Silver sulphides

Kunlze finds that the precipitate of silver sulphide, which is formed hy both melhods under Ihc same conditions, is always contaminated wilh more or less silver salt of allyl-ihiocarbamiuic acid ester. 

Fig. 12.14 Spread of silver sulphide from discontinuities in gold electrodeposits on silver substrates. The gold was deliberately scratched and the specimen exposed for 24 h to an atmosphere containing 10< o SO2. Immediately after this the sulphide stain extended 0-2 mm. Five years later, the stain extends to about 13 mm, after storage in a normal indoors atmosphere...

This complex ion dissociates to give silver ions, since the addition of sulphide ions yields a precipitate of silver sulphide (solubility product 1.6 x 10 49 mol3 L 3), and also silver is deposited from the complex cyanide solution upon electrolysis. The complex ion thus dissociates in accordance with the equation ... 

Thiosulphate as Ag2S. Add a slight excess of 0.1 M silver nitrate solution to the cold, almost neutral, thiosulphate solution. Heat at 60 °C in a covered vessel and, after cooling, filter and wash the silver sulphide precipitate with ammonium nitrate solution, water and finally with ethanol. Dry at 110°C and weigh as Ag2S (Section 11.76). 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

If the pellet contains a mixture of silver sulphide and silver chloride (or bromide or iodide), the electrode acquires a potential which is determined by the activity of the appropriate halide ion in the test solution. Likewise, if the pellet contains silver sulphide together with the insoluble sulphide of copper(II), cadmium) II), or lead) II), we produce electrodes which respond to the activity of the appropriate metal ion in a test solution. 

Fig. 21. Schematic representation of the Wagner experiment [1169] on the formation of silver sulphide at 493 K from the elements (2 Ag + S - Ag2S).

Potassium sulphide Rhenium (VII) sulphide Silver sulphide Sodium disulphide Sodium polysulphide Sodium sulphide Tin (II) sulphide Tin (IV) sulphide Titanium (IV) sulphide Uranium (IV) sulphide... 

Dimethyl mercury Iron Silver sulphide Hydrides... 

A mixture of silver sulphide heated with potassium chlorate causes a very violent reaction, which illustrates the usual incompatibility of sulphides with oxidants. 

Silver bromide Silver chloride Silver perchlorate Silver cyanide Silver fluoride Silver iodide Silver permar>gate Silver nitrate Silver carbonate Silver oxide Silver sulphate Silver sulphide Silver phosphate... 

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. 

For the halide ISEs on the basis of silver sulphide we may assume, e.g., in the case of chloride, the following exchange reaction ... 

The above equations have been derived under the condition that the mixture of halide sulphide is really polycrystalline, i.e., that no mutual compound has been formed between them, otherwise aCI and os2- (and so S0AgCi and S0Ag2s) would reflect considerable interaction. Further, the equations can be extended to any halide X instead of chloride. The aim that, in spite of the silver sulphide present, we can in fact deal with halide electrodes, can be fulfilled by the following conditions ... 

For metal ISEs based on silver sulphide we can assume for other divalent metals analogously the exchange reaction... 

The silver sulphide electrode is the most reliable electrode of this kind and is used to determine S2, Ag+ and Hg2+ ions. 

Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made using membranes fabricated from pure or doped silver sulphide (Ag2S). The membrane potential is affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that of the latter alone influences the membrane potential. The electrode will also respond to the presence of S2- ions because of their effect on the Ag+ ion activity via the solubility product expression ... 

Silver halide and thiocyanate membranes would respond in a similar way to a silver sulphide membrane, Ag+ ions being the mobile species, but by themselves make unsuitable membrane materials. A Nernstian response is, however, retained when they are incorporated into a Ag2S matrix, the membrane behaving as if it were a pure halide or thiocyanate conductor, i.e. 

Seaman and Stewart have described a radio-chemical assay for determining neomycin on cotton-fabric. Neomycin is reacted with carbon disulphide forming a dithiocarbonate which is then decomposed with [H°Ag] silver nitrate. The precipitated [H Ag] silver sulphide, which is directly related to the amount of neomycin present, is estimated by counting. 

Figure 4.11 A solid-state electrode showing a first-order response. An electrode designed to measure the activity of silver ions uses a crystalline membrane of silver sulphide. An equilibrium between the mobile silver ions of the membrane and the silver ions in the solutions results in the development of a potential difference across the membrane.

Burrow, J. H., and J. W. Mitchell Experiments with thin films of silver sulphide, silver and gold. Philos. Mag. (7) 45, 208 (1954). 

Mitchell, J. W. The properties of silver halides containing traces of silver sulphide. Philos. Mag. 40, 249 (1949). 

Buckley, A. N., 1994. A survey of the application of X-ray photoelectron spectroscopy to flotation research. Colloids Surf, 93 159 - 172 Buckley, A. N. and Woods, R., 1995. Identifying chemisorption in the interaction of thiol collectors with sulphide minerals by XPS adsorption of xanthate on silver and silver sulphide. Colloids and Surfaces A Physicochemical and Engineering Aspects, 104,2 - 3 Buckley, A. N. and Woods, R., 1996. Relaxation of the lead-deficient sulphide surface layer on oxidized galena. Journal of Applied Electrochemistry, 26(9) 899 - 907 Buckley, A. N. and Woods, R., 1997. Chemisorption—the thermodynamically favored process in the interaction of thiol collectors with sulphide minerals. Inert. J. Miner. Process, 51 15-26... 

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