Silica is the second most prevalent element in the earth's crust. In the form of silicates it is present in most natural water supplies. Typical concentrations lie between 1 and 30 mg/L. Higher concentrations may exist in brackish waters and brines.

The silica content of water should be determined prior to its use in a variety of industrial applications. Silica can form a harmful scale on equipment and heat transfer surfaces, particularly steam turbine blades.

The Colorimetric Method. CHEMetrics' test method determines "molybdate reactive silica." The heteropoly blue chemistry is employed. Silica reacts with ammonium molybdate under acidic conditions to produce heteropoly acids which are then reduced to form a blue color. Phosphate interferences are masked with the addition of citric acid. The blue color formed is directly proportional to the silica concentration of the sample. Results are expressed as ppm (mg/L) SiO₂.