Anions in surface, ground, mineral, borehole and pure waters - Determination of Anions in Natural and Treated Waters - page 460

6.3.3 Iodide
6.3.3.1 Gas chromatography
Wu et al. [25] have described a method for the derivatisation of iodide into
pentafluorobenzyl iodide using pentafluorobenzyl bromide. The derivative was analysed
at µg levels by gas chromatography with electron capture detection. The effects of
solvent, water content, base or acid concentration, amount of pentafluorobenzylbromide,
reaction time and reaction temperature were examined. Interferences by common anions
were minimal. The method was applicable to iodide determination in spring water.
6.3.4 Nitrate and nitrite
6.3.4.1 Spectrometric and ultraviolet spectroscopic methods
A spectrophotometric method using 3,3'dimethylnaphthidine as the chromogenic agent
has been used to determine down to 5.0µg L −1 nitrate in mineral waters [26].
Spectrophotometric methods based on the use of resorcinol [27] and 2, 4-xylenol [28]
have been described for the determination of nitrate in mineral and groundwaters.
Miles and Espejoi [28] compared results for nitrate in ground waters of low salinity
obtained by the 2, 4-xylenol spectrophotometric method and by ultraviolet spectroscopy.
The ultraviolet procedure is not subject to interference from the major cations present in
typical groundwaters, and interference by high levels of nitrite can be overcome by
addition of sulphamic acid. There was good agreement between nitrate levels determined
by both methods on a number of ground water samples.
Nitrite, which absorbs in a similar part of the ultraviolet region, is a potential source of
interference in this method. A solution containing 1mg L −1 of nitrite-nitrogen was
scanned between 190 and 260nm and the spectrum is shown in Fig. 6.1, together with
that of a 1mg L −1 solution of nitrate-nitrogen for comparison. The nitrite absorption
maximum is at about 211nm and it absorbs less strongly than nitrate at a similar
concentration. Solutions containing 0.5mg L −1 of nitrate-nitrogen and up 100µg L −1 of
nitrite-nitrogen were prepared and the interference from nitrite is significant only at
concentrations above 20µg L −1 . Interference by higher concentrations of nitrite can be
removed by the addition of 0.1% m/v of sulphamic acid.
Iron does not interfere at concentrations below 100µg L −1 and chloride is without
effect at concentrations below 100mgL −1
Table 6.4 compares results obtained for nitrate by ultraviolet spectroscopy and
spectrophotometry. Good agreement was obtained for all groundwater samples analysed.