Studies on the adsorption and the association of cytidine sulphate at the dropping mercury electrode with phase-sensitive ac-polarography

Abstract

The adsorption and association of cytidine sulphate have been studied by using phase-sensitive ac-polarography in acidic buffer solution (pH 3.24) at the dropping mercury electrode. At low bulk concentrations, adsorbed molecules of cytidine sulphate at maximal adsorption potential (−0.7 V) are orientated in 'dilute' adsorption layer planar to the electrode surface where the interaction of π-electrons with interface favours adsorption. At bulk concentrations above a threshold value (6.6 × 10−3 M), the stacking interactions between vertically oriented molecules lead to association and formation of a compact layer; and a pit is observed. The concentration dependence of the ac-capacity current at a maximal adsorption potential (−0.7 V) shows a two-step Frumkin isotherm due to association of the adsorbed molecules. The adsorption parameters are computed and discussed.