Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 566

14.2.3.3.
8.22.2 Polarography
Sulphite in wastewaters has been determined by dc polarography with a detection limit of
10 -6 mol L −1 and by ac polarography with a detection limit of 10 −7 mol L −1 [112].
8.22.3 Ion chromatography
The application of this technique is discussed under multianion analysis in sections 8.26.2
and 12.7.2.
8.23 Silicate
8.23.1 Spectrophotometric method
Greenberg et al. [113] and Babulak and Gildenberg [114] in official methods have
determined silicate down to 0.1mg L −1 (as Si) spectrophotometrically by the
molybdosilicate method in which the extinction of the silicomolybdate complex is
measured at 810nm. Ascorbic acid is added to reduce silicomolybdic acid to the
silicomolybdo blue complex. Phosphate interference is overcome by the addition of
oxalic acid.
8.23.2 Atomic absorption spectrometry
Looyenga and Huber [115] used atomic absorption inhibition titration to determine
silicate in waste water. The method is based on the strong inhibition by silicate of the
absorption of magnesium ions which is measured continuously during titration. The
sample solution (pH 3-4) is first passed through Amberlite IR-120 resin (H + form) to
remove magnesium and other interfering cations and the percolate is then titrated with a
standard solution (100-200mg L −1 ); the titrated solution is simultaneously aspirated into
the hydrogen air flame for measurement of the absorption at 285.2nm. The end point is
detected by a sharp increase in the magnesium signal. The detection limit is 0.1mg L −1
and there is no interference from phosphate or sulphate.
8.24 Thiocyanate
8.24.1 Spectrophotometric method
The application of this technique is discussed under multianion analysis in section
14.2.3.2.