Multianion analysis, applications of ion chromatography - Determination of Anions in Natural and Treated Waters - page 695

following the bromide peak can also be seen.
The determination of cyanide is affected by the presence of sulphide in the sample as
shown in Table 12.31. Since sulphide elutes before cyanide (Fig. 12.38), cyanide does not
interfere with the analysis of sulphide. The addition of 0.1mg L −1 sulphide enhances the
cyanide peak by 9%. This effect can be minimised or eliminated by using the standard
addition method or by matching the cyanide standards as closely as possible to the
Table 12.31 Determination of 400µg L −1 bromide in the presence of chloride
Amt of added Cl − , mg L −1 Recovery of Br − , %, ±2%
0 100
10 104
100 100
1000 115 a
a The 100mg L −1 chloride solution contains 50µg L −1 bromide, accounting for 12.5% of the 15%
excess
Source: Reproduced with permission from the American Chemical Society [23]
sample. For example, if the sample is known to contain approximately 10µg L −1
sulphide, then this amount should be added to the standards. In general, plots of the log
peak height as a function of the log of concentration are linear at low concentration. At
high concentrations an increase in concentration produces a smaller increase in current as
shown by the plateau for each ion. This plateau (also observed in flow injection analysis
studies) is caused by uncompensated resistance in the cell. The upper limit of linearity
can be extended by increasing the applied potential or by decreasing the size of the
injection loop. With a 100µL injection loop, the detection limit for cyanide is 2µg L −1 , for
sulphide 30µg L −1 , for iodide 10µg L −1 and for bromide 10µg L −1 .
12.8.3 Metal cyanide complexes
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
[23] prepared and assayed solutions of cadmium, zinc, copper, nickel, gold, iron and
cobalt cyanides. Table 12.32 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.
Category 1 includes the weakly complexed and labile cyanides Cd(CN) 2− (log4
β 4 =18.78) and Zn(CN) 4 2− (log β 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
β 4 =31.3) and Cu(CN) 4 3− (log β 4 =30.3). Although the complexes are labile, they are