Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 513

7.22.2 Ion chromatography
The application of this technique is discussed under multianion analysis in sections 7.28.2
and 12.6.1.
7.23 Selenate/selenite
7.23.1 Spectrofluorometric method
Selenate has been determined in potable water fluorometrically using naphthalene-2, 3-
diamine [76]. The complex is extracted into cyclohexane and evaluated at 521nm
(excitation 389nm). The calibration graph is rectilinear in the range 0.02-1.0µg of
selenium. The coefficient of variation for samples containing 0.5 and 1.0ng of selenium
was 6.3% and a 1000-fold excess of nitrate, barium, cadmium, calcium, chromium(III),
cobalt, copper(II), iron(III), magnesium, manganese(II), nickel(III) or zinc did not
interfere.
7.23.2 Ion chromatography
The application of this technique to the determination of selenate and selenite is
discussed under multianion analysis in sections 7.28.2, 12.2.2 and 12.6.6.
7.24 Silicate
7.24.1 Polarography
Iyer et al. [77] converted trace amounts of silica in potable water to silicomolybdate by
addition of ammonium molybdate, followed by methylethylketone to ensure that only the
beta form of silicomolybdate was present. Buffering with citrate masked any phosphate
or excess molybdate. The initial potential of the hanging mercury drop electrode was kept
at −0.2V for 2min and silicomolybdate accumulated by adsorption in the interface. Its
bulk concentration was determined by measuring the peak height at −0.36V. The
reduction of the silicomolybdate gives a well defined peak at −0.36V, followed by
reduction of the molybdate ion at −0.52kV. With increasing the concentration of silica
only the first peak at −0.36V increased linearly unlike that of the second peak.
A typical voltammogram of silicomolybdate at the hanging drop mercury electrode
obtained by following the standard procedure is given in Fig. 7.12. The reduction of the
silicomolybdate gives a well defined peak at −0.36V, followed by reduction of the
molybdate ion at −0.52V. With increasing concentration of silica only the first peak at
−0.36V increased linearly unlike the second peak.