General properties of Raman spectra of silver cyanide complexes

The chemistry of the silver cyanide bath is determined by a variety of silver cyanide complexes existing in the solution. The following equilibria for 25°C, are known in the literature [4, 5]:

Ag(CN) ~2 + CN" <-» Ag(CN) j' K = 7.01 mol"1 Ag(CN) + CN" o Ag(CN) K= 1.71 mol

From these complexes Ag(CN) exists as a white precipitate which dissolves into one of the soluble complexes in the presence of an excess of cyanide. All these complexes are supposed to exist on the surface. The formation potentials of the complexes are the following [4]:

Ag(CN) + e" -> Ag + CN" £0(SCE) = -0.266 V

Ag(CN) 2 + e" -> Ag + 2 CN" £0(SCE) = -0.56 V

Ag(CN) 3~ + e" Ag + 3 CN" £0(SCE) = -0.75 V

In principle it is possible to identify the complexes from their Raman spectra, as the cyanide group shows a characteristic vibrational band shifting to higher wavenumbers with increasing number of ligends. The following wavenumbers are knwon [6]:

From the Raman spectra obtained it is assumed in the literatur [7] that under the conditions described for the silver deposition, Ag(CN) 2{ is the predominant complex at a potential of -800 mVscE on the surface, and Ag(CN) ~2 at a potential greater than

-250 mVscE-

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