Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 545

acid.
Hartzinger [49] critically examined the D 13/3 German standard method the
determination of free and complexed cyanides in waste water. He points out that
interfering substances causing high results in the determination of cyanide in effluents by
this pyridine-benzidine method (usually carried out at pH 7) appear to be unknown N-
containing heterocyclic compounds, which, in the presence of cyanide, yield polymethine
dyes. In cases of doubt the analysis is repeated after adjustment of the pH to 5.5-5.0 by
use of an ion exchanger, thus avoiding the involved distillation procedure. The extinction
at 480nm is decreased by 70-75% at the lower pH, so that errors of greater than 0.1mg
L −1 in the cyanide concentration can be avoided.
Goulden et al. [50] determined simple and complex cyanides in waste water by
modifying the APHA standard distillation colorimetric method [51] to ensure accurate
results for less than 100µg L −1 cyanide. In the manual procedure, the sample is distilled
under reflux with sulphuric acid in the presence of aqueous magnesium chloride and
mercuric chloride, the absorption system being modified by recirculating the absorbent
(0.2mol L −1 sodium hydroxide) by a peristaltic pump through a column packed with glass
helices. The lower limit of detection is 5µg L −1 cyanide but this is lowered to 1µg L −1 by
using an AutoAnalyser system in which is included a ultraviolet irradiation cell to
decompose complex cyanides, eg those of iron and cobalt; this cell is by-passed when
only simple cyanides are being determined. The hydrogen cyanide is evolved in a
continuous distillation apparatus, a mixture of phosphoric acid (1.2N) and dilute H 3 PO 2
being used, because these acids also inhibit oxidation of cyanides during irradiation or by
any free chlorine present etc. The absorbent is 2% aqueous sodium acetate (pH 6.5), use
of which eliminates one stage in the pyridine-pyrazolone colorimetric procedure. With a
7ml sample the automated method has a detection limit of 7ng, the coefficient of
variation was 1.2% at the 40µg L −1 cyanide level.
Royer et al. [52] determined total cyanide in waste water by an Autoanalyser
procedure. They used magnesium chloride and mercuric chloride in the carrying solution
to decompose complex metal cyanides. Down to 10µg L −1 total cyanide could be

determined by this procedure.
A spectrophotometric method has been described for the determination in waste waters
of total cyanide, including free cyanide, complex cyanides and thiosyanates [53].
Thiocyanates, together with simple and complex cyanides, are reacted with acid cuprous
chloride and the hydrogen cyanide formed is absorbed in a sodium hydroxide solution
which is then made slightly acidic to prevent hydrolysis to cyanate. Chloramine-T is
added to produce cyanogen chloride, a pyridine/ pyrazolone reagent is added, and the
optical absorbance of the resulting blue dye is measured at 620nm. Volatile aldehydes
and ketones interfere in the final colour development.
Csikai and Barnard [54] used EDTA at pH 4 to displace cyanide from metal complexes
and to avoid converting thiocyanate to free cyanide where oxidants are present.
Sulphamic acid is added to prevent nitrite interference, sulphides are removed from the
sample with calcium carbonate and from distillates with cadmium nitrate. Evolved
cyanide is determined by pyridine-barbiturate photometry or potentiometrically with ion
selective electrodes.
Czikai and Barnard [54] evaluated their procedure by adding both free cyanide and