Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 284

2.91.3 Gas phase chemiluminescence analysis
Fujiwara et al. [697] described the application of gas phase chemi-luminescence analysis
to the determination of silicate in non saline waters. Silicate is converted to silane using
lithium aluminium hydride in a Teflon tube heated to 200°C The silane is collected and
mixed with ozone and analysed by chemiluminescence analyses. The detection limit is
0.5µg silicon (absolute) phosphorus and arsenic interfere.
2.91.4 Flow injection analysis
Fogg and Blelsn [5] have studied the application of this technique to the determination of
silicate.
Workers at Tecator Ltd UK [698,699] have developed flow injection methods for the
determination of 0.03-1.2mg L −1 silicate (as SiO 2 ) in non saline waters. In these methods
ammonium molybdate at pH of approximately 1.2 reacts with silica and also with any
phosphate present to produce heteropoly acids. Oxalic acid is added to destroy the
molybdo-phosphoric acid but not the molybdosilicic acid. The yellow colour produced is
reduced by means of stannous chloride to heteropoly blue. The blue colour is more
intense than the yellow colour and is proportional to the 'molybdate-reactive' silica. The
colour is measured at 695nm.
The application of this technique is discussed under multianion analysis in sections
14.4.1.3 and 14.4.1.4.
2.91.5 Atomic absorption spectrometry
Two methods based on atomic absorption spectroscopy [700,701] have been discussed
for the determination of silicate as silicon in water.
Simmons [700] described a digestion technique using a hydrofluoric/ nitric acid
mixture in which the hydrofluoric acid decomposes silicates by the reaction of fluoride
with silica to form silicon tetrafluoride. The organic matter including the cellulose nitrate
filter is destroyed by the oxidising action of nitric acid. The excess hydrofluoric acid is
complexed with boric acid and the resulting solution is sampled automatically and
analysed for silicon by atomic absorption spectrophotometry using a nitrous oxide
acetylene flame.
T he detection limit of this method is 0.1mg L −1 SiO 2 and the reproducibility at the 1-
150mg L −1 SiO 2 range is ±5%.
2.91.6 Ion chromatography
Okada and Kuwamoto [702] have described an ion chromatographic method for the
determination of silicate in non saline waters.
The application of this technique is also discussed under multianion analysis in section
12.2.5.