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This part of ISO 6703 specifies three methods for the determination of total cyanide (see clause 2) in water.

The methods are applicable to water containing less than 100 mg of cyanide per litre, but higher concentrations may be determined by suitable dilution of the sample.

The methods and corresponding ranges of cyanide contents for which they are suitable are as follows:

• Photometric method with pyridine/barbituric acid : 0,002 to 0,025 mg;
• Titrimetric method using the Tyndall effect: > 0,005 mg;
• Titrimetric method using an indicator: > 0,05 mg.

A large number of ions and compounds interfere with the determination. These are listed in the table 1, together with the concentrations below which they do not interfere (the list is not exhaustive). If present singly or in combination, up to the limiting concentrations, they do not interfere with the separation of hydrogen cyanide. The presence of aldehydes, e.g. formaldehyde, causes low cyanide values because of the formation of cyanohydrin.

If any of the limiting concentrations of the interferences is likely to be exceeded, the sample shall be diluted with distilled water before stabilization (see clause 6).

Other interference may arise from the presence of fatty acids, which will distil and form soaps during titration of alkaline solution, and from the presence of elementary sulfur.