Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 739

preconcentration will reduce these limits to approximately 5 and 25µg L −1 and a 1000-
fold preconcentration will achieve 1 and 5µg L− 1.
15.1 Non saline waters
15.1.1 Arsenate and arsenite
Howard et al. [1] electively preconcentrated arsenite onto mercapto modified silica gel.
Arsenate, monomethylarsonate, and dimethyl arsenite, which commonly occur in arsenic
contaminated non saline water samples, do not interfere in this procedure.
15.1.2 Borate
Yoshimura et al. [3] have described a highly sensitive method for preconcentrating borate
based on adsorption on Sephadex G-25 gel in alkaline medium and reversible desorption
acid medium. The borate is then determined spectrophotometrically by the azomethine-4
method.
Jun et al. [4] preconcentrated boron from non saline waters as its complex with
chromotropic acid and octyltrimethyl ammonium chloride on an anion exchange column
(TSK gel, I.C. Anion PW).
The eluted concentrate was analysed by high performance liquid chromatography.
15.1.3 Bromide
Carlsson et al. [5] preconcentrated bromide and iodide on an anion exchanger, oxidised
by per-oxodisulphate to bromate, treated with iodide and measured the absorbance of the
resulting tri-iodide. The sum of bromate and iodate produced in the oxidation was
determined by treating the oxidised sample with iodide in hydrochloric acid. The iodate
was determined separately by applying the reaction in acetic acid. The working range of
the spectrophotometric method was 1-15µM, the limit of the determination was 0.7µM
for iodate and for iodate plus bromate and the enrichment factor in the preconcentration
step was 50.
15.1.4 Chromate
The preconcentration of chromium(IV) (chromate) in non saline waters by
coprecipitation with barium sulphate has been shown to be a selective and accurate
procedure [6]. Analysis of the preconcentrate is carried out by determining down to
0.02µg L −1 chromium spectrophotometrically. The method is based on the similarity
between the solubility products of barium sulphate and barium chromate. Interference
from ferric iron, aluminium, and trivalent chromium was overcome by using salicylic
acid as a masking agent. The method was used on river water samples and the results
obtained were in good agreement with those obtained by an aluminium hydroxide
coprecipitation method. In order to prevent con-tamination, the membrane filter, the
sample bottle, and other vessels were carefully precleaned with concentrated