The Adsorption of Methylene Blue on a Positively Charged Mercury Surface

Direct measurement of the surface concentration Г of methylene blue, adsorbed from a phosphate buffer solution onto an ideally polarized mercury electrode, has produced a Lang-muir-type isotherm. The maximum value of Г, Г m , was found to be dependent on the potential applied (E). Surface tension measurements showed that the rate-controlling process is not diffusion, but, presumably, the exchange of methylene blue cation (MB + ) with phosphate ions. A model for this process has been proposed. This allowed the calculation of the rate constant of adsorption (k a ) from the surface tension values as a function of time (k a ~ 5 × 10 −5 cm/sec; dk a /dE ~ 30 × 10 −5 ). There is some evidence for the occurrence of a consecutive process, most likely the combination of adsorbed MB + to dimers.