Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 283

formation of silicomolybdenum blue. Reagents used for the reduction of β-silicomolybdic
acid to methylene blue include sodium sulphite-4-amino-3-hydroxynaphthalene-1-
sulphonic acid [682,683], sodium sulphite-metal [684] stannous chloride-glycerol [685]
and stannic chloride-ascorbic acid-oxalic acid [684].
Pavlova et al. [685] extract the silicomolybdic acid into butanol or isoamyl alcohol
prior to reduction to methylene blue in order to improve the sensitivity of the method.
Kasahara et al. [686] collect the methylene blue on a solvent soluble membrane filter
which is then dissolved in N,N-dimethylformanide and determined

spectrophotometrically. The detection limit of this method is 0.5µg SiO 2 L −1 .
Yoshimura et al. [687] determined silicic acid in non saline water at the µg L −1 level or
lower by gel-phase colorimetry based on the direct measure-ment of absorbance of
molybdenum blue adsorbed on Sephadex gel.
Pakalns [688] studied the effect of surfactants on the spectrophotometric determination
of silica. Three molybdenum blue methods using different reducing agents were used; 1-
amino-2-naphthol-4-sulphic acid [689], sodium sulphite [690] and ascorbic acid [691].
Pakalns [692] studied anionic, cationic and non-ionic detergents also sodium
triphoylphosphate, and sodium pyrophosphate. Interferences were very large for the
cationic and non-ionic detergents, but negligible for the biodegradable linear alkyl
sulphate type detergents. The amino-naphthol-sulphonic acid method is recommended.
The National Water Council UK [693] has published two spectrophoto-metric methods
for the determination of soluble molybdate reactive silicon (mainly monomeric and
dimeric silicic acids and silicate); the first is based on the use of ascorbic acid for the
reduction step, and the second is based on the use of 1-amino-2-naphthol-4-sulphonic
acid to effect the reduction when analysing clean waters, especially those of low silicon
content. In addition three alternative pretreatments are described for converting other
forms to soluble molybdate-reactive silicon. The method with ascorbic acid as the
reducing agent is particularly suited to a wide range of silicon concentrations up to
100mg L −1 measurements being made in 10mm cells at 700nm, but it is claimed to be
easily modified for higher or low sensitivity.
Greenberg et al. [694] and Babulak and Gildenberg [695] determined silicate
spectrophotometrically by means of the molybdenum blue method in which the
extinction of the silicomolybdate complex is measured at 810nm. Ascorbic acid is added
to reduce the silicomolybdic acid to the silicomolybdoblue complex. Phosphate
interference is avoided by the addition of oxalic acid.

2.97.2 Spectrofluorometric method
A special and indirect method for the determination of silicon consists in the formation of
molybdosilic acid, extraction with isoamyl alcohol, re-extraction with ammonia and
determination of molybdenum by the formation of the molybdenum-carminic acid
complex [696]. This method is used for the determination of silicon in hydrofluoric acid,
ammonia, sodium hydrogen carbonate and ammonium molybdate.