The unequal distribution of a diffusible ion across a semi-permeable membrane when an impermeable electrolyte, such as a protein, is also present on one side; e.g. a solution of an anionic protein with Na⁺ as a counter-ion, in contact through a membrane with a NaCl solution, will result in the transfer of Cl^- into the protein compartment and an equivalent amount of Na⁺ out of it, such that the products of concentrations, (Na⁺)3(Cl^-), in each compartment are equal.