Redetermination of the thermodynamic properties of the solid-solid transition of adamantane by adiabatic calorimetry to investigate the suitability as a reference material for low-temperature DSC-calibration

Sub-ambient calibration of DSCs is a problem because there are few reference substances, and every available substance has its drawbacks. One of the possibilities is the low-temperature solid-solid transition of adamantane. High-accuracy adiabatic calorimetry results are, however, only available from one source. Therefore it was decided to measure the transition with our adiabatic calorimeter. Although our results show some deviation from the results reported in the literature source - our transition temperature (208.0 K) is 0.6 K lower and our enthalpy of transition (3213 J mol-1) is 5% smaller - we strongly believe this transition can be useful for the calibration of DSCs. Because the heat capacity of the low-temperature phase significantly increases well below the transition temperature, we propose to use a linear base line drawn as a tangent to the (almost) linear heat capacity curve above the transition region and start the integration at the temperature at which the drawn base line crosses the measured curve (at about 190 K). With this integration we found a heat of transition of 3094 J mol-1 (=22.7 J g-1)