Free Cyanide (CNF)

Only hydrogen cyanide and the cyanide ion in solution can be classed as “free” cyanide. The proportions of HCN and CN in solution are according to their equilibrium equation; this is influenced by the solution pH. Methods used to detect free cyanide should not alter the stability of weaker cyanide complexes, as they may otherwise be included in the free cyanide result. Methods used to detect free cyanide should be clear of interferences due to the presence of high concentrations of more stable cyanide complexes or other cyanide forms. If not, the interference must be quantified and allowed for in the result.

Weak Acid Dissociable Cyanide (CNWAD)

Unlike the definition of “free cyanide” which identifies the specific cyanide species being measured, WAD cyanide refers to those cyanide species measured by specific analytical techniques. WAD cyanide includes those cyanide species liberated at moderate pH of 4.5 such as HCN(aq) and CN, the majority of Cu, Cd, Ni, Zn, Ag complexes and others with similar low dissociation constants. Methods used to measure WAD should be free from interferences due to the presence of high concentrations of more stable cyanide complexes or other cyanide forms. If not, the interference must be quantified and allowed for in the result.

Total Cyanide (CNT)

This measurement of cyanide includes all free cyanide, all dissociable cyanide complexes and all strong metal cyanide including ferro-cyanide Fe(CN)6-4, ferri-cyanide Fe(CN)6-3, and portions of hexacyano cobaltate Co(CN)6-3 and those of gold and platinum. Only the related or derived compounds cyanate (CNO) and thiocyanate (SCN) are excluded from the definition of total cyanide. Methods used to determine total cyanide must be shown to be capable of quantitatively determining all stable complexes of cyanide, including the cobalt cyanide complex. If methods determine other analytes as well (e.g. include SCN), those analytes need to be determined separately and allowed for in the total result.