Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 753

and monomethylphosphate, phosphite and a and β glycerophosphates at pH 4.0-6.8.
Dimethylphosphate and hypophosphite were only slightly adsorbed at all pH values
examined. The adsorbed ions were desorbed readily into a small volume of alkaline
solution.
A solution of iron containing ferric chloride and acidified with hydrochloric acid is
treated with aqueous ammonia to precipitate ferric hydroxide coprecipitated with ferric
phosphate produced by any phosphate ions in the sample [79]. The precipitate is filtered
off and dissolved in dilute hydrochloric acid and ammonia and uranyl acetate added to
produce uranyl phosphate which is estimated polarographically. To determine phosphate
plus arsenate the ferric hydroxide precipitate, containing coprecipitated iron phosphate
and arsenite, is made acid with hydrochloric acid and potassium iodide added. The
arsenic trichloride produced is extracted with carbon tetrachloride and this solution back
extracted into hydrochloric acid. Application of the ammonia ferric chloride precipitation
technique to this gives a preconcentrate containing arsenic only.
In one procedure [80,81] phosphate is adsorbed onto anion exchange resin.
Orthophosphate is quantitatively adsorbed by a A,G Dowex 1-X8 anion exchange resin,
then eluted and reacted with an acid molybdate reagent for estimation. Arsenic and
organic phosphorus compounds did not interfere with the estimation of orthophosphate
while polyphosphates did interfere if present in equal or greater amounts than
orthophosphate. It is concluded that the use of the anion exchange technique results in a
more valid estimate than direct action with the acid molybdate reagent.

15.1.9 Selenate and selenite
2,2 ′ -Diethylaminocellulose filters enabled selenate (SeO 4 2− ) and selenite (SeO 3 2− ) to be
preconcentrated from non saline water [82,83] at pH 3-6 with a detection limit of 0.05µg
L −1 .
Selenite has been preconcentrated on a bismuthiol 1L modified anion exchange resin
(Amberlite 1 RA-400) [84] followed by fluorometric estimation using
diaminonaphthalene. Selenite adsorbed on the column as selenotrisulphate was desorbed
with a small volume of 0.1mol L −1 penicillamine or 0.1mol L −1 cysteine prior to

fluorometric determination of selenium [85].
Selenate and selenite can be determined by energy dispersive X-ray spectroscopy after
preconcentration of elementary selenium on activated carbon, ascorbic acid being used to
reduce selenium to its elemental form [86-89].
Fung and Dao [2] used Chelex-100 resin to remove selenate from non saline water
prior to desorption from the column and determination at the sub µg L −1 level.

15.1.10 Silicate
Silicate can be preconcentrated [89] from water in microgram amounts by conversion to
molydosilicic acid in a medium 0.1N sulphuric acid or hydrochloric acid containing
ammonium molybdate. The heteropoly acid is extracted into butanol or isoamyl alcohol
and reduced to molybdenum blue using stannous chloride prior to spectrophotometric
determination.