Chemistry Reference
In-Depth Information
2.56.3
Gas chromatography
The application of this technique is discussed under multianion analysis in section
14.1.1.1.
2.57
Metal cyanide complexes
2.57.1
Ion chromatography
The cyanide ion in inorganic cyanides can be present as both complexed and free
cyanide. In order to study the chromatography of metal cyanides, Rocklin and Johnson
[374] prepared and assayed solutions of cadmium, zinc, copper, nickel, gold, iron and
cobalt cyanides. Table 2.17 lists the percentage of total cyanide detected.
The results suggest that the complex cyanides can be grouped into three categories
depending on the cumulative formation constant and stability of the complex.
Table 2.17
Percentage of total cyanide in metal complexes determined as 'free' cyanide
log β
t
Metal complex
%
Cd(CN)
4
2−
18.8
102
Zn(CN)
4
2−
16.7
102
Ni(CN)
4
3−
31.3
81
Cu(CN)
4
3−
30.3
52
Cu(CN)
3
2−
28.6
42
Cu(CN)
2
−
24.0
38
Au(CN)
2
−
38.3
0
Fe(CN)
6
3-
42
0
Co(CN)
4
3-
64
0
Source: Reproduced with permission from the American Chemical Society [374]
Category 1 includes the weakly complexed and labile cyanides Cd(CN)
4
2−
(log
B
4
=18.78) and Zn(CN)
4
2−
(log B
4
=16.7). These complexes completely dissociate under
the chromatographic conditions used, the cyanide being indistinguishable from free
cyanide.
C
ategory 2 includes the moderately strong cyanide complexes Ni(CN)
4
2−
(log
B
4
=31.3) and Cu(CN)
4
3−
(log B
4
=30.3). Although these complexes are labile, they are
retained on the column and slowly dissociate during the chromatography. This slow
dissociation produces tailing which lasts for several minutes as the free cyanide elutes
and is detected. As the results presented in Table 2.17 demonstrate, the tailing and the
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