92168.pdf

40 Kinetic properties

the solute with respect to solvation, etc., which was used in

establishing the concentration of the solution. For polydispersed

systems a number average is measured.

Measurement of osmotic pressure

A great deal of work has been devoted to the preparation of suitable

semipermeable membranes and to perfecting sensitive methods for

measuring osmotic pressure^33 '^34.

Certain practical difficulties arise if the solution is simply allowed

to rise and seek its own equilibrium level:

1. If the liquid rises up a narrow tube and the total volume of
   solution is large, the liquid level may be too sensitive to
   temperature fluctuations during the course of the experiment to
   permit reliable measurements. In addition, a capillary rise
   correction must be made under these conditions. To overcome the
   difficulty associated with a sticking meniscus, which is often
   encountered in the study of aqueous solutions, a liquid of low
   surface tension and good wetting properties, such as toluene or
   petrol ether, is used in the capillary.

(1)

Figure 2.8 Dependence of reduced osmotic pressure on concentration: (1) a linear

high polymer in a good solvent; (2) the same polymer in a poor solvent; (3) a globular

protein in aqueous solution