Transitiometry Basic principles

     Scanning transitiometry is a new technique which consists in inducing a transition or a change of state in a substance under investigation by a controlled variation of an independent thermodynamic variable, while the other independent variable is kept precisely constant. The output signals simultaneously recorded of such a process are the heat effect and the variations of the dependent variable.
Fig. 1 presents a scheme of thermodynamic relations between variations of the chosen independent variable (p, V or T) and the resulting rate of heat exchange (q) (thermal power). One can see that beside the well known temperature-controlled scanning calorimetry (e.g. classical DSC), where the recorded thermal power signal is proportional to the respective heat capacity,

Fig. 1. A scheme of thermodynamic foundations for various types of pVT-controlled-scanning calorimetry (for more explanation and further details see ref. 3).

there are two other important techniques: 1.) pressure-controlled scanning calorimetry, where the recorded thermal power signal is proportional to the thermal expansion and 2.) volume-controlled scanning calorimetry, where the recorded thermal power signal is proportional to the pressure coefficient.
Fig. 2 presents relations between the scanned independent variable and the dependent variable, while the other independent variable is kept precisely constant. The chosen independent variables are scanned at a very low rate in order to keep the system under investigation near the equilibrium as close as possible.

Fig. 2. Relations between the scanned independent variable and the dependent variable while the second independent variable is kept constant.

Fig. 3. A thermodynamic functional scheme of scanning transitiometry (for more explanation and further details see refs. 23, 24).

      Combining the relations between the scanned variable and the rate of heat exchange (thermal power) presented in Fig. 1 with the relations between the scanned independent variable and the dependent variable presented in Fig. 2, one can realise four metrological situations presented in Fig. 3. Depending on the choice of the variables, a pair of thermodynamic derivatives (always thermal and mechanical) is simultaneously determined in each situation. This practically leads to a simultaneous determination of the two most important contributions to the thermodynamic potential change for the process under investigation: thermal and mechanical. This was the principal reason to call the new technique transitiometry, from Latin transitio - change and Greek με'τρον - measure. Defining it in a more general way, such transitiometric measurements can lead directly to a simultaneous determination of thermal and mechanical equations of state for a substance under investigation.

Fig. 4. Educational presentation: transitiometric investigation of fusion of benzene at various thermodynamic conditions (for further details see refs. 7, 23).