ASTM D859-16(2021)e1

Standard Test Method for Silica in Water

SIGNIFICANCE AND USE
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.
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 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, health, and environmental 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 w...

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 13.060.50 - Examination of water for chemical substances
• Technical Committee: D19 - Water
• Drafting Committee: D19.05 - Inorganic Constituents in Water

Relations

• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D1066-18e1 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• Refers: ASTM D1066-18 - Standard Practice for Sampling Steam
• Effective Date: 01-Aug-2018
• Refers: ASTM D4841-88(2013) - Standard Practice for Estimation of Holding Time for Water Samples Containing Organic and Inorganic Constituents
• Effective Date: 01-Jan-2013
• Refers: ASTM D4841-88(2013)e1 - Standard Practice for Estimation of Holding Time for Water Samples Containing Organic and Inorganic Constituents
• Effective Date: 01-Jan-2013
• Refers: ASTM D2777-12 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jun-2012
• Refers: ASTM D1066-11 - Standard Practice for Sampling Steam
• Effective Date: 15-Jun-2011
• Refers: ASTM D5810-96(2011) - Standard Guide for Spiking into Aqueous Samples
• Effective Date: 01-May-2011
• Refers: ASTM D3370-10 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2010
• Refers: ASTM D1129-10 - Standard Terminology Relating to Water
• Effective Date: 01-Mar-2010
• Refers: ASTM E275-08 - Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
• Effective Date: 15-Oct-2008
• Refers: ASTM D3370-08 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Oct-2008
• Refers: ASTM D4841-88(2008) - Standard Practice for Estimation of Holding Time for Water Samples Containing Organic and Inorganic Constituents
• Effective Date: 15-Jul-2008
• Refers: ASTM D2777-08 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jan-2008
• Refers: ASTM D3370-07 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2007

Overview

ASTM D859-16(2021)e1: Standard Test Method for Silica in Water provides an internationally recognized procedure for the colorimetric determination of molybdate-reactive silica in water and wastewater. Developed by ASTM International, this standard is widely used in environmental monitoring, industrial water treatment, and water quality assessment. Silica, largely in the form of silicon dioxide (SiO₂), is common in the Earth's crust and occurs in water mainly through the degradation of silica-containing minerals. This method is particularly valuable for its sensitivity, enabling the detection of silica concentrations as low as 20 µg/L.

Key Topics

• Significance of Silica in Water
  Silicon is the second-most abundant element in the lithosphere, and silica’s presence in water can come from minerals in rocks and soil. The solubility and form of silica are influenced by water pH and the type of mineral contact.

• Industrial and Environmental Importance
  Silica concentration is crucial in various industries such as steam generation and cooling water systems. High silica content can cause scaling and deposits on equipment, while controlled silica additions may enhance corrosion resistance.

• Test Method Summary
  The method uses a colorimetric approach: soluble silica reacts with molybdate ion, forming a colored complex which is measured photometrically. The technique detects molybdate-reactive silica, including dissolved simple silicates, monomeric silica, and a part of polymeric silica.

• Scope and Limitations

  • Applicable to most waters; may require filtration and dilution for colored or turbid samples.
  • Effective for concentrations from 20 to 1000 µg/L (at 815 nm) or 0.1 to 5 mg/L (at 640 nm).
  • Suitable for treated water, natural water, and wastewater; requires matched standards for seawater and brines.

• Quality Control
  The standard includes robust quality assurance requirements such as calibration verification, blank and spike sample analysis, and ongoing performance checks to ensure accurate, reliable results.

Applications

• Water Treatment Facilities
  Monitoring silica levels helps prevent scale formation in boilers, cooling systems, and reverse osmosis units, contributing to equipment longevity and operational efficiency.

• Industrial Process Waters
  Industries using high-pressure steam turbines monitor silica to mitigate deposits which can degrade performance and lead to costly maintenance.

• Environmental and Regulatory Compliance
  Accurate measurement of silica supports compliance with environmental quality standards for discharge to natural waters or municipal systems.

• Research and Development
  Laboratories employ this method to study geochemical processes, water-rock interactions, and to support the development of new water treatment technologies.

Related Standards

• ASTM D1066 - Sampling Steam: Guidance on proper sampling techniques for steam condensate analysis.
• ASTM D1129 - Terminology Relating to Water: Definitions of relevant terms for water analysis.
• ASTM D1193 - Specification for Reagent Water: Specifications for water used in laboratory testing.
• ASTM D2777 - Determination of Precision and Bias: Techniques for establishing the reliability of test methods.
• ASTM D3370 - Practices for Sampling Water: Procedures for collecting water samples from flowing streams.
• ASTM D4841 - Holding Time for Water Samples: Guidance on maximum sample holding times to preserve analyte integrity.
• ASTM D5810 - Spiking into Aqueous Samples: Techniques for establishing recovery rates through sample spiking.
• ASTM D5847 - Writing Quality Control Specifications: Creating effective QC measures for water analysis.
• ASTM E60 / E275 - Spectrophotometry: Specifications for instruments and practices in photometric analysis.

