Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 482

throughout the analysis. Nitrogen via a fine bleed or glass frit is bubbled for 20min
through the cooled sample solution, which contains 0.5-10mmol L −1 total chloride, to
displace unhydrolysed chlorine. 5ml aliquots are analysed for chloride by the addition of
0.01mol L −1 silver nitrate solution until one drop causes no further precipitation of silver
chloride. Near the end-point, three drops of nitric acid (5mol L −1 ) are added to ensure
complete precipitation. Centrifugation enables the endpoint to be clearly distinguished.
Separate 5ml aliquots are analysed for hypochlorous acid by the addition of 10mg of
potassium iodide and titration with 0.01mole L −1 sodium thiosulphide solution. One drop
of starch solution is added just before the end-point to clarify when it has been reached.
The difference between two concentrations thus determined gives the concentration of
chloride exclusive of that produced by chlorine hydrolysis.
7.8.2 Ion chromatography
The application of this technique is discussed under multianion analysis in sections
7.28.2, 12.6.1 and 12.6.2.
7.8.3 High performance liquid chromatography
The application of this technique is discussed under multianion analysis in section
13.1.2.1.
7.8.4 Gas chromatography
Bachmann and Matusca [15] have described a gas chromatographic method for the
determination of µg L −1 quantities of chloride, bromide and iodide in potable water. This
method involves reaction of the halide with an acetone solution of 7-oxabicyclo-(4,1,0)
heptane in the presence of nitric acid to form halogenated derivatives of cyclohexanol
and cyclohexanol nitrate.
The column effluent passes through a pyrolysis chamber at 800°C and then through a
conductivity detector. The solution is injected on to a gas chromatographic column (OV-
10-Chromosorb W-HPl, 80-100 mesh, 150cm, 0.2cm dia.) operated at 50°C. Hydrogen is
used as carrier gas. Chloride contents determined by this method were 5.0±1.8µg L −1
chloride in potable water, 20.0±1.5µg L −1 in drinking water. A typical gas chromatogram
is shown in Fig. 7.7 showing peaks for the three halogens.