Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 275

use of an acidic supporting electrolyte. Generally, hydrochloric or perchloric acids are
used.
Christian et al. [671] reported that the polarographic reduction of selenite in
hydrochloric acid exhibits two main waves corresponding to the following reactions.
at E ½ =−0.01V vs SCE, followed by depolarisation of mercury by the reduction product:
The second wave at E ½ =−0.54V vs SCE corresponds to the further reduction of the HgSe
to hydrogen selenide.
The only elements and anions likely to interfere in the determination of selenite are
arsenite, copper(II), iron(III), lead(II) and zinc(II). Arsenite and copper(II) strongly
enhance the selenium peak, while iron(III) has a suppressant effect.
2.89.4 X-ray fluorescence spectroscopy
Robberecht and Van Grieken [672] determined µg L −1 levels of total dissolved selenium
and selenite in non saline waters by X-ray fluorescent spectrometry. Detection limits for
tetravalent selenium and total selenium are 50ng L −1 and 60ng L −1 respectively.
Coefficients of variation are 10% for tetravalent and hexavalent selenium at the 0.5-1µg
selenium L −1 level.
2.89.5 Ion chromatography
The application of this technique is discussed under multianion analysis in sections
12.2.2, 12.2.4 and 12.2.5.
2.89.6 Gas chromatography
Uchida et al. [673] have described a method for determination of selenium in river and
sea water by gas chromatography with electron-capture detection. The specific reaction
of 1, 2-diamino-3, 5-dibromo-benzene with selenium(IV) is used, the product (4, 6-
dibromopiazselenol) being extracted into toluene and determined from its peak height in
the gas chromatogram. Selenium (−II, 0) is oxidised to selenium(IV) by bromine