Thermodynamic investigation of disorder in the hydrates of disodium hydrogen phosphate

Abstract

Measurements have been made of the molar heat capacity of the dihydrate, heptahydrate and dodecahydrate of disodium hydrogen phosphate from ∼10 to ∼300°K, of the integral heat of solution in water at 25°C of these three hydrates and of the anhydrous salt, and of the water vapour dissociation pressure at 25°C for the salt pairs anhydrous salt + dihydrate, dihydrate + heptahydrate, and heptahydrate + dodecahydrate. The calorimetric entropies at 25°C have been compared with the values of the second-law entropies obtained from the experimental entropy changes for the three equilibria and the calorimetric entropy of the anhydrous salt. For the dihydrate and heptahydrate, the calorimetric and second-law entropies agree within experimental error, implying that there is no disorder in these salts at 0°K. For the dodecahydrate, however, the calorimetric entropy is less than the second-law entropy by 3.5 ± 0.5 cal/mole deg. Possible causes of the residual disorder in this hydrate are briefly discussed.