The extraction constant at zero ionic strength, $K_{\text{ex}}^{\text{0}}$, is the equilibrium constant of the distribution reaction expressed in terms of the reacting species. Thus, for the gross reaction: $${\text{M}}_{\text{aq}}^{n+}+n{\text{HL}}_{\text{org}}\rightleftharpoons {\text{ML}}_{n,\text{org}}+n{\text{H}}_{\text{aq}}^{+}$$ in which the reagent HL initially dissolved in an organic phase reacts with a metal ion Mⁿ in aqueous solution to form a product ML_n which is more soluble in the organic phase than in water, $$K_{\text{ex}}^{\text{0}}=\frac{a_{\text{ML }n,\text{org}}{a}_{{\text{H}}^{+},\text{aq}}^n}{a_{{\text{M}}^{n+},\text{aq}}{a}_{\text{HL},\text{org}}^n}$$