High pressure–low temperature calorimetry: I. Application to the phase change of mercury under pressure

The melting and crystallization behaviour of pure mercury under pressure was experimentally investigated using a scanning calorimeter, which is a sensitive Calvet type differential calorimeter combined with a computer controlled high-pressure pump driven by a stepping motor. In order to have a good control of calorimetric block temperature, starting from −75 °C, the jacket of calorimetric block was connected to a powerful cryostat, which is also computer controlled. This set-up allows to determine the latent heat of fusion/crystallization of mercury and the associated volume changes. Typically, one of the independent variable (p, V, or T) is kept constant, another one is changed with time, and both the enthalpy effect and the change of the remaining third variable are measured with high accuracy. Fusion/crystallization of mercury was investigated both during temperature and pressure scans and n-propanol was used as pressurization fluid. Both methods gave high accuracy data of latent heat of fusion/crystallization of pure mercury. The data obtained by the two different methods are comparatively discussed and making use of Clapeyron equation, the pressure and temperature derivatives of the mercury melting temperature were calculated. A special attention was paid to the manner in which the pressurization rate affects the calorimetric signal.