Solubility product

The solubility product constant, Ksp, is the equilibrium constant for a sparingly soluble ionic solid dissolving into its constituent ions in a saturated solution.

Solubility equilibrium: The dynamic balance between a dissolved substance and its undissolved phase in a saturated solution. Ksp quantifies the equilibrium between an ionic solid and its dissolved ions.
Ionic compound: A substance composed of positively and negatively charged ions held together by electrostatic attraction. Its ion stoichiometry determines the dissolution reaction represented by Ksp.
Selective precipitation: A separation method that precipitates one dissolved species preferentially by controlling solution conditions. Different Ksp values help determine which ions precipitate first.
Ion product: The product of dissolved ion activities, each raised to its stoichiometric coefficient, at any solution composition. Unlike Ksp, the ion product need not describe an equilibrium saturated solution.
Equilibrium constant: A number expressing the ratio of product to reactant activities at chemical equilibrium. Ksp is a specialized equilibrium constant for dissolving ionic solids.
Chemical equilibrium: A state in which forward and reverse reaction rates are equal, leaving macroscopic composition constant. Ksp describes the equilibrium state of solid dissolution.
Gravimetric analysis: A quantitative analytical method that determines an analyte's amount from the mass of an isolated compound. Ksp helps assess whether precipitation is complete enough for accurate mass measurement.
Conditional formation constant: An equilibrium constant adjusted for specified solution conditions such as pH or competing reactions. Complexation can make observed solubility differ from a value inferred using Ksp alone.
Reaction quotient: A ratio of product to reactant activities calculated for a system that may not be at equilibrium. Comparing Qsp with Ksp predicts whether precipitation or dissolution will proceed.
Chemical activity: A thermodynamic measure of a species' effective concentration in a mixture. Activities, rather than raw concentrations, define the rigorous Ksp expression.

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