Silver cyanide adsorption

In a much more through study [68,262-264], Pitt and coworkers examined the kinetics and thermodynamics of both gold and silver cyanide adsorption. They confirmed the decrease in cyanide uptake at high pH (.see Fig. 13) and speculated that this must involve a change in the concentration of active adsorption sites, ... 

FIG. 14 Schematic representation of the adsorption model for silver cyanide on activated carbons. (From Ref. 264.)... 

Fleming and Nicol [267] reported that adsorption of gold cyanide onto activated carbon is not an irreversible process, and thus confirmed similar findings for silver cyanide 1263]. They also confirmed that the process, like ad.sorption of silver cyanide [263] is exothermic, in contrast to the behavior of some other inorganic ions [125,144,268,172,269] (see also Section lll.B). 

TABLE 12 Effects of pH and the Presence of O on the Adsorption and Desorption of Gold and Silver Cyanide for Two Coconut-Shell-Based Activated Carbons... 

Two representative examples of this behavior are reflected in two distinct different chemical systems, namely (a) copper deposition from an acid sulfate electrolyte containing the co-inhibitors PEG-C1 and a bi-functional catalytic species SPS-C1 [12, 136, 243, 264] and (b) silver deposition from a cyanide electrolyte where inhibition is provided by adsorption of silver cyanide species and catalysis is achieved through adsorption of selenocyanate, SeCN [72-75]. Similar behavior is evident in some electrolytes used for the deposition of bright soft gold films [121, 180, 255-261, 267]. 

Silver deposition is important for technical applications and is achieved by electroless deposition (from a silver-cyanide bath) or bulk electrodeposition from the same bath. It has been studied by a large number of techniques [59,62,63,87], and most of them on gold and platinum in sulfate or perchlorate electrolytes [88-95]. However, there is lack of information on the problem of co-adsorption of metal and anions and the surface restructuring caused by the presence of strong adsorbable anions such as halides. 

The efficiency of the adsorption of many gases on solid adsorbents may be increased by using specific impregnation substances. These are applied onto carriers or directly onto the charcoal. In this way it is possible to increase strongly, for instance, the adsorption efficiency for ammonia (with the help of bromine), formaldehyde (with the help of sodium sulphide) and hydrogen sulphide (with the help of silver cyanide or lead acetate). 

This could not be explained on the basis of the Garten and Weiss mechanism, because both Au(CN)2 and Ag(CN)2 are univalent negative ions. Similarly, the adsorption of silver cyanide Ag(CN)2 at pH 11 to 11.5, where the zeta potential of the carbon is -50 to -60 mv, and where the carbon surface will have more negative than positive sites, could not be explained by the electrostatic forces. 

FIGURE 5.3 Schematic representation of the adsorption model for silver cyanide on activated carbons. (Adapted from Radovic, L.R., in Chemistry and Physics of Carbon, Vol. 27 L.R. Radovic, Ed., Marcel Dekker, New York, 2001. With permission.)... 

Bozzini, B., D Urzo, L., Mele, C. and Romanello, V. (2008) A SERS investigation of cyanide adsorption and reactivity during the electrodeposition of gold, silver, and copper from aqueous cyanocomplexes solutions. The Journal of Physical Chemistry C, 112, 6352-6358. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

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... 

The inhibiting influence of sodium diisobutyldithiophosphinate [385] and 2-mercaptobenzothiazole [386] on silver dissolution in aqueous solutions of pH = 11 has also been investigated. Inhibition was caused by adsorption of both compounds, accompanied with the displacement of cyanides from the silver surface. 

Other metals, such as copper, nickel, or silver, have been used as electrode materials in connection with specific applications, such as the detection of amino acids or carbohydrates in alkaline media (copper and nickel) and cyanide or sulfur compounds (silver). Unlike platinum or gold electrodes, these electrodes offer a stable response for carbohydrates at constant potentials, through the formation of high-valence oxyhydroxide species formed in situ on the surface and believed to act as redox mediators (40,41). Bismuth film electrodes (preplated or in situ plated ones) have been shown to be an attractive alternative to mercury films used for stripping voltammetry of trace metals (42,43). Alloy electrodes (e.g., platinum-ruthenium, nickel-titanium) are also being used for addressing adsorption or corrosion effects of one of their components. The bifunctional catalytic mechanism of alloy electrodes (such as Pt-Ru or Pt-Sn ones) has been particularly useful for fuel cell applications (44). 

The adsorption of CN on a polycrystalline copper electrode has been studied experimentally by Lee et al. [116], Since copper oxidizes more readily than gold and silver, the results depend strongly on the cyanide concentration. At low cyanide concentration in solution (2x 10 M), a potential dependent band between 2084 and 2120cm was assigned to the linearly adsorbed CN ion (Fig. 33, p. 172). 

Reactions were also monitored by SERS. Yamada et discussed the reaction of rose bengal adsorbed on a ZnO electrode. This is an example of the use of the resonance enhancement to gain enough sensitivity. Billmann et followed the chemisorption of cyanide on silver, and detected the formation of the various cyanides on the surface. Similarly, Loo investigated halide complexes on a gold electrode. Pemberton and Buck " used SERS to see the adsorption of diphenylthiocarbazone and its oxidation into... 

Donnan, from Laplace s theory of the internal pressure of liquids, arrived at a theory of negative surface tension which was supposed to explain why substances disperse to form colloidal rather than true solutions, a minimum size of particle being stable. Donnan and Barker investigated adsorption (see p. 742), Donnan and Le Rossignol the kinetics of the reaction between ferri-cyanide and iodide (see p. 660), and Donnan and Miss K. A, Burke the kinetics of the reaction between silver nitrate and alkyl iodides. 

---