Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 157
struments with pressure compensation were sometimes used, e.g., Baxendale et al. 59 Modern
electronic pressure sensors can be thermostated within certain temperature ranges. If
pressure measurement is made outside the thermostated equilibrium cell, the connecting
tubes must be heated slightly above the equilibrium temperature to avoid condensation.
The measurement of polymer solutions with lower polymer concentrations requires
very precise pressure instruments, because the difference in the pure solvent vapor pressure
becomes very small with decreasing amount of polymer. At least, no one can really answer
the question if real thermodynamic equilibrium is obtained or only a frozen non-equilibrium
state. Non-equilibrium data can be detected from unusual shifts of the χ-function with some
experience. Also, some kind of hysteresis in experimental data seems to point to non-equilibrium
results. A common consistency test on the basis of the integrated Gibbs-Duhem
equation does not work for vapor pressure data of binary polymer solutions because the vapor
phase is pure solvent vapor. Thus, absolute vapor pressure measurements need very
careful handling, plenty of time, and an experienced experimentator. They are not the
method of choice for high-viscous polymer solutions.
(ii) Differential vapor pressure measurement
The differential method can be compared under some aspects with the absolute method, but
there are some advantages. The measuring principle is to obtain the vapor pressure difference
between the pure solvent and the polymer solution at the measuring temperature. Figure
4.4.4 explains the basic principle as to how it is used by several authors. References 66-75
provide a view of a variety of differential vapor pressure apparatuses developed and used by
different authors.
The polymer sample is put, after weighing, into
the sample flask and the apparatus is evacuated. Degassed
solvent is distilled into the measuring burette
and from there a desired amount of solvent is distilled
into the sample flask. The Hg-manometer is
filled from the storage bulb and separates the polymer
solution from the burette. Care must be taken to
avoid leaving any solvent in the manometer. The apparatus
is kept at constant measuring temperature,
completely immersed in a thermostat for several
days. After reaching equilibrium, the pressure is
read from the manometer difference and the concentration
is calculated from the calibrated burette meniscus
difference corrected by the amount of
vaporized solvent in the unoccupied space of the
equipment. The pure solvent vapor pressure is usually
precisely known from independent experiments.
Difference/differential manometers have some
Figure 4.4.4. Schematic diagram of a differential
vapor-pressure apparatus: 1 - connection
to vacuum pump, 2 - Hg-storage bulb, 3
- burette, 4 - Hg-manometer, 5 - polymer solution.
The whole apparatus is kept constant
at the measuring temperature within a thermostat.
advantages from their construction: They are comparatively
smaller and their resolution is much
higher (modern pressure transducers can resolve differences
of 0.1 Pa and less). However, there are the
same disadvantages with sample/solution preparation
(solutions of grams of polymer in some cm 3 vol-