Solute flux:

The solute also diffuses by the membrane at the same time with water and the solute flux, (J_S) that is a measure of the quantity of solute, expressed within mass or molar basis, diffusing by the membrane per unit time per unit area, is given through

J_S     = D_S K_S ?C_S / ?x

where, ?C_S is the difference in solute concentration across the membrane, K_S is the distribution coefficient for the solute between feed solution near the membrane surface and the membrane which is given again as a dimensionless parameter and D_S refers to the diffusion coefficient of solute in the membrane phase.

The quantity D_SK_S/?x   can be treated as solute permeation constant (B) whose magnitude is characteristic of the membrane and the solute under consideration. Accordingly, Eq.may be written as

J_S    = B (?CS)

Eq. indicates that the solute flux through the membrane increases with increase in solute concentration difference across the membrane which will manifest in lower observed solute separation. A magnitude of solute flux depends on the value of solute permeation constant (B).The desirable properties of semipermeable membranes for use in reverse osmosis procedure is higher water flux and solute separation that is probable for a membrane along with a higher value of A and a lower value of B. Therefore, the ratio of the two constants A/B is often used to describe the semipermeable character of a membrane. Higher the value of A/B, the more chosen is the membrane for water against solute.