Osmometer principles

Osmosis is the passage of a pure solvent into a solution separated from it by a semipermeable membrane, which is permeable to the solvent but not to the polymeric solute. The osmotic pressure n is the pressure that must be applied to the solution in order to stop the flow. Equilibrium is reached when the chemical potential of the solvent is identical on either side of the membrane. The principle of a membrane osmometer is sketched in Figure 2. 

Based on the theory of colligative properties and the principles of osmometry, it is understood that osmometer will read osmolalities and not osmolarities because colligative properties are directly proportional to the total solute concentration expressed in molality [see Eqs. (1)-(16)]. The relationship between osmolality and osmolarity and its significance can be found in the Remington s Pharmaceutical Sciences and in a review article by Deardorff. However, it is more convenient to use osmolarity because it is based on weight/volume rather than on weight/weight as in... 

Because of their large molecular weight, proteins contribute only about 1 mOsmol/kg H2O to the total serum osmolality measured by freezing point depression. Occasionally, one may be asked to determine the contribution of macromolecules to the serum osmolality. Colloid osmotic pressure (COP) is a direct measure of the contribution of macromolecules (primarily proteins) to the serum osmolality. It is used primarily in the assessment of pulmonary edema or other abnormahties of water balance and serum protein concentrations. However, its utility has been questioned and the method is seldom used. Previous editions of this textbook describe the principles of a COP osmometer. 

Dynamic osmometers reach equilibrium pressures in 10 to 30 minutes and indicate osmotic pressure automatically. Several types are available. Some commonly used models employ sensors to measure solvent flow through the membrane and adjust a counteracting pressure to maintain zero net flow. A commercially available automatic osmometer operates on the null-point principle. In this high-speed membrane osmometer schematically represented in Fig. 4.4, the movement of an air bubble inside the capillary immediately below the solvent cell indicates the solvent flow to the solution cell. Such movement is immediately detected by a photocell, which in turn is coupled to a servomechanism. If any movement of the air bubble is detected by a photocell, the servomechanism is stimulated to move the solvent reservoir upward or downward in order to adjust the hydrostatic pressure such that the solvent flow is completely arrested. The pressure head of the reservoir gives the osmotic head. Some osmometers also use strain gauges on flexible diaphragms to measure the osmotic pressure directly. 

Figure 4.4.14. Principle scheme of a vapor-pressure osmometer of the hanging drop type. T - measuring temperature, AT - obtained temperature difference (time dependent), measurements are made at atmospheric pressure where T determines the partial vapor pressure of the solvent P, in air.

Figure 4.4.16. Principle scheme of a membrane osmometer 1 - solvent, 2 - polymer, 7C - osmotic pressure. Ah - hydrostatic height difference, - ordinary pressure or measuring pressure, Vj- partial molar volume of the solvent in the polymer solution.

Laboratory designed instruments were developed in the 40 s and 50 s, e.g. by Zimm or by Flory. Later on, high speed membrane osmometers are commercially available, e.g., from Knauer, Hewlett-Packard or Wescan Instruments. External pressures may be applied to balance the osmotic pressure if necessary, e.g., Vink. The principle scheme of a membrane osmometer together with the corresponding... 

Technical details of the different apparatuses will not be presented here, however, the principle construction of the measuring cell and the heating thermostat of the Knauer membrane osmometer A 300 is shown in Figure 4.4.17 for illustration and as example. 

A static capillary osmometer is illustrated in Fig. 2.18. 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 dynamie equilibrium method can be used. Here a counter pressure is applied to maintain equal levels of the liquid in both capillaries and prevents flow of the solvent. Different types of dynamic membrane osmometers are available commercially. The principle is illustrated in Fig. 2.19. 

Most modern osmometers do not require a static equilibrium to be reached and they use the principle of stopping the flow of solvent through the membrane by applying an external pressure to the polymer solution. This method has the advantage of much greater speed and there are several commercial instruments now available that use this dynamic equilibrium principle. ... 

Up to this point, the discussion has concerned systems with one or more polymer species dissolved in a pure solvent. However, thermodynamic behavior in the more general case of systems with multiple solvents involves additional effects that are of considerable interest. Here, for simplicity, only a three-component system of a polymer (component 2) in two low molecular weight solvents (components 1 and 3) is considered but an exhaustive analysis of the general multicomponent case is available. The classification of solvents and solute is of course arbitrary, but it has utility with respect to specific experimental measurements. For instance, the operative principle in an osmotic measurement is that solvents pass through the membrane that confines a polymeric solute. The obvious effect of the additional thermodynamic degree of freedom in the three-component system is the possibility of selective interaction (preferential solvation or binding ) of the solute with a solvent component. In an osmotic experiment this will be manifested by equilibrium solvent compositions that are different in the solute and solvent compartments of the osmometer. It is possible, though not always very useful, to formally redefine a solute component so as to include in it any excess (or deficiency) of a solvent component required to make the free solvent mixture appear... 

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