Thermodynamics of the monohydrogen difluorides V. Melting thermodynamics of NH4HF2

The heat capacity of ammonium hydrogen difluoride was extended from 300 to 450 K by adiabatic calorimetry in order to study the melting phenomenon and provide thermodynamics into the liquid region. The corrected enthalpy of melting at the triple-point temperature, 399.6 K, was found to be (4564 ± 2) calth mol−1 and the corresponding entropy increment is (11.42 ± 0.01) calth K−1 mol−1. Our enthalpy of melting is at variance with a published value (3940 calth mol−1) derived from cryoscopic measurements and is large compared with the sum of transition and melting enthalpies for the nearly isostructural alkali hydrogen difluorides. Heat capacities in excess of 104 calth K−1 mol−1 in the essentially isothermal region and significant premonitory thermal effects were observed.