Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 544

ferrocyanides. The methods discussed in the previous section determine only free
uncomplexed cyanide. To determine uncomplexed plus complexed cyanide, ie total
cyanide, it is necessary prior to analysis to decompose the complexed cyanides to free
cyanide.
Overbach [38] discussed problems encountered in the determination of low
concentrations of cyanide in photographic processing waste waters. Determination of
both total and free cyanide in the same sample differentiates between the toxic free
cyanide and the complexed ferrocyanide. Because of possible instability of the cyanide,
he recommended that samples should be analysed on site, for instance by the
microdiffusion technique.
The determination of concentrations of total and labile cyanide is very important for
many environmental and industrial applications. The sample pretreatment to obtain free
cyanide is perhaps the most important aspect in cyanide analysis. The standard methods
for cyanide analysis use acid distillation procedures to decompose metal cyanide
complex. The standard methods have several drawbacks. The distillation procedure is
very time-consuming and labour-intensive. The methods also suffer seriously from a
number of interferences such as sulphide and thiocyanate. A different approach of sample
pretreatment based on the photo-dissociation reactions of metal-cyanide complexes under
ultraviolet-visible irradiation has been studied for the determination of total cyanide.
Those studies have shown that complete decomposition of stable complexes such as Fe
(CN) 6 3− can be obtained in a few minutes when the sample solution is exposed to intense
irradiation from high pressure mercury lamps of several hundred watts. However,
thiocyanate interferes since it is converted to cyanide under ultraviolet irradiation when
quartz tubings or vessels are used as reactors. The Pyrex glass reactor avoids the
photoconversion of thiocyanate to cyanide.
The separation of cyanide ion from interferences is required in the determination of
cyanide because the measurement methods are usually prone to interferences such as
sulphide and thiocyanate. The tedious procedures such as precipitation and extraction are
used in standard methods. Gas diffusion methods using hydrophobic porous membranes
have also been studied. Cyanide is separated as hydrogen cyanide gas from the acidic
solution and hydrogen cyanide gas is collected in the basis solution on the opposite side
of the membrane. The gas diffusion method separates cyanide from most ionic species in
the sample solution. The drawback of the method is that it does not separate cyanide from
sulphide since hydrogen sulphide gas also diffuses across the membrane, ion
chromatography methods have been shown to be advantageous in the separation of
cyanide from sulphide (see section 8.10.1). However, these methods may be subject to
interferences.
8.9.1 Spectrophotometric method
Kollau and Reidt [48] determined total cyanide and cyanogen chloride in amounts down
to 0.1ml L −1 in waste water. To determine total cyanide, hydrogen cyanide is liberated by
distillation with sulphuric acid and collected in aqueous sodium hydroxide. Cyanide is
determined spectrophotometrically at 578nm after chloramine-T oxidation of cyanide to
cyanogen chloride and formation of the coloured complex with pyridine and barbituric