Supersaturation

Article By:

Gordon, Louis Formerly, Case Institute of Technology, Cleveland, Ohio.

Murray, Royce W. Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.670000

A solution is at the saturation point when dissolved solute in it crystallizes from it at the same rate at which it dissolves. Under prescribed experimental conditions of temperature and pressure, a solution can contain at saturation only one fixed amount of dissolved solute. However, it is possible to prepare relatively stable solutions which contain a quantity of a dissolved solute greater than that of the saturation value provided solute phase is absent. Such solutions are said to be supersaturated. They can be prepared by changing the experimental conditions of a system so that greater solubility is obtained, perhaps by heating the solution, and then carefully returning the system to or near its original state. The addition of solute phase will immediately relieve supersaturation. Solutions in which there is no spontaneous formation of solute phase for extended periods of time are said to be metastable. There is no sharp line of demarcation between an unstable and a metastable solution. In fact, the latter is poorly defined and much influenced by many factors such as mechanical shock and the presence of minute quantities of foreign materials. The process whereby initial aggregates within a supersaturated solution develop spontaneously into particles of new stable phase is known as nucleation. The greater the degree of supersaturation, the greater will be the number of nuclei formed. This should be avoided in gravimetric analysis because of the formation of many small crystals that tend to coprecipitate excess amounts of foreign ions by virtue of their great surface area. See also: Gravimetric analysis; Nucleation; Phase equilibrium; Precipitation (chemistry)

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