Conductivity and kinetic studies of silver-sensing electrodes

Abstract

The mixed conductivity (ionic and electronic) of Ag 2S was measured at room temperature by a square wave method. Various types of cells were used. They allowed blocking of either the ionic or electronic current in the steady state. The electronic conductivity was very different depending upon whether the Ag 2S was contacted with metallic silver or not, the ratio of electronic to ionic conductivity being 3·22 or 0·015, respectively. The ionic conductivity was the same in both cases, in agreement with theoretical expectations for a system with high ionic disorder and relatively small electronic disorder. The measurements yielded also information about the kinetics of the exchange of Ag + -ions at the interface Ag 2S Ag aq. It was estimated that the exchange current density is at least 100 mA cm 2 for a 0·1 M AgNO 3 solution. In contrast to this the kinetics of a redox process (Fe 3− aq + e → Fe 2+ aq) were found to be very slow. These two features are favourable from the viewpoint of the use of Ag 2S as an ion selective electrode. The kinetics of the Ag/Ag + aq interface were also studied. Especially with AgNO 3 solutions the polarisation resistance is extremely high near the equilibrium potential (at cd's of the order of μA/cm 2) and drops suddenly at potentials above about 5–10 mV. The phenomena observed are probably due to adsorption effects. The difference between the behaviour of the Ag and Ag 2S interfaces is discussed.