here XS denotes the adsorbed state. The symbols in square brackets [ ] denote the volume or surface concentrations, as appropriate, of the relevant species. In this model any reverse reaction has been assumed extremely slow and so it is neglected. Since this is modelled as an irreversible reaction, it will continue to completion until one of X or S, or both are fully consumed. In what follows X can be thought of as Hg 2+ or HgCl2.