Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 161
Figure 4.4.7a. Isopiestic vapor -
sorption apparatus using a quartz
spring: 1 - connection to the vacuum
line, 2 - connection to the
thermostating unit which realizes
the constant measuring temperature
T 2 (the correct value of
T 2 is obtained by a Pt-100 resistance
thermometer within the
cell that is not shown), 3 - closing
plug, 4 - quartz spring (reading
of its extension is made by a
cathetometer), 5 - sample pan
with the polymer solution, 6 -
pure solvent reservoir at temperature
T 1. [Reprinted with permission
from Ref. 82, Copyright
1982, Wiley-VCH].
Figure 4.4.7b. Dynamic isopiestic
vapor-sorption apparatus using
a quartz spring (drawing
provided by G. Sadowski): a)
evaporator, b) superheater, c)
measuring cell, d) condenser, e)
quartz spring, f) polymer sample/solution,
g) Pt-100 resistance
thermometer. [Reprinted with
permission from Ref. 87, Copyright
1995, Wiley-VCH].
ture. Changes by thermal ageing
during the experiment must be
avoided. The temperatures provided
by the thermostats must not
fluctuate more than ±0.1 K. Condensation
of solvent vapor at
points that become colder than T 2
has to be avoided by slight overheating
(this problem may arise at
the closing plug and at the connection
between the pure solvent reservoir
flask and the double-walled
jacket). An intelligent improvement
of this compact apparatus
was made by Sadowski, 87 see Figure
4.4.7b, where this vapor-sorption
apparatus is combined with
technical solutions from
ebulliometry (more about
ebulliometers can be found below).
In comparison to the usual
ebulliometric equipment where
the polymer solution is placed into
the evaporator, only pure solvent
is evaporated. The vapor flows
through the cell and is condensed
at its head-condenser to flow back
into the reservoir at the bottom.
The vapor pressure is kept constant using a manostat and is
measured additionally outside the apparatus after the condenser.
Equilibrium times decrease somewhat, degassing of
the solvent is not necessary, air leakage does not play any role.
As was stated by different authors, additional measure-
ment of the vapor pressure inside the isopiestic sorption apparatus seems to be necessary if
there is some doubt about the real pressure or if no reliable pure solvent vapor pressure data
exist for the investigated temperature range. Figure 4.4.8 shows an apparatus used by the
author for measurements between room temperature and 70 o C and pressures up to 1.5 bar. It
combines mercury float valves with Hg-manometers to avoid the use of any grease within
the measuring system, a kind of equipment proposed earlier by Ashworth and Everett. 88 Up
to four quartz springs can be inserted into the equilibrium cell (only one is shown). Reading
of the manometer and of the extension of the quartz spring was made using a cathetometer.
The direct pressure measurement has the advantage that absolute pressures can be obtained
and pressure fluctuations can be observed. More modern equipment applies electronic
pressure sensors instead of Hg-manometers to avoid the problems caused by the