Surface studies of the silver sulfide ion selective electrode membrane

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) have been used to examine the surface layers of the silver sulfide ion selective electrode membrane. It has been found that the outermost surface layer of the membrane is hydrated, and that equated ions do not penetrate the membrane beyond this "skin" layer. The mercuric ion undergoes ion-exchange reactions with the Ag2S surface. A study of mercuric ion interference shows that reductive Hg2+ ion exchange occurs in the presence of light, whereas a metathetic displacement reaction occurs in the dark. Ferric and cupric ion interference studies show that these ions exhibit relatively weaker effects. Exposure of Ag2S to solutions of Cl-, Br-, and I- in the presence of light has been found to cause crystals of the corresponding silver halide to grow on (and out of) the membrane surface. It is postulated that the reaction mechanism of the membrane involves photooxidation of Ag2S to produce some metal deficient sulfide(s) (e.g. Ag2S2 and/or elemental S).