Cyanide Analysis Chemistry
Cyanide refers to a monovalent anion consisting of carbon and nitrogen atoms with triple covalent bonds.
Transition metals form strong bonds with the carbon atom of cyanide. Alkali metals form ionic bonds with the CN molecule. The relative stability of metal-cyanide compounds and complexes in water (listed in order of increasing stability) is presented below. Cyanide Species 1. Free Cyanide CN - , HCN 2. Simple Cyanide Compounds a) Readily Soluble NaCN, KCN, Ca(CN) 2 , Hg(CN) 2 b) Relatively Insoluble Zn(CN) 2 , CuCN, Ni(CN) 2 , AgCN 3. Weak Metal-Cyanide Zn(CN) 4 2 - , Cd(CN) 3 - ,Cd(CN) 4 2 - 4. Moderately Strong Metal-Cyanide Complexes Cu(CN) 2 - , Cu(CN) 3 2 - , Ni(CN) 4 2 - , Ag((CN) 2 - 5. Strong Metal-Cyanide Fe(CN) 6 2 - , Fe(CN) 6 4 - , Co(CN) 6 4 - , Au(CN) 2 - ,Hg(CN) 4 2 - Free Cyanide Free cyanide refers to the sum of hydrogen cyanide (HCN) and cyanide ion (CN - ) in a sample. Free cyanide is bioavailable and toxic to organisms in aquatic environments. At a pH of 7 or less free cyanide consists entirely of HCN. 4 For analytical purposes free cyanide is defined as the amount of HCN liberated from a solution at pH 6.0. Weak Acid Dissociable (WAD) Cyanide WAD cyanide refers to metal cyanide complexes (Zn, Cd, Cu, Hg, Ni, and Ag) that dissociate under weak acid conditions of pH 4.5 to 6. Cyanide Amenable to Chlorination (CATC) CATC refers to cyanide complexes that dissociate and oxidize when exposed to chlorine under alkaline conditions. 4 CATC consists of free cyanide and weak acid dissociable cyanide complexes. Testing for CATC is intended to measure the effectiveness of cyanide destruction by chlorination. To determine CATC a sample is split into two portions, with one portion tested for total cyanide and the second portion treated with calcium hypochlorite (to destroy cyanide) prior to a total cyanide test. The difference in total cyanide contents constitutes CATC cyanide.
:C N
Cyanide is very reactive and readily forms metal-cyanide complexes and organic compounds. The chemical composition of cyanide in environmental samples is affected by factors such as pH, temperature, trace metal content, and the presence of sulfur or sulfur compounds. A multitude of cyanide species of varying stability may be present in a sample. For these reasons cyanide testing can be a challenging analytical task. Methods for the determination of cyanide are defined by the relative strength of metal complexes. From an analytical perspective cyanide compounds are broadly classified into three categories; total cyanide, available or weak acid dissociable (WAD) cyanide, and free cyanide.
Strong Metal-Cyanide Complexes of Fe
Weak and Moderately Strong Metal-Cyanide Complexes of Ag, Cd, Cu, Hg, Ni and Zn
Total Cyanide WAD
Cyanide
CN - HCN
Free Cyanide
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