Silver sulphide conductivity

Homogeneous polycrystalline membrane electrodes [see Fig. 2.10 (3)J. The relatively high electrical conductance of monoclinic / -Ag2S and its extremely low solubility product led to the development of halide and other metal ISEs with addition of silver sulphide. 

Silver sulphide exists in two modifications, a-Ag2 S, the cubic form, which is an electronic conductor and is stable above 176°C, and monoclinic/l-AgjS, an ionic conductor, which is stable at lower temperatures [316]. In this latter modification, Ag is almost the only charge carrier [141, 325,428], The good conductivity and negligible solubility of the compact membrane make the Ag2 S ISE one of the most reliable sensors. 

Other electrodes are based on silver salts or metal sulphides, and are prepared by pressing the salts into a disc together with a polymeric support matrix made of rubber, silicone or PVC, for example. Silver salts conduct via Ag+ ions, and silver sulphide is added to the metal sulphides to improve conductivity. Examples are given in Table 13.1. 

As seen in Table 13.1 and already stated, silver sulphide is added to electrodes based on sulphides of copper, cadmium, or lead to improve conductivity. In fact the functioning is not altered as the solubility product of silver sulphide is significantly lower than that of the other sulphides. The expression for the potential is... 

Some substances manifest both electronic and electrolytic conductance at the same time. Among such conductors are some metal compounds such as solid silver sulphide, cuprous oxide, zinc oxide, cuprous chloride and similar substances. A special group includes solutions of alkali- and alkaline-earth metals in amines, namely in liquid ammonia, where appart from cations of metals and electrons bonded to the ammonia also free electrons are present at higher concentrations of solutions, whereby the conductance is considerably increased. 

Where the crystal membrane contains both Ag2S and a silver hahde, the electrode is then selective to the hahde involved. The silver hahde wiU have higher solubhity than the silver sulphide. It wdl, however, have an equihbrium solubihty such that the hahde activity in the membrane wiU be significantly less than its activity in the test solution. The extent of the difference in activities on either side of the interface generates a potential difference resulting in the movement of the conducting silver ions. The extent of the difference in potential relates through this movement to the activity (concentration) of the hahde in the test solution. 

The method can also be used to determine the ratio between the ionic and electronic conductivities by measuring the change of the anode relative to the total charge passed. This was first done by Hittorf (1851) , who was able to follow the change of silver sulphide from a predominantly ionic conductor at low temperatures to a predominantly electronic conductor at high temperatures. 

Copper and silver tarnish readily in sulphide atmospheres, and copper in contact with sulphur-vulcanised rubber will sometimes react with the sulphur, devulcanising it in the process. The growth of conducting sulphide whiskers on silver is noteworthy as these whiskers may give rise to short circuits across silver-plated contacts. Ammonia has little effect on most metals, but traces will tarnish many copper alloys and cause stress-corrosion cracking of certain stressed brasses. 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

In the last 20 years, new sulphide, sulphate, molybdate, halide, etc., based compositions have been obtained in the glassy state (Ingram, 1987). They have much higher ionic conductivity than most oxide glasses at ambient temperature, e.g. from 10 to 10 S cm in the case of some lithium or silver conducting glasses (Fig. 4.1(h)). 

Fig. 2.23 shows the separation achieved on a 12 anion standard by this procedure. Sulphide, cyanide, bromide, and sulphite are detected at the silver electrode while nitrite, nitrate, phosphate and sulphate produce no response. Due to the low dissociation of hydrogen sulphide and hydrogen cyanide following protonation by the suppressor column, they are not detected by the conductivity detector. 

Amperometric detectors, or specifically their electrodes, need much more care and maintenance and are therefore used only in cases where conductivity measurements are not possible or sensitive enough. Analytes range from sugars, metal-oxo-anions, to toxic anions, such as cyanide and sulphide. The determination of cyanide and sulphide in a drug substance is shown in Fig.5. The sample is analyzed after on-line matrix removal and accumulation of the analyte (a system similar to that described by Reust et al. in ) amperometrically on a silver electrode. 

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