Equations are derived for the calculation of the solubility product, K_sp, of a sparingly soluble salt A_mB_n from e.m.f. measurements with an electrode responding to ion A before and after the addition of V ml of a standard solution of A to V₀ ml of a saturated solution of A_mB_n. K_sp is expressed in terms of the e.m.f. shift, ΔE, and an independently determined slope factor.