Silver sulphide formation

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).

The attractive appearance of silver has caused it to be in great demand for ornamental purposes. As has been mentioned, its main disadvantage lies in the ease with which it tarnishes, particularly in our centres of industry because of the presence of sulphur compounds in the atmosphere which induce the formation of a black, dull superficial layer of silver sulphide. 

Reaction (13) is particularly problematic in photographic silver recovery as a rapid chemical reaction results in the formation of silver sulphide ... 

SO that the ester which is always present even in freshly-prepared alcoholic solutions of mustard oil does not decompose quantitatively with ammonia- cal silver solution, with formation of sulphide of silver, but is partly pre- cipitated as a silver salt of the above-mentioned ester. Whilst this admixture, owing to its molecular weight 252, which differs but little from that of silver sulphide 248, cannot cause an appreciable error in the gravimetric determination, it cans s a considerable difference in the volumetric estimation, because in the formation of this ester salt only 1 atom of silver corresponds to 1 molecule of mustard oil, whereas for the conversion into silver sulphide, 2 atoms of silver are required, so that the titration results are obviously too low. 

The tarnishing of copper and silver in dry air containing traces of hydrogen sulphide (Table 2.6) is another example of film growth by lattice diffusion at ambient temperatures. In these cases defects in the sulphide lattice enable the films to grow to visible thicknesses with the consequent formation of tarnish films which are aesthetically objectionable and may have a significant effect on the behaviour of the metals in particular applications, e.g. electrical contacts. 

Dilute binary alloys of nickel with elements such as aluminium, beryllium and manganese which form more stable sulphides than does nickel, are more resistant to attack by sulphur than nickel itself. Pfeiffer measured the rate of attack in sulphur vapour (13 Pa) at 620°C. Values around 0- 15gm s were reported for Ni and Ni-0-5Fe, compared with about 0-07-0-1 gm s for dilute alloys with 0-05% Be, 0-5% Al or 1-5% Mn. In such alloys a parabolic rate law is obeyed the rate-determining factor is most probably the diffusion of nickel ions, which is impeded by the formation of very thin surface layers of the more stable sulphides of the solute elements. Iron additions have little effect on the resistance to attack of nickel as both metals have similar affinities for sulphur. Alloying with other elements, of which silver is an example, produced decreased resistance to sulphur attack. In the case of dilute chromium additions Mrowec reported that at low levels (<2%) rates of attack were increased, whereas at a level of 4% a reduction in the parabolic rate constant was observed. The increased rates were attributed to Wagner doping effects, while the reduction was believed to result from the... 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

In some cases, such as with the oxides of silver and copper, the sulphate can undergo further reduction by sulphur with formation of sulphide and sulphur dioxide the oxides of zinc, tin and iron are not greatly attacked by sulphur, whilst chromium trioxide reacts so violently as to cause inflammation of the sulphur.6... 

Sulphates of the alkali and alkaline earth metals, when heated with sulphur, are converted into sulphide, polysulphide and thiosulphate, with simultaneous formation of sulphur dioxide many other sulphates, e.g. those of copper, mercury, silver and lead, yield only sulphide.7 Other salts of the metals behave in a similar- manner, undergoing transformation into sulphides, the change being effected more readily with the salts of the heavy metals, many of which indeed react slowly with sulphur even at 100° C. in the presence of water.8 At 150° to 200° C. mercuric, stannic and ferric salts in aqueous solution are quantitatively reduced by sulphur mercurous, cupric, bismuth and lead salts arc slowly but quantitatively precipitated as sulphides. Nitrates, permanganates and iodates cause oxidation of the sulphur to sulphuric acid. 

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