Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 551

technique.
Bakensky [60] discussed inaccuracies in complex formation methods used to
determine free and bound cyanide.
Pohlandt et al. [61] have critically evaluated 92 methods applicable to the separate
determination of free cyanides and complex cyanides in waste waters and process
streams. These include titrimetric, spectrophotometric, potentiometric, amperometric,
polarographic, voltammetric, ion selective electrodes, indirect atomic absorption
spectrometric, gas chromatographic and automated techniques. From this work it appears
that most titrimetric, colorimetric and electrochemical methods (including potentiometry
and the use of cyanide-selective electrodes) for the determination of ionic cyanide are
liable to interference from ionisable metal cyanide complexes, eg Zn(CN) 4 2− . These
methods can therefore be employed only if such complexes are known to be absent.
Potentially accurate methods for the determination of ionic cyanide include ion
chromatography (because of its ability to effect separations) and indirect atomic
absorption spectrophotometry based on the selective formation of (Ag(CN) 2 ) − .
Similarly, none of the methods mentioned gives a reliable result for the total amount of
cyanide present in a sample. The decomposition of stable complexes and the separation
from interfering substances by distillation is therefore necessary. However, most
distillation procedures cannot effect the decomposition of all the metal cyanide
complexes and they suffer from interferences. The most useful distillation procedure
appears to be the ligand displacement technique [61]. With this method, interference from
sulphide and thiocyanate can be avoided and all the ionic and coordinated cyanide—
except cyanide from (Co(CN) 4 ) 3− —present in a sample can be distilled out.
Because of the speed in decomposing cyanide complexes, irradiation with ultraviolet
light warrants further attention. A combination of this method with the separation, by ion
chromatography, of interfering species might result in a fast method for the determination
of total cyanide.
8.10 Cyanide, free and total
8.10.1 Ion chromatography
This is emerging as the method of choice for the determination of cyanides. Nonomura
[62] determined free cyanide and metal cyanide complexes in wastewaters by ion
chromatography with conductively detection.
The cyanide is not detected by the conductivity detector of the ion chromatograph due
to its low dissociation constant ( p K=9.2). Nonomura [62] described an ion
chromatographic method for the determination of free cyanide and metal cyanide
complexes that uses a conductivity detector. It is based on the oxidation of cyanide ion by
sodium hypochlorite to cyanate ion ( p K=3.66). Therefore, cyanide ion can now be
measured indirectly by the conductivity detector. In this procedure, optimum operating
conditions were examined. In addition, the interferences from anions and reducing agents
were investigated. The method was applied to the determination of metal cyanide
complexes. The coefficients of variation (%) for CN − (1.05mg L −1 ), Zn(CN) 4 2− (CN − ,