The role of the ice rules in the thermal properties of potassium dihydrogen phosphate

Calorimetric properties of potassium dihydrogen phosphate are examined by analysis of the heat capacity data taken from the literature and from a recent measurement. The analysis is based on an extensive use of harmonic heat capacity functions to separate the effect of the phase transition from the vibrational contribution. The transition enthalpy and entropy derived are 421 J mol(-1) and 3.79 J K-1 mol(-1), respectively. Characteristic temperatures of the lattice vibrations including the Debye temperature (254+/-18) K were determined. The transition entropy, exceeding the value compatible with the ice-rules, is consistent with the temperature dependence of the heat capacity. The implication of the result is discussed by comparison with the hydrogen bond networks in copper formate tetrahydrate and thallium dihydrogen phosphate.