In the experimental protocols described above the starting volume concentrations of the aqueous species X and W are known. However, the starting surface concentrations and the effective surface area of the coating will not usually be known. Indeed, to some degree, these are idealised or conceptual quantities anyway. To overcome this difficulty the starting stoichiometric ratio 𝑟 % = number of available (free) adsorption sites S number of entities X is introduced as another model parameter, in addition to the reaction rates.