Osmometry Some Applications

To carry out an osmotic pressure experiment, we need to prepare a solution, find a suitable semipermeable membrane, achieve isothermal equilibrium, and measure the equilibrium pressure. Aside from noting that the pressures produced by colloidal solutes are large enough to be measurable, we have not yet considered any of the experimental aspects of osmometry. This is our present task. 

a suitable solvent and membrane must be found. The solvent must dissolve enough solute to produce an adequate pressure. The results of measurements made at relatively high concentrations may be extrapolated, as discussed in Section 3.2d and illustrated below, to zero concentration, so we need not worry about the effects of nonideality. As low a solvent viscosity as possible is desirable to minimize the time required for equilibration. 

It is important that the materials be free of contaminants in osmotic pressure experiments. Suppose, for example, that the solvent contains a small amount of impurity that, like the colloidal solute, is retained by the membrane. Then, as far as the osmotic pressure is concerned, that impurity will contribute to the osmotic pressure in the same way that the colloid does. Since the osmotic pressure responds to the number of solute particles present, a low molecular weight impurity in extremely small amounts may contain as many or more particles as a dilute solution of a colloidal solute of very high molecular weight. Quite large errors in molecular weight may arise in this way. The confusion may be compounded if the same system is investigated using a different membrane material. It is conceivable that another membrane 

A great many different materials have been used in osmotic pressure experiments. Various forms of cellophane and animal membranes are probably the most common membrane materials. Various other polymers, including polyvinyl alcohol, polyurethane, and polytrifluorochl-oroethylene, have also been used along with such inorganic substances as CuFe(CN)6 precipitated in a porous support. 

In successive runs, the capillary on the solution side of the membrane is filled with solution to some initial setting that will be above or below the equilibrium location of the meniscus. At various times after the initial settings, the height of the liquid column is mea- 

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