Two-Phase Isochoric Heat Capacity Measurements for Nitrogen Tetroxide in the Critical Region and Yang–Yang Relation

The two-phase isochoric heat capacity of nitrogen tetroxide was measured in the temperature range from 261.74 K to the critical temperature (431.072 K) at densities between 201.21 and 1426.5 kg } m&3 using a high-temperature and high-pressure adiabatic calorimeter. The measurements were performed in the two-phase region for 26 isochores (15 liquid and 11 vapor densities) including the coexistence curve and critical region. Uncertainties of the measurements are estimated to be 20. The original temperatures and CV data were converted to the ITS-90. The liquid and vapor two-phase isochoric heat capacities, tempera- tures, and densities at saturation were extracted from experimental data for each measured isochore. From measured (TS , \S , C$V2 , C"V2) data, the values of second temperature derivatives of vapor-pressure d 2PS dT 2 and chemical poten- tial d 2+ dT 2 were derived using the Yang Yang relation. The results were com- pared with values calculated from other vapor-pressure equations. The values of saturated densities and critical parameters derived in calorimetric experiments were compared with literature data. The unusual temperature behavior of d 2PS dT 2 and d 2+ dT 2 was found at low temperatures around 351 K and near the critical point.