A measure of the amount of a solute in a feed solution fed to a semi-permeable membrane that does not permeate the membrane in a separation process such as ultrafiltration:
where R is the solute rejection coefficient, CP is the concentration of the solute in permeate, and CR is the concentration of solute in the retentate. For a given membrane with a pore size distribution there is a relationship between the rejection coefficient and the solute molecular weight.