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Cyanide
Cyanide, what is it? What are its chemical forms? And how should we measure it?
Aquatic Free Cyanide
Cyanide is about 1000 times more toxic to aquatic life than it is to humans. For this reason, it is important that free cyanide be measured in fragile aqueous environments. Free Cyanide is defined as the amount of HCN that is liberated from a solution at pH 6.0, and should only measure cyanide present in solution as HCN or as CN-.
Gas-Diffusion Amperometric Flow Injection analyzers accurately measure aquatic free cyanide by ASTM D7237-06.
Simple Cyanide Compounds
Simple Cyanides are the readily soluble salts of cyanide such as sodium, potassium, and calcium and along with free cyanide are the only cyanide complexes likely to be present in Drinking Water.
Available Cyanide, Weak and Moderately Strong Metal-Cyanide Complexes
Weak to moderately strong metal-cyanide complexes are compounds that could readily release hydrogen cyanide gas from an acid. Weak And Dissociable (WAD), Cyanide Amenable to Chlorination (CATC), and Ligand Exchange methods have been devised in an effort to quantify these complexes.
Gas-Diffusion Amperometric Flow Injection analyzers accurately measure Available Cyanide in wastewater by OIA1677 or ASTM D6888 and in ore processing and related metallurgical materials. Gas-diffusion Amperometric flow injection methods are the only interference free approved methods for the determination of available cyanide in wastewater that do not require time consuming and expensive preliminary distillations.
Weak metal-cyanide complexes are those in which cyanide ions are weakly bound to a metal such that they can dissociate at pH < 6 to form HCN. This is in contrast to strong metal-cyanide complexes that require additional energy, other than lowering of pH, to liberate Hydrogen Cyanide gas. There are three cyanide techniques designed to measure weak-metal complexes plus free cyanide.
Weak metal-cyanide complexes are:
[Zn(CN)4]-2
[Cd(CN)4]-2
[Cu(CN)4]-3
[Ag(CN)2]-1
[Ni(CN)4]-2
[Hg(CN)4]-2
Hg(CN)2
The three techniques used to analyze for the cumulative sum of these cyanide species present in a sample are referred to as WAD (Weak Acid Dissociable), CATC (Cyanide Amenable to Chlorination, and Available Cyanide (Ligand Exchange, Gas Diffusion-Amperometry).
All three techniques recover most of these species quantitatively. The only technique that recovers every species quantitatively is ligand exchange gas diffusion amperometry.
Total Cyanide, Strong Metal - Cyanide Complexes
Commonly known as "total" cyanide and defined by the USEPA as cyanide ion and complexes converted to hydrogen cyanide gas by reflux distillation in the presence of strong acid and the magnesium ion. Methods for the analysis of "total cyanide" include ferrous and ferric complexes plus available and free cyanide. Thiocyanate or cyanate ions are not included. Use ASTM D7284 to measure total cyanide after distillation, or avoid distillation completely and use ASTM D7512. These gas diffusion amperometric methods will ensure accurate and precise results in the toughest of matrices.
Cyanide in Drinking Water
Gas-Diffusion Amperometric Flow Injection analyzers accurately measure free cyanide in drinking water by OIA1677-DW or ASTM D6888-04. These flow injection methods are the only interference free approved methods for the determination of cyanide in drinking water that do not require time consuming and expensive preliminary distillations.