Summary

Compliance with ASTM D859-16(2021)e1 ensures consistent, accurate, and reliable determination of silica in water, supporting operational efficiency in industry, regulatory adherence, and high-quality environmental monitoring. The colorimetric method's practical range, sensitivity, and robust quality controls make it an industry standard for laboratories and water treatment professionals worldwide. For optimal results, users must also follow relevant related standards for sampling, quality control, and safety.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: D859 − 16 (Reapproved 2021)
Standard Test Method for
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 U.S. Department of Defense.
ε NOTE—The WTO caveat was added editorially in December 2021.
NOTE 1—For many natural waters, a measurement of molybdate-
1. Scope
reactive silica by this test method provides a close approximation of total
1.1 This test method covers the determination of silica in
silica, and, in practice, the colorimetric method is frequently substituted
water and waste water; however, the analyst should recognize for other more time-consuming techniques. This is acceptable when, as
frequently occurs, the molybdate-reactive silica is in the milligram per
that the precision and accuracy statements for reagent water
litreconcentrationrangewhilethenonmolybdate-reactivesilica,ifpresent
solutions may not apply to waters of different matrices.
at all, is in the microgram per litre concentration range.
1.2 This test method is a colorimetric method that deter-
1.7 Former Test Method A (Gravimetric—Total Silica) was
minesmolybdate-reactivesilica.Itisapplicabletomostwaters,
discontinued. Refer to Appendix X1 for historical information.
but some waters may require filtration and dilution to remove
1.8 This international standard was developed in accor-
interferences from color and turbidity. This test method is
dance with internationally recognized principles on standard-
useful for concentrations as low as 20 µg/L.
ization established in the Decision on Principles for the
1.3 This test method covers the photometric determination
Development of International Standards, Guides and Recom-
of molybdate-reactive silica in water. Due to the complexity of
mendations issued by the World Trade Organization Technical
silica chemistry, the form of silica measured is defined by the
Barriers to Trade (TBT) Committee.
analytical method as molybdate-reactive silica.Those forms of
silica that are molybdate-reactive include dissolved simple 2. Referenced Documents
silicates, monomeric silica and silicic acid, and an undeter-
2.1 ASTM Standards:
mined fraction of polymeric silica.
D1066 Practice for Sampling Steam
1.4 The useful range of this test method is from 20 to 1000
D1129 Terminology Relating to Water
µg/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L at
D1193 Specification for Reagent Water
the lower wavelength (640 nm). It is particularly applicable to
D2777 Practice for Determination of Precision and Bias of
treated industrial waters. It may be applied to natural waters
Applicable Test Methods of Committee D19 on Water
and wastewaters following filtration or dilution, or both. For
D3370 Practices for Sampling Water from Flowing Process
seawater or brines, this test method is applicable only if
Streams
matched matrix standards or standard addition techniques are
D4841 Practice for Estimation of Holding Time for Water
employed.
Samples Containing Organic and Inorganic Constituents
D5810 Guide for Spiking into Aqueous Samples
1.5 The values stated in SI units are to be regarded as
D5847 Practice for Writing Quality Control Specifications
standard. No other units of measurement are included in this
for Standard Test Methods for Water Analysis
standard.
E60 Practice for Analysis of Metals, Ores, and Related
1.6 This standard does not purport to address all of the
Materials by Spectrophotometry
safety concerns, if any, associated with its use. It is the
E275 Practice for Describing and Measuring Performance of
responsibility of the user of this standard to establish appro-
Ultraviolet and Visible Spectrophotometers
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
3. Terminology
3.1 Definitions:
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
in Water. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2021. Published December 2021. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1945. Last previous edition approved in 2016 as D859 – 16. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D0859-16R21E01. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D859 − 16 (2021)
3.1.1 For definitions of terms used in this standard, refer to 7. Apparatus
Terminology D1129.
7.1 Spectrophotometer or Filter Photometer (see Note
2)—To obtain maximum sensitivity and reproducibility, a
4. Summary of Test Method
spectrophotometer suitable for measurements at 815 nm is
4.1 This test method is based on the reaction of the soluble
required. Measurements may be made at 640 nm with a
silica with molybdate ion to form a greenish-yellow complex,
spectrophotometer, or 640 to 700 nm with a filter photometer
which in turn is converted to a blue complex by reduction with
if less sensitivity is preferred. Precision and bias information
1-amino-2-naphthol-1-sulfonic acid.
on this test method (see Section 13) is based on data obtained
at 815 nm. A direct reading spectrophotometer or filter pho-
