AvapH data for various groups of substances in the interval T = 298-373 K with an accuracy
o f 0.1-0.3 per cent.
2.7.2 Drop Calorimetry Method
The method of drop calorimetry has been proposed for measurement of vaporization enthalpy
[ 103-105]. The sample at room temperature is dropped into a Calvet-type calorimeter with a
certain enhanced temperature, which should be sufficiently high to provide a rapid vaporization
or sublimation of a sample. The substance is then rapidly heated to the temperature of the
calorimeter and vaporizes. The amount of heat transferred is obtained from the peak area in
the calorimetric signal. This method is very simple and fast, the measuring error is 1-1.5
per cent for a minimal sample amount of less than 5 mg for calorimeter temperatures of
320-1000 K. Kiyobayashi and Minas da Piedade [105] have described in detail the apparatus,
operating procedure and tests.
2.7.3 Differential Calorimetry
The measurement of vaporization enthalpies using a commercial Calvet-type microcalorimeter
(Setaram BT 2.15) without modification and using an original design of the vaporization
vessel is reported by Viton et al. [106]. A modified commercial heat-flow differential
microcalorimeter of the Calvet type was developed for the determination of enthalpies of
vaporization [ 107]. A special calorimetric cell (see Figure 2.5) was designed according to the
requirements of maximal thermal contact in the measuring zone and prevention of heat loss
from the measuring zone. The evaporation can occur either from the broken glass ampoules,
or from the special metal ampoules through the orifice in the thin nickel membrane. The glass
ampoule is broken with a metal rod. The latter is fixed on a spring, which is moved with an
auxiliary Teflon rod. If metal ampoules are used, the orifice is made in advance and is covered
with a silicon cap placed on the butt-end of the metal rod. The hole is opened when the measurements
begin. It is more convenient if the membrane is pierced with a needle placed on the
rod during the experiment as shown in Figure 2.5. The power or the heat flow W is evaluated
from the relation W - AE/K, where AE corresponds to the temperature difference between the
core of the calorimeter and the evaporating camera and K is the thermal constant of the cell.
The constant for the cells is determined from the results of measurements of enthalpies of
vaporization for the reference substances.
2.8 Conclusions
Many experimental techniques for the determination of the vapour pressure are described in
the literature, but no single method is applicable for the entire vapour pressure range of industrially
significant compounds (--- 105-10 -6 Pa). The choice of a method depends largely on the
temperature and pressure range concemed, although there may be considerable overlap