17.1.3  Solutes and osmotic pressure

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Substances in aqueous solution lower the free-energy status of solvent molecules (water), and the resulting osmotic pressure of that solution (where pure water is zero) can be derived from physical chemistry. Osmotic pressure (P) generated by a salt that dissociates into ions in aqueous solution, or by undissociated organic molecules in solution, can be calculated from the historic van’t Hoff expression (Equation 17.2). This relates osmotic pressure of an ideal solution (i.e. a dilute solution of an ideal solute) to the concentration of that solute expressed in terms of its osmolality and RT (where R is the universal gas constant and T is absolute temperature in degrees Kelvin). Thus,


where Π is osmotic pressure (expressed in megapascals, MPa), c is concentration (osmoles per litre of water) and RT is 2.48 at 25°C (litre-MPa per mole).


Table 17.2

The osmotic pressure of a solution can be calculated from solute concentration per litre (or per kilogram) of water (see Table 17.2 for an example based on sucrose). With organic compounds such as sucrose that do not dissociate into ions in water, osmolality (osmoles per litre of water) is the same as molality (moles per litre of water), which is almost the same as the more familiar term molarity (moles per litre of solution). At 100 mM, the discrepancy between osmolality and osmolarity is only about 1%.

When calculating the osmotic pressure of a salt solution (e.g. a solution of NaCl) each molecule of NaCl dissociates into two ions. In a solution of CaCl2, each molecule dissociates into three ions. Osmotic impact varies according to that dissociation, and must be taken into account when calculating osmotic pressure (see osmoles per mole in Table 17.2). A molecule of NaCl dissociates into two ions in solution, so that osmolality is almost two times the molality. However, osmotic effects from that dissociation are not complete, and deviations from ideal behaviour are accommodated by empirically deter-mined osmotic coefficients. Calculated values of osmotic pressure for solutions of NaCl and CaCl2 are given in Table 17.2.