The product of the concentrations of ions present in a given solution taking the stoichiometry into account. For a sodium chloride solution the ionic product is [Na⁺][Cl⁻]; for a calcium chloride solution it is [Ca²⁺][Cl⁻]². In pure water, there is an equilibrium with a small amount of self-ionization:

The equilibrium constant of this dissociation is given by

since the concentration [H₂O] can be taken as constant. K_W is referred to as the ionic product of water. It has the value 10⁻¹⁴ mol² dm⁻⁶ at 25°C. In pure water (i.e. no added acid or added alkali) [H⁺] = [OH⁻] = 10⁻⁷ mol dm⁻³. In this type of self-ionization of a solvent the ionic product is also called the autoprotolysis constant. See also solubility product; pH scale.