aq) and Na2HPO4(aq). These new results show that predicted values from the Helgeson−Kirkham−Flowers model overestimate V° for these two species. For the monosodium salt, the difference ranged from 5 cm3·mol−1 at 473 K to 24 cm3·mol−1 at 573 K, and for the disodium salt, the difference was 6 cm3·mol−1 at 473 K and 16 cm3·mol−1 at 570 K. The values for disodium phosphate show evidence of significant contributions from an ion pair, probably NaHPO4−(aq). The apparent molar volumes of N...