Practical Aspects of Osmometry

Practical Aspects of Osmometry In static osmometers, the heights of liquid in capillary tubes attached to the solvent and solution compartments (Fig. 4.3) are measured. At equilibrium, the hydrostatic pressure corresponding to the difference in liquid heights is the osmotic pressure. The main disadvantage of this static procedure is the length of time required for attainment of equilibrium. 

It may be noted that the osmotic pressure is essentially the extra pressure that must be applied to the solution to maintain equilibrium when solution and pure solvent are separated by a semipermeable membrane. This extra pressure can be measured by attaching a counter pressure device to the solution tube (Fig. 4.3). 

This method of determining the osmotic pressure is conveniently referred to as the dynamic equilibrium technique. It is especially useful when rapid determinations of osmotic pressure are required. Dynamic osmometers reach equilibrium pressures in 10 to 30 minutes, as compared to hours in the static method, and indicate osmotic pressure autornatically. There are several types. Some models employ sensors to measure solvent flow through the membrane and adjust a counteracting pressure to maintain a zero net flow. In a commercially available high-speed membrane osmometer, schematically shown in Fig. 4.4, the movement of an air bubble inside the capillary immediately below the solvent cell is used to indicate this solvent flow. Such movement is immediately detected by a photocell, which in turn is coupled to a servomechanism that controls the flow. 

The data obtained by osmotic pressure measurements are pressures osmotic heads) in terms of heights h) of solvent colunms at different concentrations (c) of the polymer solution. In applying the data, hjc is plotted against c and extrapolated to c = 0, yielding the value of (/z/c)o. The column height h is then converted to osmotic pressure II by II = hpg, where p is the density of the solvent and g is the gravitational acceleration constant, and Mn is calculated from Eqs. (4.41)-(4.43), which in the limit of c — 0 reduce to 

The second virial coefficient can be obtained from the slope of the straight line portion of the (II/c) versus c plot by removing the terms in Eqs. (4.41)-(4.43). When plotted according to Eq. (4.41), the osmotic pressures of solutions of the same polymer in different solvents should yield plots with the same intercept (at c = 0) but with different slopes (see Fig. 4.5), since the second virial coefficient, 

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