The conditions of Donnan distribu- tion of solutes (such as alkali, salt and dyes) across the membrane are applicable. The most useful condition can be simply stated. At equilibrium, the product of the concentrations of each of the ions of a solute are equal in both the external and internal phases. The only caution needed here is that solutes producing more than two ions, even if two or more are the same, must be treated as if each ion were different. For example, sodium sulfate, Na2S04, ionizes to give three ions, two sodium ions and one sulfate. The product of the concentrations of these three ions is: [Na'] X [Na'] X [SOT] or [Na+12 X [SO%]. The same considerations apply