where f 0 is the free energy of the system when a site is deprotonated, f 1 is when it is protonated, and ∆µ = f 1 -f 0 is the free energy change of the system due to the removal of proton from the bulk of the cell and its reaction with the adsorption site. In the initial state the site is deprotonated and is solvated by the ions of electrolyte solution, resulting in solvation free energy (see Appendix A):