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
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).