Thermodynamic investigation of sodium pyrophosphate decahydrate for residual disorder

Abstract

The heat capacity of sodium pyrophosphate decahydrate has been measured from ∼10 K to ∼300 K. Measurements have also been made, at 25°C, of the heat of solution of the decahydrate and of the anhydrous salt, and of the dissociation pressure in the equilibrium between the decahydrate, anhydrous salt, and water vapour. The value of 164.77 ± 0.2 cal K^–1 mol^–1 for the entropy of the decahydrate obtained from the heat capacity results agrees, within the limits of experimental uncertainty, with that of 164.62±0.3 cal K^–1 mol^–1 derived from the equilibrium study. The conclusion that the decahydrate achieves perfect order at 0 K is consistent with previous X-ray structural studies.