Studies of chloride adsorption on the Ag/AgCl electrode

Experimental results supporting a non-Faradaic potential origin for the Ag/AgCl electrode and Cl−-ISE resulting from chloride ion adsorption on the AgCl membrane are presented. In this paper we show that the salt bridge commonly used in potential measurements, necessary for ion transfer in cell redox reactions, can be replaced by a platinum wire or graphite fiber. Immersion of an Ag/AgCl electrode in a KCl solution at different electrode depths, results in more negative potential reading due to a larger exposed electrode surface. Indeed, the more chloride ions adsorbed on the electrode surface the more negative is the potential. In a different experiment, when the same electrode is dipped into increasing KCl concentration solutions but kept at a constant level inside these solutions, the potential increases as function of stirring speeds except for 10−4 M KCl where the trend was opposite. One possible explanation of this behavior can be related to the counter triple layer highly dependent on the K+ concentration and its capacity to replace Cl− under increasing stirring speeds influencing this way the electrode surface charge density. We also show that Ag is not a determinant component in the electrode making. Similar experiments performed on a Cl−-ISE show stable potential reading and good calibration curves under identical experimental conditions. As expected, when the KCl solution is not stirred, the Cl−-ISE potential decreases to more negative values for increasing KCl concentrations. Electrode surface charge distribution was also followed by experiments based on stirring force effect on the ISE potential. The results are discussed here in terms of adsorption isotherm and chemical capacitor behavior of the AgCl electrode membrane.