Osmosis and osmotic pressure

The next topic in this chapter, possibly familiar from other physical chemistry courses, is the phenomenon of osmosis and the associated osmotic pressure. This is very important in colloid science. 

The osmotic pressure of a solution is generally described as a function of concentration, c (mass 

90% (0.15 M) sodium chloride solution is known as a physiological saline solution because it is isotonic with blood plasma that is, it has the same concentration of NaCl as blood plasma. Because each mole of NaCl yields about 2 mol of ions when in solution, the solute particle concentration in physiological saline solution is nearly 0.30 M. Five-percent-glucose solution (0.28 M) is also approximately isotonic with blood plasma. Blood cells neither swell nor shrink in an isotonic solution. The cells described in the preceding paragraph swell in water because water is hypotonic to cell plasma. The cells shrink in 5%-urea solution because the urea solution is hypertonic to the cell plasma. To prevent possible injury to blood cells by osmosis, fluids for intravenous use are usually made up at approximately isotonic concentration. 

All solutions exhibit osmotic pressure, which is another colligative properly. Osmotic pressure is a pressure difference between the system and atmospheric pressure. The osmotic pressure of a system can be measured by applying enough pressure to stop the flow of water due to osmosis in the system. The difference between the applied pressure and atmospheric pressure is the osmotic pressure. When pressure greater than the osmotic pressure is applied to a system, the flow of water can be reversed from that of osmosis. This process can be used to obtain useful drinking water from seawater and is known as reverse osmosis. Osmotic pressure is dependent only on the concentration of the 

Human red blood cells. Left In an isotonic solution, the concentration is the same inside and outside the cell (0.9% saline). Center In a hypertonic solution (1.6% saline), water leaves the cells, causing them to crenate (shrink). Right In a hypotonic solution (0.2% saline), the cells swell as water moves into the cell center. Magnification is 260,000x. 

Laboratory demonstration of osmosis As a result of osmosis, water passes through the membrane, causing the solution to rise in the thistle tube. 

Osmosis has a role in many biological processes, and semipermeable membranes occur commonly in living organisms. An example is the roots of plants, which are covered with tiny structures called root hairs soil water enters the plant by osmosis, passing through the semipermeable membranes covering the root hairs. Artificial or synthetic membranes can also be made. 

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S. Murad, J. G. Powles. A computer simulation of the classic experiment on osmosis and osmotic pressure. J Chem Phys 99 7271, 1993. 

Organic chemistry or organic molecules Osmosis and osmotic pressure... 

Osmosis and osmotic pressure. Osmosis is the movement of solvent from a dilute solution to a more concentrated solution through a semipermeable membrane. The pressure that must be applied to the more concentrated solution to stop this flow is the osmotic pressure. The osmotic pressure, like the pressure exerted by a gas, may be treated quantitatively by using an equation similar in form to the ideal gas equation tt = MRT. By convention the molarity of particles that is used for osmotic pressure calculations is termed osmolarity (osmol). 

An extremely thin piece of material such as a sheet of synthetic polymer or animal tissue can allow some molecules to pass through it. Such a material is called a membrane and is said to be permeable to those molecules and ions that can pass through. Permeability is dependent on the presence of tiny passages within the membrane. A membrane permeable to water molecules but not to ions or molecules larger than water molecules is called a semipermeable membrane. Many membranes made from synthetic polymers have this characteristic. One such polymer is cellulose acetate. If a semipermeable membrane is placed between seawater (brine) and pure water, the pure water will pass through the membrane to dilute the seawater. This is a process called osmosis. The hquid level on the seawater side rises as more water molecules enter than leave, and pressure is exerted on the membrane until the rates of diffusion of water molecules in both directions are equal. Osmotic pressure is defined as the external pressure required to prevent osmosis. Figure 11.10a and 11.1 Ob illustrates the concepts of osmosis and osmotic pressure. 

Kahlenberg, L. (1906). On the nature of the process of osmosis and osmotic pressure with observations concerning dialysis. J. Phys. Chem. 10, 141. 

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