Anions in surface, ground, mineral, borehole and pure waters - Determination of Anions in Natural and Treated Waters - page 458

addition of N, N ′ -bis-(2-hydroxypropyl)-o-phenylene diamine, which combines with
cerium(IV) to give a red colour, the intensity of which is proportional to the iodide
concentration. The standard deviation of the method ranges from 0.3ng (10ng iodide) to
3.8ng (100ng iodide).
6.2.5 Nitrate
6.2.5.1 Ion selective electrode
Robowsky [17] evaluated a nitrate-selective electrode for use with groundwater samples.
For concentrations in excess of 10mg of nitrate L −1 the recoveries are ~99.6% and the
coefficient of variation is ~2.5%; at lower nitrate concentrations there is a positive error
in the determinations due to interference by chloride.
6.2.6 Phosphate
6.2.6.1 Spectrophotometric method
Stauffer [18] has carried out studies to optimise the determination of phosphate in
groundwater using reduced molybdenum blue. High levels of silica may be responsible
for phosphorus determinations being raised by three orders of magnitude.
Yoshimura et al. [19] used gel phase colorimetry to determine phosphate in
groundwaters with molybdenum blue. In this method phosphate reacted with molybdate
in acidic conditions to produce 12-molybdophosphate. The blue species of
molybdophosphate were reduced by ascorbic acid in the presence of antimonyl ions and
adsorbed on Sephadex G-25 gel beads. Attenuation at 836 and 416nm (absorption
maximum and minimum wavelengths) was measured and the difference used to
determine trace levels of phosphate. Conditions were optimised (acidity, stirring time,
standing time, temperature). The effects of nitrate, sulphate, silicic acid, arsenate,
aluminium, titanium, iron, manganese, copper and humic acid on the technique were
examined.
6.2.7 Selenate and selenite
6.2.7.1 Ion chromatography
Reddy et al. [20] studied the speciation of selenium in groundwaters by adsorption of
selenite and selenate onto copper oxide particles followed by hydride generation atomic
absorption spectrometry and ion chromatography.
The application of this technique is also discussed under multianion analysis in section
6.2.9.1.