Osmotic Pressure

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osmotic pressure

[äz′mäd·ik ′presh·ər]
(physical chemistry)
The applied pressure required to prevent the flow of a solvent across a membrane which offers no obstruction to passage of the solvent, but does not allow passage of the solute, and which separates a solution from the pure solvent.
The applied pressure required to prevent passage of a solvent across a membrane which separates solutions of different concentration, and which allows passage of the solute, but may also allow limited passage of the solvent. Also known as osmotic gradient.

Osmotic Pressure

 

the force that tends to reduce the concentration of a solution that is in contact with a pure solvent by the reciprocal diffusion of molecules in the solute and solvent. If the solution is separated from the pure solvent by a semipermeable membrane, diffusion is possible in only one direction: the solvent is osmotically drawn across the membrane into the solution. In this case the osmotic pressure may be measured directly as the excess pressure that exists on the side of the solution at osmotic equilibrium.

Osmotic pressure results from a reduction in the chemical potential of a solvent in the presence of a solute. The tendency of a system to have equal chemical potentials over its entire volume and to reach a state of lowest free energy gives rise to the osmotic diffusion of matter. In ideal and dilute solutions, the osmotic pressure is independent of the nature of the solvent and solutes. At constant temperature it is determined only by the number of kinetically active particles—ions, molecules, associated species, and colloidal particles—in a unit volume of the solution.

The first measurements of osmotic pressure were made in 1877 by W. Pfeffer, who studied aqueous solutions of cane sugar. In 1887 his data were used by J. H. van’t Hoff to establish the dependence of osmotic pressure on the concentration of the solute; van’t Hoff’s expression for this dependence is identical in form to the Boyle-Mariotte law for ideal gases. The osmotic pressure (π) was found to be equal to the pressure that would be imparted by the solute, if, at a given temperature, the solute were an ideal gas and occupied a volume equal to that of the solution. For very dilute solutions of nondissociating compounds, osmotic pressure is described with sufficient accuracy by the equation πV = nRT, where n is the number of moles of solute, V is the volume of the solution, R is the universal gas constant, and T is the absolute temperature. When the solute dissociates into ions, a factor i—the van’t Hoff coefficient—is introduced into the right-hand side of the equation; i is greater than 1 with dissociation and less than 1 with association of the solute. The osmotic pressure of a real solution (π ) always exceeds that of an ideal solution (π″), and the ration π′/π″, which is called the osmotic coefficient g, increases with increasing concentration.

Solutions with the same osmotic pressure are called isotonic, or isosmotic, solutions. Various blood substitutes and physiological solutions are isotonic relative to the internal fluids of organisms. A solution with a higher osmotic pressure relative to another solution is hypertonic, while a solution with a lower osmotic pressure is hypotonic.

Osmotic pressure is measured with an osmometer. A distinction is made between static and dynamic methods of measurement. By the static method, the excess hydrostatic pressure is represented by the height H that a column of liquid in an osmometer tube reaches at osmotic equilibrium with the equal external pressures pA and pB acting on chambers A and B (Figure 1). The dynamic method entails measurement of v—the rates of ascent and descent of the solvent within the osmotic cell—at several values of excess pressure Δ p, which is the difference PApB; subsequently the results are extrapolated to v = 0 at Δp = π.

Figure 1. Schematic diagram of an osmometer: (A) solution chamber; (6) solvent chamber; (M) semipermeable membrane; (pA) and (pB) external pressure on chambers A and B, respectively. At osmotic equilibrium, H is the height of the columns of liquid that counterbalances the osmotic pressure when PA = pb; when pApB, that is, when π = pApB, the height is b.

Many osmometers are able to measure by both methods. One of the major difficulties in the measurement of osmotic pressure is the selection of a semipermeable membrane. Porous ceramic or glass partitions or films made of cellophane or of natural or synthetic polymers are usually used. Osmometry is the study of the techniques used in measuring osmotic pressure.

Osmometry is used primarily to determine the molecular weight (M) of polymers. M is calculated using the formula

where c is the concentration of the polymer by weight, and A is a coefficient that depends on the structure of the macromolecule.

