ABSTRACT
Dissolved phase cyanide can exist in various inorganic and organic forms. Inorganic forms of cyanide are usually classified into three forms, namely, (i) free cyanide, (ii) weak metal-cyanide complexes; and (iii) strong metal-cyanide complexes [1]. The simplest and the most reactive class of cyanide species is free cyanide, which includes both the cyanide anion, CN−, and its protonated form HCN(aq). The next most reactive species are the soluble complexes of the cyanide ion with certain transition metals like copper, zinc, cadmium, nickel, and silver. This group of transition metalcyanide complexes is commonly known as weak metal-cyanide complexes or weak acid dissociable cyanides because of their ability to release the cyanide ion in aqueous solutions under weak acid
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List of Common Dissolved Cyanide Species Associated with Contaminated Waters
conditions. Finally, there are strong cyanide complexes with other transition metals, for example, gold, platinum, cobalt, and iron. Because of strength of the metal-cyanide bonds in these complexes, they often require strong acidic pH conditions (pH < 2) and heat to dissociate and release the cyanide ion. As a result, these complexes are often referred to collectively as strong metal-cyanide complexes or strong acid dissociable cyanides. Table 7.1 lists some common inorganic cyanide species encountered in contaminated waters. A more complete list is provided in Chapter 5.
