In this section, cyanide refers to forms of inorganic cyanide (CN−) in water. Cyanide can exist in aqueous media as free undissociated hydrogen cyanide (HCN), free cyanide ion (CN−), and anionic complexes of cyanide with various metal cations.
Hydrogen cyanide is a very weak acid with a dissociation pKa of 9.2. In water samples with approximately neutral pH, HCN is the predominant form1 rather than free CN− anion.
Simple cyanide salts (NaCN and KCN) dissociate completely in water unless some other metal cations are present; then, anionic cyanide complexes can be formed. The stability of these complexes varies widely, with thermodynamic formation constants ranging from 1041.5 for Hg(CN)42− to 1016.7 for Zn(CN)42−. Their stability depends on pH; most dissociate appreciably in acidic solutions. The most stable anionic cyanide complexes are formed with cobalt, gold or iron; when water contains excess cyanide, all available iron are bound in either the ferrocyanide [Fe(CN)64-] or ferricyanide [Fe(CN)63-] complex.
Molecular HCN is highly toxic to aquatic life2–5; it is formed in solutions of cyanide by hydrolytic reaction of CN−anions with water. CN− anion is less toxic than HCN; it usually is unimportant because most of the free cyanide (CN group present as CN− or as HCN) exists as HCN2–5 because most natural waters have a substantially lower pH than the pKa for molecular HCN. The toxicity to fish of most tested solutions of complex cyanides is attributable mainly to the HCN resulting from dissociation of the complexes.2,4,5 Analytical distinction between HCN and other cyanide species in solutions of complex cyanides is possible.2,5–10