Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 85

2.4.6 High performance liquid chromatography
The application of this technique is discussed under multianion analysis in section
13.1.1.5.
2.4.7 Ion exclusion chromatography
The application of this technique is discussed under multianion analysis in section
13.2.1.2.
2.4.8 Gas chromatography
Fukai et al. [10] have described a procedure based on gas chromatography of 2.3
mercaptopropanol complexes for the determination of arsenate, arsenite and
monomethylarsonic acid in non saline waters. The 2,3 mercaptopropanol is extracted
with benzene and introduced into a gas chromatograph equipped with a flame-
photometric detector for the determination of sulphur. One aliquot of sample is treated
with stannous chloride solution and potassium iodide solution to reduce arsenate and
monomethylarsonic acid, then 2,3 mercaptopropanol is added and the complexes are
extracted with benzene. The extract is analysed for total inorganic arsenic plus mono-
methylarsonic acid. Magnesia mixture and phosphate solution are added to another
aliquot to remove arsenate by co-precipitation with magnesium ammonium phosphate.
The precipitate is filtered off and arsenite determined in the filtrate. The detection limits
are 0.02ng of arsenic for arsenate and arsenite and 0.04ng of arsenic for
monomethylarsonic acid.
2.4.9 Miscellaneous
2.4.9.1 Determination of arsenate and arsenite
In the method of Johnson and Pilson [11], one aliquot of sample arsenite is oxidised to
arsenate, in a further aliquot arsenate is reduced to arsenite and a third aliquot is
untreated. Each aliquot is submitted to a molybdenum blue procedure. The first gives a
value for phosphate plus arsenate plus arsenite, the second for phosphate and the third a
value for phosphate plus arsenate. The content of each anion is calculated from these
results. The method is applicable at the 3µmol L −1 level.
Johnson [12] has also described a spectrophotometric method for the determination of
arsenate and phosphate in non saline water in which total arsenate-plus phosphate are
determined by classical molybdenum blue procedure [13]. Arsenate is then reduced to
arsenite which does not form a coloured complex with molybdic acid. A difference
technique then enables the separate determination of phosphate and arsenate.