Water quality -- Determination of cyanide -- Part 2: Determination of easily liberatable cyanide

Document Number: NL ISO 6703 2 : 2009
Sector: Food Technology
TC: No
ICS: 13.060.50
This part of ISO 6703 specifies three methods for the determination of easily liberatable cyanide (see clause 2) in water. The methods are applicable to water containing less than 50 mg of easily liberatable cyanide (as cyanide ions) per litre, and less than 100 mg of total cyanide (as cyanide ions) per litre, but higher concentrations may be determined by suitable dilution of the sample. The methods and corresponding ranges of easily liberatable cyanide contents for which they are suitable are as follows: — Photometric method with pyridine/barbituric acid: 0,002 to 0,025 mg of cyanide; — Titrimetric method using the Tyndall effect: > 0,005 mg of cyanide; — Titrimetric method using an indicator : > 0,05 mg of cyanide. A large number of ions and compounds interfere with the determination. These are listed in table 1, together with the concentrations below which they do not interfere. If present singly or in combination, up to limiting concentrations, they do not interfere with the separation of hydrogen cyanide. The list is not exhaustive. Table 1 — Interferences Interference Limiting concentration, mg/l Sulfide ions 1 000 Polysulfide ions 500 Sulfide and polysulfide ions 1 000 Sulfide ions 500 Thiosulfate ions 1 000 Thiocyanate ions 1 000 Carbonate ions 1 000 Cyanate ions 1 000 Nitrate ions 500 Nitrite ions 500 Ammonium ions 2 000 Iron(II) and iron(III) ions 5 000 Copper(II) ions 100 Nickel(II) ions 50 Silver ions 50 Mercury ions 50 Chromate ions 300 Propionic acid 1 000 Phenol 1 000 Anthracene 100 Naphthalene 100 Anisaldehyde 10 Piperonal 10 Pyrrole 100 Pyridine 10 Chlorine (elemental) 250 Hydrogen peroxide 10 Perborate ions 10 If any of the limiting concentrations of the influences are likely to be exceeded, dilute the sample with distilled water before stabilization (see clause 6). Prussiates (pentacyano complexes with iron), which cannot be destroyed by chlorination under normal conditions of waste water treatment, partly decompose (up to 50 %), releasing hydrocyanic acid under the conditions specified. If it is desired to exclude prussiates, the procedure specified in clause 6 and 7.1 has to be used. This procedure is only applicable, however, if the concentration of copper ions in the sample is less than 1 mg/l. The presence of aldehydes, e.g. formaldehyde, may give lower cyanide values because of the formation of cyanohydrin.