Handbook of Solvents - George Wypych - ChemTech - Ventech!

156 Christian Wohlfarth
Figure 4.4.2. Schematic of the common principle of an
absolute vapor pressure apparatus: 1 - polymer solution,
2 - connection to the manometer, 3 - Hg-manometer, 4 -
heating coils. The whole construction is thermostated at
the measuring temperature, the connection to the manometer
is kept slightly above the measuring temperature
to avoid condensation.
be eliminated. Equilibration of all prepared
solutions is very time-consuming (liquid oligomers
need not so much time, of course).
Increasing viscosity makes the preparation
of concentrated solutions more and more difficult
with further increasing amount of
polymer. Solutions above 50-60 wt% can
hardly be prepared (depending on the solvent/polymer
pair under investigation). The
determination of the volume of solvent vaporized
in the unoccupied space of the apparatus
is difficult and can cause serious errors
in the determination of the final solvent concentration.
To circumvent the vapor phase
correction, one can measure the concentration
directly by means, for example, of a differential
refractometer. The contact of
solvent vapor with the Hg-surface in older
equipment may cause further errors. Complete
thermostating of the whole apparatus is
necessary to avoid condensation of solvent
vapors at colder spots. Since it is disadvantageous
to thermostat Hg-manometers at
higher temperatures, null measurement in-
Figure 4.4.3. Schematic diagram of a modern absolute vapor pressure apparatus: T - temperature meter, P-vapor-pressure
meter, V - vacuum meter, Z - measuring cell, M - magnetic stirrer, GI and GII - degassing units for the
solvent and for the polymer. [Reprinted with permission from Ref. 58, Copyright 1990,Wiley-VCH].