The analytical chemistry of cyanide in acetonitrile-water mixtures

A method for the simultaneous determination of total gold and available cyanide in gold cyanidation solutions has been investigated. Total gold and available cyanide in cyanidation solutions are determined by ion exchange chromatography-HPLC under slightly acidic conditions. Cyanide is determined by measuring the peak height associated with the formation of the nickel-cyanide complex which is formed by adding an excess of nickel(II) to the gold cyanidation solution. The nickel(D)-cyanide complex is not strong enough to dissociate the very strong dicyanoaurate(I). Anion exchange is used for the separation of the cyano complexes of gold(I) and nickel, using a solvent comprising acetonitrile-water and containing citrate as counterion at pH 6.0 with UV-VIS detection at 229 nm. This provides two HPLC peaks, one giving the dicyanoaurate( I) complex and the other giving the concentration of the "available cyanide" as its nickel complex. The method is fast, accurate, sensitive, relatively cheap and can easily be made free from interfering anions commonly found in gold cyanidation solutions. In order to provide a basis for further development of this procedure in the presence of potentially interfering cations such as zinc(II) and copper(I), the formation constants of the metal-cyanide complexes were determined by potentiometric titrations in five different AN/H2O mixtures at 25°C and I = 1 M (Na)C1O4. Only the formation constants of cyanide complexes of Cd(II), Zn(II) and Cu(I) could be determined at the present study. Other metals of interest including nickel(II) were too strong to be studied by glass electrode potentiometry. Evidence was obtained for the formation of ternary Mn+/CN-/OH- complexes at high pH. However, only the formation constants of the binary Mn+/CN- complexes could be determined due to the lack of quantitative information for the hydroxo complexes in these solvent mixtures. The M'7CN equilibria shift strongly in favour of the formation of the higher-order complexes (n=3,4) at high acetonitrile concentrations. These changes are rationalised in terms of the Gibbs energies of transfer of the various species from water to the AN/H2O mixtures