Zinc cyanide complexes

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

In a mixed copper-zinc solution of complex cyanide, however, the Cu ion concentration can be reduced to the order of lO mol/L and the concentration ratio (zinc ion)/(copper ion) will be made very large. A detailed calculation for this case is given by Faust in the 1974 edition of Modem Electroplating (1). It is shown there, and in detail below, that the copper cyanide complex is Cu(CN)3 , for which the dissociation value is known. The dissociation constant for the zinc cyanide complex, Zn(CN)4 , is also well known. Using those values that determine the fraction concentration of the free metal ion in solution and assuming an initial specific molar concentration, it is shown below that their respective reversible electrode potentials [see also Eq. (11.1)] can be brought together. 

The zinc-cyanide complexes are on the interface of organic and inorganic chemistry. We include them here because of their Zn—C bond. (Perhaps this will encourage thermo-chemical investigations on the isoelectronic alkynylzinc species, much as there has been interest in both cyano and alkynyl silver-containing species. ) The species Zn(CN)2, Zn(CN)3 and Zn(CN)4 have all been thermochemically characterized in aqneons soln-tion by thermometric titration. The enthalpy of reaction values kJmoL are shown for reactions 9-11. 

The sorption of cyanide ions and copper and zinc cyanide complexes was studied on soils (Table 3.13) and sediments (Table 3.15). Distilled water and natural river water were applied as the solution phase. Sorption was studied by radioisotopic labeling with 14CN ions. 

It should be noted that this discussion is based on a comparison of the equilibrium constants of enzymic zinc-cyanide complexation versus aqueous zinc-cyanide complexations. Cyanide has a high affinity for the soft zinc ion under both conditions (stability constant of [Zn(CN)4p = 7.7 x lO" ) hence it should not be concluded that there is any lack of affinity for cyanide in the enzyme. 

Chemical oxidation is very effective in destroying free cyanide as well as cadmium, copper, and zinc cyanide complexes. However, nickel cyanide is incompletely destroyed, and iron cyanide complexes are apparently unaffected by chlorine or ozone. The ozone-UV radiation process (i.e., advanced oxidation process) is effective for treatment of complexed cyanide, such an ferric cyanide, copper cyanide, and nickel cyanide. Performance data of oxidation processes from the following industries are presented in the appendixes ... 

In a cyanide-containing bath, the copper potential is sufficiently negative so cementation does not occur and copper can be successfully deposited onto zinc. This is due to the fact that the cyanide complexes of copper are very strong so the potential of copper in such a solution is much more negative than in simple salt solutions. On the other hand, the zinc cyanide complex is relatively weak and the potentials of two metals become comparable so an external power supply is required to deposit copper on the zinc from cyanide. 

The water solubiUty of zinc compounds varies greatly, as shown in Table 1. Water-soluble compounds not Hsted are zinc formate [557-41-5] chlorate [10361-95-2] fluorosihcate [16871 -71 -9] and thiocyanate [557-42-6]. Also, the water-soluble amino and cyanide complexes have many uses. 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

The paper [8] includes results of investigating electron mechanisms of the impact of active particles, radicals, hydrated electrons artificially generated by plasma on the behavior of cyanide complexes of zinc in water solutions. The above investigation was conducted using quantum chemistry methods. Quantum-chemical calculation of electron structure of the complexes Zn(CN)42 4EP-20H- with complete optimization of all geometric parameters [9] was performed. 

Cyanide occurs most commonly as hydrogen cyanide in water, although it can also occur as the cyanide ion, alkali and alkaline earth metal cyanides (potassium cyanide, sodium cyanide, calcium cyanide), relatively stable metallocyanide complexes (ferricyanide complex [Fe(CN)6]-3), moderately stable metallocyanide complexes (complex nickel and copper cyanide), or easily decomposable metallocyanide complexes (zinc cyanide [Zn(CN)2], cadmium cyanide [Cd(CN)2]). Hydrogen cyanide and cyanide ion combined are commonly termed free cyanide. The environmental fate of these cyanide compounds varies widely (Callahan et al. 1979). 

In the cyanidation process the ore is crushed and roasted with sodium chloride and then treated with a solution of sodium cyanide. Silver forms a stable silver cyanide complex, [Ag(CN)2]. Adding metallic zinc to this complex solution precipitates sdver. 

Additive or more-than-additive toxicity of free cyanide to aquatic fauna has been reported in combination with ammonia (Smith et al. 1979 Leduc et al. 1982 Alabaster et al. 1983 Leduc 1984) or arsenic (Leduc 1984). However, conflicting reports on the toxicity of mixtures of HCN with zinc or chromium (Towill et al. 1978 Smith et al. 1979 Leduc et al. 1982 Leduc 1984) require clarification. Formation of the nickelocyanide complex markedly reduces the toxicity of both cyanide and nickel at high concentrations in alkaline pH. At lower concentrations and acidic pH, solutions increase in toxicity by more than 1000-fold, owing to dissociation of the metallo-cyanide complex to form hydrogen cyanide (Towill et al. 1978). Mixtures of cyanide and ammonia may interfere with seaward migration of Atlantic salmon smolts under conditions of low dissolved oxygen (Alabaster et al. 1983). The 96-h toxicity of mixtures of sodium cyanide and nickel sulfate to fathead minnows is influenced by water alkalinity and pH. Toxicity decreased with increasing alkalinity and pH from 0.42 mg CN/L at 5 mg CaCOj/L and pH 6.5, to 1.4 mg CN/L at 70 mg CaCOj/L and pH 7.5 to 730 mg CN/L at 192 mg CaCOj/L and pH 8.0 (Doudoroff 1956). 

Very recently Geus and co-workers [44, 45] have applied another method based on chemical complexes. This is the complex cyanide method to prepare both monocomponent (Fe or Co) and multicomponent Fischer-Tropsch catalysts. A large range of insoluble complex cyanides are known in which many metals can be combined, e.g. iron(n) hexacyanide and iron(m) hexacyanide can be combined with iron ions, but also with nickel, cobalt, copper, and zinc ions. Soluble complex ions of molybdenum(iv) which can produce insoluble complexes with metal cations are also known. Deposition precipitation (Section A.2.2.1.5) can be performed by injection of a solution of a soluble cyanide complex of one of the desired metals into a suspension of a suitable support in a solution of a simple salt of the other desired metal. By adjusting the cation composition of the simple salt solution, with a same cyanide, it is possible to adjust the composition of the precursor from a monometallic oxide (the case when the metallic cation is identical to that contained in the complex) to oxides containing one or several foreign elements. 

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