Physico-chemical expressions are discussed for three coefficients Lf, σ and P, to characterize membranes. The filtration coefficient Lf consists of two parts: the one related to Poiseuille's hydrodynamic flow and the other to migration of water molecules by diffusion and the latter is responsible to the selective permeability of reverse osmosis membranes. Since the tortuosity is not the same in these two processes, the membrane coefficient f is larger for the latter than for the former. The reflection or rejection coefficient cr is smaller than 1 for imperfect semi-permeable membranes. In this connection, the membrane equilibrium, dialysis, and ultrafiltration are discussed. The membrane permeation coefficient P is approximated by the product of the membrane coefficient f, the partition coefficient at the membrane surface σ and the diffusion coeffi-cient in the membrane Ds. The value of P of a charged membrane for an electrolyte decreases with decrease of the electrolyte concentration because β decreases by the effect of membrane potential. Reverse permeation (P<0) may be caused either by the difference of β between two surfaces of a membrane, or, if the solutes are electrolytes, by a negative value of Ds due to electrical potential gradient.
