Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 595

10.16 Silicate
10.16.1 Spectrophotometric method
A modification of the standard molybdenum blue Spectrophotometric method has been
used to determine silicate in industrial effluents [46].
10.16.2 Atomic absorption spectrometry
Flameless atomic absorption spectrometry has been shown to be capable of determining
down to 25µg L −1 silica in industrial process waters [47].
10.17 Sulphate
10.17.1 Titration method
Methods have been described for determining sulphate in electroplating works effluents
[48], chromium plating bath effluents [49] and industrial effluents [50]. For electroplating
works effluents [48], the sample was first passed down a column of ion exchange resin in
the H + form to remove interfering cations. Sulphate in the eluate is then determined by
titration at pH 5.5 to 6.5 with 0.01N barium chloride to the carboxyarsenazo endpoint. 3-
(2-Carboxylphenylazo), chromotropic acid and 3,6-bis (4-chloro2-phosphorophenylazo)
chromotropic acid, and, in the presence of phosphate or arsenate, nitroorthaonilic S (3,6-
bis-(4-nitro-2-sulphophenylazo) chromotropic acid have also been used as indicators in
the direct titration of sulphate with barium chloride. An alternate method is to precipitate
sulphate as barium sulphate, dissolve the precipitate in excess 0.02mol L−1 ammoniacal
EDTA and titrate unconsumed EDTA with standard magnesium chloride solution to the
Eriochrome Black T end-point.
Standard UK official titration methods for the determination of sulphate have been
published [51,52]. In one method [52] interfering ions are first removed by cation
exchange in Amberlite IR 120H in the hydrogen form. Sulphate ions are then precipitated
by excess barium chloride and the residual excess of barium ions determined by titration
with standard 0.01M sodium ethylene diamine tetraacetic acid solution. The method is
capable of determining up to 1400mg L −1 sulphate and has a detection limit of 1.5mg L −
1 sulphate. Standard deviations range from 0.36mg L−1at the 19.6mg L −1 sulphate level
to 0.68mg L −1 at the 98.4mg L −1 sulphate level Phosphate and chromate are known to
interfere in this procedure but compensation procedures are available. Coloured
substances may interfere with end-point detection.
In a second method [52] following treatment with amberlite IR 120H resin as above
sulphate ions are titrated directly with 0.005M barium chloride titrant to the
carboxyarsenazo-bromocresol purple end point. The range of application is up to 20mg
L −1 sulphate using 0.005M barium chloride titrant. Down to 0.2mg L −1 sulphate can be
determined. Standard deviations range from 0.15mg L −1 sulphate at the 5mg L −1 sulphate