ABSTRACT
Silica occurs in high abundance all over the earth. It occurs in the form of sand and quartz. It is also present in rocks and silicate minerals. It is found in natural waters at varying concentrations from 1 to 100 mg/L. Silica in water may be analyzed by the following methods: gravimetric method, ammonium molybdate colorimetric method, and atomic absorption spectrophotometric method. This chapter discusses the first two methods in detail. In gravimetric method, silica and silicates react with hydrochloric or perchloric acid to form silicic acid, H2SiO3. The evaporation of the solution precipitates dehydrated silica. In ammonium molybdate colorimetric method, silica reacts with ammonium molybdate to form a yellow colored heteropoly acid, silicomolybdic acid under acid condition. Certain forms of silica and many polymeric silicates do not react with ammonium molybdate. These complex silicates may be decomposed to simple molybdatereactive silica by high-temperature digestion or fusion with sodium bicarbonate.
