Static capillary osmometer

A static capillary osmometer is illustrated in Fig. 1.7. Rather than rely on the liquid to rise in the capillary on the side of the solution in response to osmotic pressure, as is done in the static method, a dynamic equilibrium method can be used. Here a counterpressure is applied to maintain... 

The simplest static osmometers are of the basic form indicated schematically in Fig. 3.9 (e.g. Pinner-Stabin osmometer) and have relatively large cell volumes (typically 3-20cm ). The volume of solvent transported across the membrane at equilibrium is reduced by using capillaries for the pressure head. However, these capillary osmometers are cumbersome to use and give relatively long equilibration times (typically several hours). Modern static osmometers have much smaller cell volumes and pressure sensors which monitor the pressure in the solution cell, usually indirectly (e.g. Wescan and Knauer osmometers). It normally is not necessary to correct the solution concentration for dilution due to movement of solvent into the solution cell because the effect is usually negligible. 

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. 

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. 

Instrumentation A typical static osmometer design includes a membrane that permeates only the solvent, a capillary, and a reference capillary. Typical membranes are made from regenerated cellulose or other microporous materials. Such an instrument usually requires 24 h to reach equilibrium (20). 

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