Time dependence of the adsorption of valeric acid at the mercury-sodium chloride interface

Abstract

The time dependence of adsorption of valeric acid at a mercury electrode-0.5-mole dm −3 sodium chloride solution interface at pH 2 was investigated using phase sensitive ac polarography at E = −0.6 V (vs an Ag/AgCl reference electrode). Up to 4 s of adsorption time, a Frumkin-Fowler-Guggenheim (FFG) type of semistable state of adsorption was observed and at longer times a slow transition process took place. The adsorption equilibrium constant for such a primary adsorption step was estimated to be B ≈ 100 dm 3 mole −1 with an FFG interaction coefficient of a ≈ 2. Up to 300 s the adsorption process was consistent with a model of the slow first order transition of adsorbed molecules to a final state. However, at longer adsorption times (up to about 15,000 s), a persistent time dependence was observed followed by a constant decrease in FFG interaction coefficients. The parameters of the corresponding final state were estimated as B ≈ 1500 dm 3 mole −1 and a ≈ 0.6. The change of state of adsorbed molecules was supported by experiments of inhibition of Cd(II) reduction by adsorbed molecules of valeric acid. Due to relatively weak adsorption, mass transport of valeric acid could not have caused the observed time dependence of adsorption. The test for impurities by gas chromatography showed that the content of higher fatty acids was less than one pro mille. Several methods for interpretation of the experimental results were illustrated on the case of adsorption of valeric acid.