The differential capacitance of mercury in pure and coumarinated I.0 N aqueous solutions of Na,SO,, HZS04, KNO+ KCl, HCl and KBr respectively was traced over appropriate potential range at 1 K Hz and 25°C. In NalSO., and H,SO,, assumed not to exhibit specific anion adsorption, the results were most informative with respect to adsorption and orientation of coumarin. Relative coverages at saturation and the associated minimum capacitances could be correlated with molecular areas for planar and vertical orientations.
The variation of the relative coverage '0 with either potential or surface charge density at different coumarin concentrations displayed almost the same general features. The ti/concentration isotherms at constant potential, however, could be readily identified with pure planar, pure vertical or mixed adsorption and therefore were more informative. With the assumption that an adsorbed planar molecule displaces four water molecules and a vertical one two molecules, the pure isotherms were shown to fit reasonably the Flory-Huggins isotherm. A modification of this isotherm explained only qualitatively the mixed isotherms in absence of specific anion adsorption, whde the results were far less convincing in the presence of specific anion adsorption.