The adsorption of iodine by potassium iodide

Abstract

Opinion has been divided for some years as to whether the adsorption of a vapour on a solid surface is confined to a monomolecular layer until the vapour pressure reaches the saturation value, or whether multimolecular layers are formed. Experimental work has generally failed to provide an unequivocal answer to this point, often because the true area of the surface of the adsorbent could not be estimated. It is now recognized that the true area of a surface may be many times the apparent area, and the ratio of these will depend on the previous treatment of the surface. The position has been complicated further by the difficulty of determining the amount of adsorption on a small area of a plane surface when the adsorption is so small as to be undetectable without sensitive apparatus; consequently, the bulk of the experimental work has been carried out on finely divided adsorbents, of large surface. This breaking up of the adsorbent, whether accomplished by chemical or mechanical means, is bound to have an unpredictable effect on the ratio of the true to apparent area of the surface, as distinct from the purely geometrical multiplication. The use of porous or finely divided adsorbents may also introduce the additional complication of capillary condensation of vapours in the spaces between the particles. It is clear, as was first emphasized by Langmuir in 1918, that the investigation of adsorption on plane surfaces, using more delicate technique, is the first step towards a solution of the problem. Langmuir measured the adsorption of gases on large plane surfaces of mica, glass and platinum at low temperatures and pressures. He showed that the adsorption reached a maximum, which corresponded to slightly less than one complete monomolecular layer, and that the extent of adsorption was related to the pressure by an equation of the form x = abp /1 + ap where a and b are constants involving the condensation coefficient, the time of life of adsorbed molecules in the surface, and the number of “ adsorbing centres.”