Interface capacitance at mercury and iron electrodes in the presence of organic compound

Abstract

A new experimental setup that determines simultaneously the electrochemical impedance spectrum and the modulation of interface capacitance induced by the impedance measurement itself is described. Then, the signal processing used to determine the traditional impedance spectrum and the modulation of interface capacitance at the same frequency as for the impedance measurement is discussed. Two model systems have been used to illustrate the implications of faradaic processes for the interface capacitance. First, an ideally polarisable interface, an Hg electrode in KCl solution, was examined. It was found that the interfacial capacitance changed instantaneously, and the sign of the capacitance modulation changed when the electrode potential crossed the potential of zero charge (PZC). In the presence of n-butyl alcohol in the same system, however, the capacitance modulation response was quite different. A negative slope of the capacitance - potential curve was verified only at extremely low frequencies, although the system still remained an ideally polarisable one. The second system model examined was Fe in H₂SO₄ solution at the open circuit corrosion potential. In the absence of inhibitor, only one time constant was observed, and this phenomenon was attributed to the relaxation of surface coverage by H_ads. In the presence of propargylic alcohol, two relaxation phenomena were observed, both in the impedance spectrum and in the capacitance modulation. The faster relaxation process was attributed to that of the reaction intermediate of Fe dissolution, and the slower phenomenon to the adsorption - desorption of the inhibitor.