Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 585

nitric acid and then titrated with 0.1N mercurous nitrate in the presence of
diphenylcarbozide indicator.
Official methods issued by the Department of the Environment UK [7] describe
various methods for the determination of chloride in sewage, trade effluents and waters.
These methods are silver nitrate titration with chromate indicator (Mohr's method),
mercuric nitrate titration with diphenyl-carbazone indicator, potentiometric titration with
silver nitrate, and an automated mercuric-ferric thiocyanate colorimetric method. With
one or other of these methods, concentrations from 1 to 1000mg L −1 chloride may be
determined. Brief notes are given in respect of the chloride ion selective electrode, silver
coulometry and ion chromatography, the latter being relevant at very low concentrations
down to the µg L −1 level.
10.3.2 Ion chromatography
The application of this technique is discussed under multianion analysis in sections
10.22.1 and 12.8.1.
10.4 Cyanide (free)
10.4.1 Titration method
Kogan et al. [8] titrated down to 1mg L −1 cyanide in coke works effluents,
amperometrically in a basal electrolyte of 0.1N sodium hydroxide using silver nitrate as
titrant. Titration was carried out at −0.2V (versus SCE) using a vibrating platinum
electrode. Up to 500mg L −1 thiocyanate, phenol and ammonia and 100mg L −1 of chloride
or sulphate do not interfere. Lead carbonate is added to avoid interference by sulphide.
Cadmium carbonate-magnesium chloride mixtures have also been used to prevent
interference by sulphide [9] and more theoretical yields are claimed for this procedure.
In their method for the determination of cyanide in industrial effluents, Hewitt and
Austin [10] distil the acidified sample in the presence of zinc and lead acetates (to
prevent decomposition of complex cyanides and sulphides) and collect the liberated
hydrogen cyanide in sodium hydroxide. Cyanide is then estimated by titration with
standard silver nitrate solution using 4-dimethylaminobenzylidenerhodamine as indicator.
The application of this technique is discussed under multianion analysis in section
14.3.2.2.
10.4.2 Spectrophotometric method
Dimethylaminobenzylidenerhodamine [11] and pyrazolone [12] have been used as
chromogenic reagents in the determination of cyanide in industrial effluents. In the latter
method [12], free cyanide is separated from complex cyanides by distillation under
reduced pressure in the presence of zinc and lead acetates. The hydrogen cyanide evolved
is absorbed in sodium hydroxide solution, and Chloramine T is added, after slight
acidification with acetic acid to produce cyanogen chloride. A pyridine/pyrazolone