ionproduct

Ion product, abbreviated IP, is the instantaneous product of the activities of the ions in solution for a given dissolution or precipitation equilibrium. It is a diagnostic quantity used in aqueous chemistry to assess whether a solution is undersaturated, saturated, or supersaturated with respect to a particular solid.

For a dissolution process of a salt AB(s) ⇌ A^z+ + B^w-, the ion product is IP = a_A^z+ × a_B^w-, where a_i denotes the activity of ion i. Activities relate to concentrations by a_i = γ_i [i], with γ_i the activity coefficient. In dilute solutions, IP ≈ [A^z+] × [B^w-]. More generally, for a stoichiometry pA^n+ + qB^m- forming a solid, IP = (a_A)^p × (a_B)^q.

A common example is CaF2(s) ⇌ Ca2+ + 2F−, where IP = a_Ca2+ × (a_F−)^2. The solubility-product constant Ksp is the equilibrium value of IP for that solid at a given temperature; at equilibrium IP = Ksp. If IP > Ksp, the solution is supersaturated and precipitation tends to occur; if IP < Ksp, the solution is undersaturated and dissolution tends to occur; if IP = Ksp, the solution is saturated with respect to the solid.

The concept extends to the ion-product of water, Kw = a_H+ × a_OH−. At 25 °C in dilute aqueous solutions, Kw ≈ 1.0 × 10^−14. Temperature and ionic strength affect activity coefficients, so IP and related equilibria are not fixed constants but depend on the solution conditions.