Osmotic pressure can reach considerable values. For example, a 4 percent solution of sugar at room temperature has an osmotic pressure of about 0.3 meganewtons per square meter (MN/m2); a 53 percent solution, about 10 MN/m2. The osmotic pressure of sea water is about 0.27 MN/m2.

L. A. SHITS

The osmotic pressure in biological fluids and in the cells of animals, plants, and microorganisms depends on the concentration of solutes in the liquid mediums. The salt composition of biological fluids and cells—which is characteristic for each type of organism—is maintained by active transport of ions and by the selective permeability of biological membranes to various salts. Osmotic pressure is kept at a relatively constant level by water-salt metabolism, that is, by the uptake, distribution, use, and elimination of water and salts. Internal osmotic pressure is greater than external osmotic pressure in hyperosmotic organisms, while the reverse is true of hypoosmotic organisms; the internal and external osmotic pressures of poikilosmotic organisms are equal.

Ions are actively absorbed and retained by hyperosmotic organisms, while water passively crosses the biological membranes according to the osmotic gradient. Hyperosmotic regulation is characteristic of all plants, freshwater organisms, and marine chondrichthians, including sharks and members of the suborder Batoidei. Organisms with hypoosmotic regulation are adapted to actively eliminate salts. Teleosts eliminate the most common marine ions—sodium and chlorine—through gills, while birds and marine reptiles—such as marine snakes and turtles—eliminate these ions through salt glands that are located in the head region. In these organisms, magnesium, sulfate, and phosphate ions are eliminated through the kidneys.

Osmotic pressure in hyperosmotic and hypoosmotic organisms may be created either by ions that are predominant in the environment or by products of metabolism. For example, in sharks and members of the suborder Batoidei, 60 percent of the osmotic pressure is maintained by urea and trimethylammonia; in blood plasma of mammals, mainly by sodium and chlorine ions; and in insect larvae, by various low-molecular-weight metabolites. In poikilosmotic organisms, including unicellular marine organisms, echinoderms, cephalopods, and members of the subclass Myxini, the osmotic pressure is determined by and is equal to the osmotic pressure of the environment; other than certain intracellular processes, osmoregulatory mechanisms do not exist in these animals.

The range of mean osmotic pressures in the cells of organisms that are incapable of osmotic homeostasis is rather broad and depends on the type and age of the organism, the type of cells, and the osmotic pressure of the environment. Under optimal conditions, the osmotic pressure in the cytoplasm of cells of surface organs varies from 2 to 16 atmospheres (atm) in swamp plants and from 8 to 40 atm in plains plants. The osmotic pressure in a plant may differ sharply from cell to cell. For example, in mangroves the osmotic pressure in the cytoplasm is about 60 atm, while in the xylem channels it does not exceed 1–2 atm.

Among homoiosmotic organisms, which are capable of maintaining a relatively constant osmotic pressure, the average value and range of osmotic pressures differ, for example, from 3.6 to 4.8 atm in earthworms, from 6.0 to 6.6 atm in freshwater fish, from 7.8 to 8.5 atm in marine teleosts, from 22.3 to 23.2 atm in elasmobranchs, and from 6.6 to 8.0 atm in mammals.

Except for such glandular fluids as saliva, sweat, and urine, the osmotic pressure of most mammalian bodily fluids is equal to that of the blood. In animal cells, the osmotic pressure that is contributed by high-molecular-weight compounds, including proteins and polysaccharides, is insignificant, although these substances are metabolically important in that they create the oncotic pressure.

IU. V. NATOCHIN and V. V. KABANOV

REFERENCES

Moelwyn-Hughes, E. A. Fizicheskaia khimiia, vols. 1–2. Moscow, 1962. (Translated from English.)
Kurs fizicheskoi khimii, vols. 1–2. Edited by la. I. Gerasimov. Moscow-Leningrad, 1963–66.
Pasynskii, A. G. Kolloidnaia khimiia, 3rd ed. Moscow, 1968.
Prosser, C. L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Griffin, D. and A. Novick. Zhivoi organizm. 1973. (Translated from English.)
Nobel, P. Fiziologiia rastitel’noi kletki (fiziko-khimicheskii podkhod). Moscow, 1973. (Translated from English.)