Special treatment is accorded to equilibrium constants that determine the maximum concentrations in water of the cation and anion of a solid ionic compound. Alternatively, this equilibrium can be regarded as determining the maximum concentrations of the cation and anion in water without precipitating the solid compound of the two species. Dissolution of an ionic solid in water is more complex than dissolution of a nondissociating species (such as sugar). In the latter case, there exists a unique solubility at a fixed temperature. For the dissociating solid, on the other hand, all that thermodynamics provides is the product of the concentrations of the cation and anion. The actual solubility of the solid, expressed as the aqueous concentration of the cation, depends on the concentration of the anion. The latter may be varied by the addition to the solution of other ionic species that have an anion in common with the solid compound.
Consider the metal hydroxide that dissolves in water according to the reaction: M(OH)z(s) = Mz+ + zOH" The law of mass action for this reaction, assuming the solid to be pure, is:
where the equilibrium constant KSP is called the solubility product. Although no oxidation or reduction of any of the elements is involved in the reaction, the solubility product can be determined from the standard electrode potentials of following half-cell reactions:
M + zOH- — M(OH)z(s) + ze for which s2 =s2 -^^log-1
Equating sj and s2 and solving for the product of the concentrations of the cation and the square of the anion yields:
This result could also have been obtained by direct application of Eq (10.32).
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.