Kinetic properties 41- If the liquid rises up a wide tube and the volume of solution is
small, significant changes in concentration will take place and the
establishment of equilibrium will be extremely slow,
A frequently adopted procedure for overcoming these difficulties is
to set the liquid level in turn at slightly above and slightly below the
anticipated equilibrium level and then plot its position as a function
of time. If the estimation of the final equilibrium level has been
reasonably accurate, the two curves will be almost symmetrical, and
by plotting the half-sum as a function of time the equilibrium level
can be reliably estimated after relatively short times (Figure 2.9).
This procedure permits the use of a moderately wide capillary tube;
however, good thermostatting and a firmly supported membrane are
still required.
TimeFigure 2.9 Estimation of osmotic pressure by the half-sum method
In the Fuoss-Mead osmometer^139 the membrane is firmly clamped
(and also acts as a gasket) between two carefully machined stainless
steel blocks provided with channels into which small volumes of
solvent and solution are introduced (Figure 2.10). Owing to the high
ratio of membrane surface area to solution volume (c. 75 cm^2 :
15 cm^3 ), the approach to equilibrium is rapid. With last membranes,
and using the half-sum method, measurements in well under 1 h are
possible. Over such a short time the membrane can be slightly
permeable to the macromolecules without introducing serious errors.