A dsor#tion at Inorganic Szcr faces. Part I .held water. The film formed in chromic acid is the purest and is almost entirely y-alumina (s.g. ca. 2-7) with only ca. 0-1% of impurity (as Cr). Some partial hydration of the oxide may take place during electrolysis but the present work gives no evidence of this. The degree of crystallinity appears to increase with rise in temperature of the electrolyte. Dry heating above 100" drives off any loosely held water, but does not decrease the adsorptive powers; treatment with steam or with water above W", however, causes the f i l m to lose its adsorptive powers entirely. This effect, known as " sealing," is caused by blocking of the pores through the formation of a thin layer of a-alumina monohydrate (boehmite) containing an additional molecule of lattice water. Dry heating above 100" subsequent to sealing removes water and reconverts a proportion of the monohydrate into y-alumina. The adsorptive capacity of the film is thus partially restored. Heating to temperatures up to 650" does not appreciably alter the film, but at ca. 900" the internal structure changes entirely, and a-alumina is produced (

No abstract

Quantitative adsorption tests with aromatic solutes (mainly sulphonated dyes) and chromic acid anodic alumina film are described. The adsorption mechanism of the non‐chelating dyes is probably covalent bond formation (each sulphonate group replacing a hydroxy‐group or a chromate group attached to aluminium), together with some ion‐exchange adsorption of anionic micelles. Chelating dyes form an aluminium chelate‐complex with the film. The sequence of events in adsorption from water appears to be: (i) film etching rapid dissolution (<5 min.) of a surface layer of alumina, (ii) rapid adsorption of an external layer of solute, (iii) slow inward diffusion of solute from the external layer to complete a monolayer, (iv) (with sulphonates) penetration (and ultimate breakdown) of the oxide crystal structure, (v) slow subsequent ‘sealing’ of the film pores by growth of boehmite crystals. Process (i) influences the available surface area, which increases proportionately (and very considerably) with the volume of liquid used for adsorption. The apparent activation energy of diffusion in the film falls with increase in size of the unsul‐phonated residue of a dye.