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Consider a porous medium whose solid matrix holds a fixed electrical charge and whose interstitial fluid consists of a solvent and two monovalent counter-ions (such as Na$^{\mathrm{+}}$ and Cl$^{\mathrm{-}})$ . The boundaries of the medium are permeable to the solvent and ions. The fixed charge density is denoted by $c^{F}$ ; it is a measure of the number of fixed charges per volume of the interstitial fluid in the current configuration. This charge density may be either negative or positive, thereby producing an imbalance in the concentration of anions and cations in the interstitial fluid. To determine the osmolarity of the interstitial fluid, it is necessary to equate the mechano-chemical potential of the solvent and the mechano-electrochemical potential of the ions between the porous medium and its surrounding bath. When assuming ideal physicochemical behavior, the interstitial osmolarity (resulting from the interstitial ions) is given by $$\begin{equation} c=\sqrt{\left(c^{F}\right)^{2}+\left(2c^{\ast}\right)^{2}}\,,\label{eq147} \end{equation}$$ where $c^{\ast}$ is the salt concentration in the bath. Alternatively, we note that the osmolarity of the bath is $\bar{c}^{\ast}=2c^{\ast}$ . Though this expression may be equated with (2.9-2), the resulting value of $c_{r}$ is not constant in this case, since ions may transport into or out of the pore space; therefore that relation is not useful here.