ASTM D859-05
(Test Method)Standard Test Method for Silica in Water
Standard Test Method for Silica in Water
SIGNIFICANCE AND USE
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.
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.
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.
SCOPE
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 [u]g/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 [u]g/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 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 ...
General Information
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
An American National Standarddo
Designation:D859–05
Standard Test Method for
1
Silica in Water
This standard is issued under the fixed designation D859; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 1.6 Former Test Method A (Gravimetric—Total Silica) was
discontinued. Refer toAppendix X1 for historical information.
1.1 This test method covers the determination of silica in
water and waste water; however, the analyst should recognize
2. Referenced Documents
that the precision and accuracy statements for reagent water
2
2.1 ASTM Standards:
solutions may not apply to waters of different matrices.
D1066 Practice for Sampling Steam
1.2 This test method is a colorimetric method that deter-
D1129 Terminology Relating to Water
minesmolybdate-reactivesilica.Itisapplicabletomostwaters,
D1193 Specification for Reagent Water
but some waters may require filtration and dilution to remove
D2777 Practice for Determination of Precision and Bias of
interferences from color and turbidity. This test method is
Applicable Test Methods of Committee D19 on Water
useful for concentrations as low as 20 µg/L.
D3370 Practices for Sampling Water from Closed Conduits
1.3 This test method covers the photometric determination
D4841 Practice for Estimation of Holding Time for Water
of molybdate-reactive silica in water. Due to the complexity of
Samples Containing Organic and Inorganic Constituents
silica chemistry, the form of silica measured is defined by the
D5810 Guide for Spiking into Aqueous Samples
analytical method as molybdate-reactive silica.Those forms of
D5847 Practice for Writing Quality Control Specifications
silica that are molybdate-reactive include dissolved simple
for Standard Test Methods for Water Analysis
silicates, monomeric silica and silicic acid, and an undeter-
E60 Practice for Analysis of Metals, Ores, and Related
mined fraction of polymeric silica.
Materials by Molecular Absorption Spectrometry
1.4 The useful range of this test method is from 20 to 1000
E275 Practice for Describing and Measuring Performance
µg/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L at
of Ultraviolet and Visible Spectrophotometers
the lower wavelength (640 nm). It is particularly applicable to
treated industrial waters. It may be applied to natural waters
3. Terminology
and wastewaters following filtration or dilution, or both. For
3.1 Definitions—For definitions of terms used in this test
seawater or brines, this test method is applicable only if
method, refer to Terminology D1129.
matched matrix standards or standard addition techniques are
employed.
4. Summary of Test Method
1.5 This standard does not purport to address all of the
4.1 This test method is based on the reaction of the soluble
safety concerns, if any, associated with its use. It is the
silica with molybdate ion to form a greenish-yellow complex,
responsibility of the user of this standard to establish appro-
which in turn is converted to a blue complex by reduction with
priate safety and health practices and determine the applica-
1-amino-2-naphthol-1-sulfonic acid.
bility of regulatory limitations prior to use.
5. Significance and Use
NOTE 1—For many natural waters, a measurement of molybdate-
reactive silica by this test method provides a close approximation of total
5.1 Silicon comprises about 28 % of the lithosphere and is,
silica, and, in practice, the colorimetric method is frequently substituted
next to oxygen, the most abundant element. It is found as the
for other more time-consuming techniques. This is acceptable when, as
oxide in crystalline forms, as in quartz; combined with other
frequently occurs, the molybdate-reactive silica is in the milligram per
oxides and metals in a variety of silicates; and in amorphous
litreconcentrationrangewhilethenonmolybdate-reactivesilica,ifpresent
forms. Silicon is the most abundant element in igneous rocks
at all, is in the microgram per litre concentration range.
and is the characteristic element of all important rocks except
1
This test method is under the jurisdiction of ASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
2
in Water. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Feb. 1, 2005. Published February 2005. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1945. Last previous edition approved in 2000 as D859 – 00. DOI: Standards volume information, refer to the
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