Abstract
A complete characterization of the adsorption of phenolphthalein at the hanging mercury drop electrode in aqueous buffer solutions containing 10% (v/v) ethanol was obtained employing cyclic voltammetry and double potential step chronocoulometry. At pH=3.4, a sharp symmetric cyclic voltammetric wave due to an irreversible weak adsorption of the monoanionic form is obtained at a concentration of 5.1 μmolċdm−3. Under these conditions, phenolphthalein adsorbs in a compact layer in which the triphenylmethan moiety is arranged perpendicular to the electrode surface. Upon increasing the concentration, the symmetry of the wave decreases; it can now be attributed to a mixed diffusion-adsorption process. In acidic solutions (pH<3.4) the observed electroactivity is ascribed to the adsorption of the neutral lactone form. In solutions of pH=10.58, no adsorption was detected.
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