Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 491

7.15 Iodate
7.15.1 High performance liquid chromatography
The application of this technique is discussed under multianion analysis in section
13.1.2.1.
7.16 Iodide
7.16.1 Spectrophotometric method
Manual [37] and automated [38] spectrometric methods have been described for the
determination of iodide in potable water. Moxon [38] describes an automated procedure
capable of determining iodide ion in potable water in amounts down to 0.4µg L −1 and
total inorganic iodine (including iodate) in amounts down to 0.2µg L −1 . The methods are
based on the catalytic effect of iodide on the destruction of the thiocyanate ion by the
nitrite ion:
The methods are applicable to normal drinking waters, with possible interfering ions
having negligible effects. The system is easy to use and can handle 20 samples per h.
When series of drinking waters were run by this method, the peaks were found to be
irregular and erratic. It was found that there was a relationship between the hardness of
the waters and the peak irregularity, and this was removed when the alkaline earth metals
were precipitated out with potassium carbonate. Subsequently, samples and standards
were made up in 0.3% potassium carbonate solution. This addition of alkali prolonged
the stability of the standard solutions from 6h to 8d, but led to a decrease in the sensitivity
of the method. Chloride at a level of 400mg L −1 gave a 20% increase in the response to a
4µg L −1 ofiodide standard solution. To overcome this interference, a large excess of
chloride in the form of sodium chloride solution was introduced into the sample stream
and it was found that, as well as removing the interference effect of chloride, the
sensitivity of the method was greatly increased.
A considerable proportion of the total inorganic iodide present in water can consist of
iodate. System A, illustrated in Fig. 7.8, recovered iodate quantitatively and gave
measure of the total inorganic iodine in solution. In order to determine free iodide only,
the oxidation-reduction potential of the reaction mixture was adjusted so that iodate was
not reduced to iodine or iodide. This was achieved by:
(a) reducing the concentration of nitric acid in the ammonium iron(III) sulphate reagent;