SURFACE STUDIES OF THE COPPER SILVER SULFIDE BASED ION-SELECTIVE ELECTRODE MEMBRANE

The effect of oxygenated solutions on the surface composition of the jalpalte-based (Ag1.5Cu0.5S) copper ion-selective electrode membrane is examined, and it is demonstrated that oxidation of the surface occurs with a consequent shift in the E-degrees of the system. It is also established that only the outermost surface layer (approximately 2 nm) is hydrated and that the oxidation process extensively corrodes the surface of the membrane, causing crystals of insoluble salts to grow on the membrane surface. There is strong evidence that the severe interference with the function of the copper-selective electrode by both Hg(II) and Fe(III) is due to a photooxidation reaction in ambient light which leads to severe corrosion of the membrane surface and the formation of surface layers of mercury(I) and silver sulfates, respectively. In the dark, corrosion of the surface is less extensive although oxidation of the surface still occurs through reaction with dissolved oxygen to form mercury(II) sulfate on the surface in the presence of Hg(II) and silver sulfate in the presence of Fe(III). The strong interference from halides in ambient light is not due to photooxidation but to coupling of oxidation involving dissolved oxygen with ion-exchange to produce surface deposits of the silver halide. A similar reaction takes place in the dark.