5.1 Silicon comprises about 28 % of the lithosphere and is, next to oxygen, the most abundant element. It is found as the oxide in crystalline forms, as in quartz; combined with other oxides and metals in a variety of silicates; and in amorphous forms. Silicon is the most abundant element in igneous rocks and is the characteristic element of all important rocks except the carbonates. It is the skeletal material of diatoms but is not known to play a significant role in the structure of processes of higher life forms.

5.2 Silica is only slightly soluble in water. The presence of most silica in natural waters comes from the gradual degradation of silica-containing minerals. The type and composition of the silica-containing minerals in contact with the water and the pH of the water are the primary factors controlling both the solubility and the form of silica in the resulting solution. Silica may exist in suspended particles, as a colloid, or in solution. It may be monomeric or polymeric. In solution it can exist as silicic acid or silicate ion, depending upon pH. The silica content of natural waters is commonly in the 5 to 25 mg/L range, although concentrations over 100 mg/L occur in some areas.

5.3 Silica concentration is an important consideration in some industrial installations such as steam generation and cooling water systems. Under certain conditions, silica forms troublesome silica and silicate scales, particularly on high-pressure steam turbine blades. In cooling water systems, silica forms deposits when solubility limits are exceeded. In contrast, silica may be added as a treatment chemical in some systems, for example, in corrosion control. Silica removal is commonly accomplished by ion exchange, distillation, reverse osmosis, or by precipitation, usually with magnesium compounds in a hot or cold lime softening process.

1.1 This test method covers the determination of silica in water and waste water; however, the analyst should recognize that the precision and accuracy statements for reagent water solutions may not apply to waters of different matrices.

1.2 This test method is a colorimetric method that determines molybdate-reactive silica. It is applicable to most waters, but some waters may require filtration and dilution to remove interferences from color and turbidity. This test method is useful for concentrations as low as 20 μg/L.

1.3 This test method covers the photometric determination of molybdate-reactive silica in water. Due to the complexity of silica chemistry, the form of silica measured is defined by the analytical method as molybdate-reactive silica. Those forms of silica that are molybdate-reactive include dissolved simple silicates, monomeric silica and silicic acid, and an undetermined fraction of polymeric silica.

1.4 The useful range of this test method is from 20 to 1000 μg/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L at the lower wavelength (640 nm). It is particularly applicable to treated industrial waters. It may be applied to natural waters and wastewaters following filtration or dilution, or both. For seawater or brines, this test method is applicable only if matched matrix standards or standard addition techniques are employed.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Note 1: For many natural waters, a measurement of molybdate-reactive silica by this test method provides a close approximation of total silica, and, in practice, the colorimetric method is frequently substituted for other more time-consuming techniques. This is acceptable when, as frequently occurs, the molybdate-reactive silica is in the milligram per litre concentration range while the nonmolybdate-reactive silica, if present at all, is in the microgram per litre concentration range.

1.7 Former Test Method A (Gravimetric—Total Silica) was discontinued. Refer to Appendix X1 for historical information.