Metal sulfide membrane electrodes

Typical response curves of metal sulfide membrane electrodes in pure aqueous solutions show a linear relation with log C, with the slope expected from the Nemst... 

AgjS can also be used as a matrix for other metal sulfides, notably CuS, PbS, and CdS, giving membranes responsive to the second metal. The electrode can be regarded as responding to the equilibrium activity of Ag" ", which is given by... 

Membranes constructed either from silverasulfide or from a mixture of silver sulfide and anollicf silver salt are useful for the determination of both. sulfide and silver ions. Toward silver Ions, the electrical response i.s similar to a metal electrode of the first kind (although the mechanism of activity is totally different). I he electrical response of a. silver sulfide membrane lo sulfide ions is similar to that of an electrode of the second kind (Section 23B-2). When the membrane is immersed in the analyte solution, a minuscule amount of silver sultlde di.ssolves and quickly saturates the tilm of liquid adjacent to the clecircxle. I he solubility, and thus the silver ion concentration, however, depends on the sulfide concentration of ihe analyte. 

The 14-membered macrocyclic phosphine 77 and corresponding sulfide 78 were prepared by the macrocyclization under high dilution conditions and further oxidation with sulfur (Scheme 12.29). Both ligands were used as ionophores in an all-solid state poly(vinylchloride) matrix membrane electrode. The sulfide 78 based electrode exhibited a Nemstian response towards copper (II) ions with a cationic slope of 30.7 mV/pCu and a detection limit of 5.97 x 10 M. The potential response remains almost unchanged over the pH range 3.9-6.4 at least 7 months. Sensors based on the macrocycles 77 and 78 demonstrate the copper (11) ion sensitivity over other metal ions [83]. 

Table 10.1 summarizes the characteristics of common ISEs and a number of new sensors in this field. We have not included in this table the liquid or polymer membrane-based electrodes which are selective, but rather fragile (for more details on such membranes see References 58,59). ISEs of the first kind are not very numerous, e.g., F -ISE (monocrystalhne membrane based on LaFj), Ag" -ISE (silver salts), or Na" -ISE (Na alumino-silicate glass or polyciystalline NASICON [Na super ionic conductor] membranes). Most of the ISEs are of the second kind and are based on insoluble silver salts for example, halide ISEs (CE, Br, I"), Cd ", Pb ", Cu ", etc. Such ISEs use mixtures of insoluble salts based on silver sulfide or silver selenide. Recently, Vlasov etal. and Neshkova have proposed several glasses sensitive to transition metals. Typical ISE devices are shown in Figure 10.5. Thin-layer chemical sensors based on chalcogenide glasses have also been developed. ... 

Due to its large compressibility, silver sulfide serves, for many manufacturers, as a basis material for halide as well as metal electrodes. In this case only about 30% of the corresponding silver halide is used relative to silver sulfide, and the mixture is compressed at higher pressures and temperatures. It is, strictly speaking, no longer a homogeneous solid-state membrane. 

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