Adsorption of methylene blue in the determination of surface areas

Data are presented for adsorption of methylene blue from aqueous solutions on two non-porous carbons and on a porous activated charcoal. From the specific surface areas of the non-porous carbons (determined by low-temperature adsorption of nitrogen) it is calculated that the adsorbed methylene blue occupies an area of between 102 and 108 sq. A/molecule. These values are compared with those derived from a structural model and with those reported in the literature. It is concluded that this type of adsorption cannot yet be used for accurate determination of surface areas of carbons, nor can it give more than semi-quantitative information when used as a ‘molecular probe’ to investigate the pore structure of porous charcoals.