Osmotic pressure apparatus

The osmotic pressure apparatus (Fig. 2.12) consists of an open ended... 

If the changes of volume are executed very rapidly, they may be made adiabatic, and a Carnot s cycle may be performed with the apparatus. We take Y, the total volume of the system in the cylinder, as the abscissa, and P, the osmotic pressure, as ordinate. Let... 

Suppose the osmotic pressure under total pressure p is P, and let Arp be the change of total volume occurring when a mol of solute is dissolved under total pressure p. Let Yp be the change of volume of the system inside the osmotic apparatus when solvent is admitted so as to dissolve a mol of solute (cf. 188). 

The osmotic pressure is a property that has proven to be especially valuable in the study of solutions of macromolecules, including those of biologic and polymeric interest. The apparatus for measuring this quantity is shown schematically in Figure 7.10. Two compartments are separated by a membrane that will allow the flow of liquid solvent between the two chambers. If solvent is added, flow will occur until the liquid level on the two sides of the membrane is the same. 

Figure 7.10 Schematic representation of the apparatus for measuring osmotic pressure. The flow of solvent through the semipermeable membrane is followed by observing the movement of the meniscus of the flow indicator. The osmotic pressure II is the pressure that must be applied to the solution to prevent the flow.

In some cases, it may be necessary to calculate the osmotic pressure from the height, h, of the solution (in an apparatus like that in Fig. 8.31) by using 11 = gdh, where d is the density of the solution and g is the acceleration of free fall (see inside back cover). 

Figure 15.3. Schematic diagram of apparatus for measurement of osmotic pressure.

In order for solvent and solution to be in equilibrium in an apparatus such as that shown in Figure 3.2, the solution side must be at a higher pressure than the solvent side. This excess pressure is what is known as the osmotic pressure of the solution. If no external pressure difference is imposed, solvent will diffuse across the membrane until an equilibrium hydrostatic pressure head has developed on the solution side. In practice, to prevent too much dilution of the solution as a result of the solvent flow into it, the column in which the pressure head develops is generally of a very narrow diameter. We return to the details of osmotic pressure experiments in the next section. First, however, the theoretical connection between this pressure and the concentration of the solution must be established. 

The phenomenon of osmotic pressure is often demonstrated with apparatus such as the following ... 

A process called reverse osmosis is used to remove salts from seawater to make drinking water for human consumption. If a pressure greater than the osmotic pressure is applied to the solution side of an apparatus such as shown in Figure 15.6, water is forced from the solution (the seawater) to the pure solvent (water) side. This process is used industrially for water purification. 

Figure 2-5 is a schematic of apparatus for the measurement of osmotic pressure. A solution is separated from its pure solvent by a semipermeable membrane, which allows solvent molecules to pass but blocks solute. Both components arc at the same temperature, and the hydrostatic pressure on each is recorded by means of the heights of the corresponding fluids in capillary columns. The solute cannot distribute itself on both sides of the membrane. The solvent Hows initially, however, to dilute the solution, and this flow will continue until suflicient excess hydrostatic... 

Boiling-point elevation can also be used to measure osmotic pressure and tonicity of a solution using just a reflux condenser and a thermometer. The commercially available instrument is the Cottrell boiling-point apparatus. However, this method is affected by the ambient barometric pressure and the presence of volatile solvents in the solution. 

Suppose 0.125 g of a protein is dissolved in 10.0 cm of ethyl alcohol (C2H5OH), whose density at 20°C is 0.789 g cm . The solution rises to a height of 26.3 cm in an osmometer (an apparatus for measuring osmotic pressure). What is the approximate molar mass of the protein ... 

The osmotic pressure of a given aqueous solution can be measured with an apparatus such as that depicted in Figure 14-16a. The solution of interest is placed inside an inverted glass (thistle) tube that has a membrane firmly fastened across the bottom. This part of the thistle tube and its membrane are then immersed in a container of pure water. As time passes, the height of the solution in the neck rises until the pressure it exerts just counterbalances the osmotic pressure. 

The first of these is so small that it would be hard to measure precisely. Even this small lowering of the vapor pressure is sufficient to raise the boiling point by an amount that could be measured, although with difficulty. The freezing point depression is greater, but still could not be measured very precisely without a special apparatus. The osmotic pressure, on the other hand, is so large that it could be measured much more precisely. Thus, osmotic pressure is often the most easily measured of the four colligative properties, especially when very dilute solutions are used. 

Suppose we place a semipermeable membrane between a saline (salt) solution and pure water. If the saline solution is pressurized under a greater pressure than its osmotic pressure, the direction of flow can be reversed. That is, the net flow of water molecules will be from the saline solution through the membrane into the pure water. This process is called reverse osmosis. The membrane usually consists of cellulose acetate or hollow fibers of a material structurally similar to nylon. This method has been used for the purification of brackish (mildly saline) water. It has the economic advantages of low cost, ease of apparatus construction, and simplicity of operation. Because this method of water purification requires no heat, it has a great advantage over distillation. 

An osmotic pressure difference, as over the semipermeable membrane in the apparatus of Figure 2.4 before equilibrium has been reached An endogenous tendency of the material to shrink, as in the syneresis of renneted milk... 

The average osmotic pressure of seawater, measured in the kind of apparatus shown in Figure 12.12, is about 30.0 atm at 25°C. Calculate the molar concentration of an aqueous solution of urea [(NH2)2CO] that is isotonic with seawater. 

Another aspect of current interest associated with the lipid-water system is the hydration force problem.i -20 When certain lipid bilayers are brought closer than 20-30 A in water or other dipolar solvents, they experience large repulsive forces. This force is called solvation pressure and when the solvent is water, it is called hydration pressure. Experimentally, hydration forces are measured in an osmotic stress (OS) apparatus or surface force apparatus (SFA)2o at different hydration levels. In OS, the water in a multilamellar system is brought to thermodynamic equilibrium with water in a polymer solution of known osmotic pressure. The chemical potential of water in the polymer solution with which the water in the interlamellar water is equilibrated gives the net repulsive pressure between the bilayers. In the SEA, one measures the force between two crossed cylinders of mica coated with lipid bilayers and immersed in solvent. 

The osmotic pressure between two solutions is measured by means of an apparatus whose principle is indicated on Fig. 5.4. In the beginning, the cells I and II, separated by a semi-permeable membrane, are filled with pure solvent. Then, the levels are equal and the pressures P 0 and / " are equal. 

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