Principles of Osmotic Pressure and Osmosis

Osmometry is a technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution. Osmotic pressure governs the movement of solvent (water in biological systems) across membranes that separate two solutions. Different membranes vary in pore size and thus in their ability to select molecules of different size and shape. Examples of biologically important selective membranes are those enclosing the glomerular and capillary vessels that are permeable to water and to essentially aU small molecules and ions, but not to large protein molecules. Differences in the concentrations of osmoticaUy active molecules that carmot cross a membrane cause those molecules that can cross the membrane to move to establish an osmotic equilibrium. This movement of solute and permeable ions exerts what is known as osmotic pressure. 

As an example, consider an aqueous solution of sucrose placed within a sac made up of a membrane permeable only to water, with an open vertical glass tube (a crude manome- 

Osmosis is the process that constitutes the movement of solvent across a membrane in response to differences in osmotic pressure across the two sides of the membrane. Water migrates across the membrane toward the side containing more concentrated solute. 

If the sucrose solution in the aforementioned membrane sac were replaced with a sodium chloride solution of the same molarity, the solution in the manometer would reach equilibrium at a point almost twice as high as that observed with sucrose because sodium chloride dissociates into two ions per molecule. If ion activity is unrestricted, the sodium chloride solution would have twice as many osmoticaUy active particles (osmoles) for the same molecular concentration as the sucrose solution. In reality, the number of active particles is less than this (0.93 for NaCl), as explained later in this chapter. The total number of individual (solute) particles present in a solution per given mass of solvent, regardless of their molecular nature (i.e., nonelectrolyte, ion, or coUoid), determines the total osmotic pressure of the solution. In blood plasma, for example, nonelectrolytes such as glucose and urea and even proteins contribute to the osmotic pressure of this body fluid. 

In addition to increasing osmotic pressure when the solute is added to the solvent, the vapor pressure of the solution is lowered below that of the pure solvent. As a result of the change in vapor pressure, the boiling point of the solution is raised above that of the pure solvent, and the freezing point of the solution is lowered below that of the pure solvent. 

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