Solubility equilibrium

We derive the solubility equilibrium from the relationship for equilibrium constants , which characterize the equilibria of chemical reactions in heterogeneous systems. In precipitation reactions in aqueous solutions used in qualitative analysis, an equilibrium is established between unreacted ions in the solution above the precipitate and the solid phase of the precipitate. E.g. for the slightly soluble compound (precipitate) Ag 2 CrO 4 , arising according to the chemical equation

Ag₂CrO₄ → 2Ag⁺ + CrO₄^2-

applies to the equilibrium state

$K={\frac {[{\mbox{Ag}}^{+}]^{2}\cdot [{\mbox{CrO}}_{4}^{2-}]}{[{\mbox{Ag}}_{2}{\mbox{CrO}}_{4}]}}$.

Since during precipitation the solution of the insoluble compound is saturated and the substance concentration of the insoluble compound is constant, the product of the equilibrium constant and the concentration of the insoluble compound will also be constant.

We get a following state

$K_{s}=K\cdot [{\mbox{Ag}}_{2}{\mbox{CrO}}_{4}]=[{\mbox{Ag}}^{+}]^{2}\cdot [{\mbox{CrO}}_{4}^{2-}]$

In general, therefore, the solubility product K s is given by the product of the equilibrium concentrations of ions in the solution above the precipitate, multiplied by the stoichiometric coefficients of the given chemical reaction. The solubility products of a number of substances are tabulated.

According to the solubility product, we can assess and calculate the solubility of substances under different conditions, it depends on temperature, pH and the presence of other foreign ions in the solution. This can be used in qualitative analysis, it can be influenced according to the need of the analytical procedures of the precipitation of poorly soluble compounds for the purpose of their better separations and evidence.

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Related Articles

• Solubility products of ionic compounds
• Approximate solubilities of ionic compounds

Source

KADLECOVÁ L.. Qualitative analysis of selected elements, important from the point of view of biology and toxicology