Accurate DSC measurement of the phase transition temperature in the TBPB-water system

There is some disagreement in the literature on the phase transition temperatures of salt semi-clathrates, which melt incongruently due to the change of salt concentration in the liquid phase. By using differential scanning calorimetry (DSC), the phase transition temperatures of tetrabutylphosphonium bromide (TBPB) hydrates were investigated at 0.10 < w_TBPB < 0.37 and atmospheric pressure. The freezing point of ice with the presence of NaCl was also studied as a reference. In this work, two DSC procedures were employed: dynamic and stepwise schemes, characterized by the maximum peak temperature T_peak and inflection point temperature T_end, and final step temperature T_step, respectively. The stepwise DSC scheme was validated by the measurement of the ice freezing point at w _NaCl = 0.10, 0.15 and 0.18. For the dynamic scheme, we developed and tested a method to estimate the melting temperature of TBPB hydrates. This method, though having a higher uncertainty, was more efficient than the stepwise scheme. The phase transition temperatures of TBPB hydrates are presented with the accuracy of 0.1 K and compared with literature data.