Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.4 Measurement of solvent activity 179
Figure 4.4.17. Description of the principal construction of a Knauer
membrane osmometer A 300: 1 - head thermostat, 2 - channel for syringe,
3 - calibration device with suction tube, 4 - calibration glass, 5
- capillary position MEASUREMENT, 6 - capillary position
CALIBRATION, 7 - tension screws, 8 - cell retaining disc, 9 - upper
half of measuring cell, 10 - sample introduction system, 11 -
semipermeable membrane, 12 - lower half of measuring cell, 13 -
pressure measuring system, 14 - cell thermostat, 15 - suction of calibration
bottle. [Reprinted from the operating manual with permission
from Dr. H. Knauer GmbH (Germany)].
thermodynamic situation is illustrated
in Figure 4.4.16.
Technical details of the different
apparatuses will not be presented
here, however, the
principle construction of the measuring
cell and the heating thermostat
of the Knauer membrane
osmometer A 300 is shown in Figure
4.4.17 for illustration and as
example.
As a general feature of most
osmometers, the membrane is
clamped into a stainless steel
thermostated chamber (the measuring
cell and the pressure measuring
system of modern
osmometers are built into a
high-grade electronically stabilized
thermostat) and serves as
barrier between the pure solvent
and the polymer solution sides of
the chamber. The solvent side
(bottom) is in juxtaposition with a
pressure sensor, e.g. the diaphragm
of a capacitance strain
gauge or a piezo-chip. The solvent
transport is measured across the
bottom side of the membrane in
the direction of the solution which
is topside the membrane. The
amount of flowing solvent is in the
range of 10 -6 ml and equilibrium is
established after some minutes
when hydrostatic pressure prevents further solvent flow. This is indicated by the electronics
of the equipment as well as any changes in equilibrium such as thermal drift of solute diffusion
through the membrane. Other osmometers apply compensation methods where the increase
of the hydrostatic height of the solution side is automatically compensated by
changing the filling height. Due to this procedure, only very small amounts of solvent have
to permeate through the membrane and equilibrium is reached within 10-20 minutes. The
classical procedure was only used in older laboratory designed instruments where one
started at zero and measured the hydrostatic height difference as a function of time until
equilibrium is reached. More details about instrumentation and techniques can be found in
the reviews by Adams, 38 Tombs and Peacock, 39 Mays and Hadjichristidis. 40 A very recent
presentation can be found in a new book edited by Pethrick and Dawkins. 26