Determination of total and free cyanide in water after distillation

Abstract

Improved analytical methods are described for the determination of total cyanide after separation from thiocyanate (SCN^–) and free cyanide [CN^–+ HCN (aq)] after separation from hexacyanoferrate [Fe(CN)₆^4– and Fe(CN)₆^3–] and thiocyanate in water by means of a standard continuous-flow system. Separation of total cyanide from thiocyanate is achieved by illumination of samples with visible light, preventing transition of SCN^– to CN^–, prior to distillation. Separation of free cyanide from hexacyanoferrate and thiocyanate is performed by distillation of samples at pH 4 in a citrate buffer solution in the presence of Cu²⁺. Distillation under these conditions prevents decomposition of hexacyanoferrate and evaporation of thiocyanate as HSCN. Both methods combine the advantages of samples distillation with a high specificity for total and free cyanide, respectively.