Available Cyanide Available cyanide encompasses free cyanide and WAD cyanide complexes which dissociate under mildly acidic conditions of pH 3 to 6. Available cyanide is determined by ligand exchange pretreatment followed by flow injection analysis of the liberated CN - . 4 Total Cyanide Total Cyanide refers to the sum of all cyanide-containing compounds in a sample, including free cyanide, WAD cyanide compounds and strong metal-cyanide complexes. Strong acidic conditions (pH<2) are required to dissociate strong metal-cyanide complexes and release free cyanide. Analytical methods for total cyanide determination employ an acid distillation or UV irradiation step to release free cyanide. 4 Interferences There are a number of known interferences associated with cyanide methods that employ an acid distillation step. In fact, the USEPA Solutions to Analytical Chemistry Problems with Clean Water Act Methods 5 (“Pumpkin Guide”) notes; “Next to oil and grease, cyanide is the pollutant for which the most matrix interferences have been reported to EPA.” For accurate results it is important to select an analytical technique and method that are free of these interferences. Sulfide – Sulfide when present in samples can cause low or high cyanide recoveries. Low recoveries are caused by sulfide reacting with cyanide to form thiocyanate. High recoveries occur when volatilized sulfide distills over and reacts with colorimetric reagents and is erroneously detected as cyanide. Thiocyanate – Distillation of samples containing thiocyanate (SCN - ) can result in low cyanide recoveries. Thiocyanate decomposes to oxidized sulfur compounds during distillation, which react with cyanide in the absorber solution and cause a negative bias. Thiosulfate – Samples containing thiosulfate (S 2 O 3 2 - ) salts can react during acid distillation to form cyanate compounds and cause a negative bias.
Nitrate and Nitrite – Samples containing nitrate and/ or nitrite and organic compounds can react during acid distillation to form cyanide and cause a positive bias. Sulfite – Wastewater effluent samples that have been chlorinated and then de-chlorinated using sulfur dioxide will contain sulfites. Acidification of sulfite during distillation forms H 2 SO 3 , which decomposes to form SO 2 gas. This SO 2 gas is absorbed in the NaOH scrubber solution, (along with HCN gas), forming Na 2 SO 3 . Because a stream of air continuously passes through the NaOH scrubber, the absorbing solution contains sulfite (SO 3 2 - ), dissolved O 2 and CN - . The sulfite and oxygen will oxidize CN - forming cyanate (OCN - ) resulting in a negative interference. Oxidizing Agents – Chlorine, hypochlorite and other oxidizing agents will decompose most cyanides. If oxidizing agents are present during acid distillation they will react with cyanide and cause a negative bias. In cases where chlorine or another oxidant is known to be present, addition of a reducing agent such as ascorbic acid, sodium arsenite (NaAsO 2 ), or sodium borohydride (NABH 4 ) is recommended in some methods. Careful testing for cyanide recovery is necessary to avoid secondary reactions between the reducing agent and other substances that may be present in a sample.
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Cyanide Analysis Chemistry
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