5. Significance and Use
tometer may be used.
5.1 Silicon comprises about 28 % of the lithosphere and is,
NOTE 2—Photometers and photometric practices shall conform to
next to oxygen, the most abundant element. It is found as the
Practice E60. Spectrophotometers shall conform to Practice E275.
oxide in crystalline forms, as in quartz; combined with other
7.2 Sample Cells—The cell size to be used depends on the
oxides and metals in a variety of silicates; and in amorphous
range covered and the particular instrument used. The higher
forms. Silicon is the most abundant element in igneous rocks
concentration range should be attainable with 10-mm path
and is the characteristic element of all important rocks except
length cells. Longer path length cells (40 to 50 mm) are
the carbonates. It is the skeletal material of diatoms but is not
recommended for concentrations below 0.1 mg/L.
known to play a significant role in the structure of processes of
higher life forms.
8. Reagents and Materials
5.2 Silica is only slightly soluble in water. The presence of
NOTE 3—Store all reagents to be used in this test method in polyeth-
most silica in natural waters comes from the gradual degrada-
ylene or other suitable plastic bottles.
tion of silica-containing minerals.The type and composition of
8.1 Purity of Reagents—Reagent grade chemicals shall be
the silica-containing minerals in contact with the water and the
used in all tests. Unless otherwise indicated, it is intended that
pH of the water are the primary factors controlling both the
all reagents shall conform to the specifications of the Commit-
solubility and the form of silica in the resulting solution. Silica
tee onAnalytical Reagents of theAmerican Chemical Society,
may exist in suspended particles, as a colloid, or in solution. It 3
where such specifications are available. Other grades may be
may be monomeric or polymeric. In solution it can exist as
used, provided it is first ascertained that the reagent is of
silicic acid or silicate ion, depending upon pH. The silica
sufficiently high purity to permit its use without lessening the
content of natural waters is commonly in the 5 to 25 mg/L
accuracy of the determination.
range, although concentrations over 100 mg/L occur in some
8.2 Purity of Water—Unless otherwise indicated, references
areas.
towatershallbeunderstoodtomeanreagentwaterconforming
5.3 Silica concentration is an important consideration in
to Specification D1193, Type I. In addition, the water shall be
some industrial installations such as steam generation and
made silica-free by distillation or demineralization and deter-
cooling water systems. Under certain conditions, silica forms
mined as such in accordance with the method of test being
troublesome silica and silicate scales, particularly on high-
used. The collecting apparatus and storage containers for the
pressure steam turbine blades. In cooling water systems, silica
reagent water must be polyethylene or other suitable plastic.
forms deposits when solubility limits are exceeded. In contrast,
Type II water was specified at the time of round robin testing
silica may be added as a treatment chemical in some systems,
of this test method.
for example, in corrosion control. Silica removal is commonly
8.3 Amino-Naphthol-Sulfonic Acid-Solution—Dissolve 0.5
accomplishedbyionexchange,distillation,reverseosmosis,or
g of 1-amino-2-naphthol-4-sulfonic acid in 50 mLof a solution
by precipitation, usually with magnesium compounds in a hot
containing 1 g of sodium sulfite (Na SO ). After dissolving,
2 3
or cold lime softening process.
add the solution to 100 mL of a solution containing 30 g of
6. Interferences
sodium hydrogen sulfite (NaHSO ). Make up to 200 mL with
water and store in a dark, plastic bottle. Shelf life of this
6.1 Color and turbidity will interfere if not removed by
reagent may be extended by refrigeration. Solution should be
filtration or dilution.
adjusted to room temperature, 25 6 5 °C, before use. Discard
6.2 The only specific substance known to interfere in the
when the color darkens or a precipitate forms.
color reaction is phosphate. Phosphate interference is elimi-
8.4 Ammonium Molybdate Solution (75 g/L) (Note 4)—
nated by the addition of oxalic acid.
Dissolve 7.5 g of ammonium molybdate ((NH ) Mo
4 6 7
6.3 Ahighdissolvedsaltsconcentration,suchasinseawater
O ·4H O) in 100 mL of water.
24 2
or brine samples, can affect color development. This can be
compensated for by preparing standards in a matrix similar to
that of samples or by using a standard additions technique.
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
6.4 Strong oxidizing and reducing agents that may be found
listed by the American Chemical Society, see Analar Standards for Laboratory
in some industrial waste waters may interfere in the reduction
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
step of the reaction. Such waste waters may also contain
and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
organic compounds that may interfere in the color formation. MD.
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D859 − 16 (2021)
TABLE 1 Overall (S ) and Single-Operator (S ) Interlaboratory TABLE 2 Recovery in Reagent Water
T o
Precision for Silica
Statistically
Amount Added, Amount Found,
Bias, % Significant
Mean Concentration (X),
µg SiO /L µg SiO /L
S ,µ g/L S ,µ g/L 2 2
T o
95 % Level
µg/L
951 941.8 −1.0 no
38.6 3.4 0.8
381 381.8 + 0.2 no
112.2 5.8 1.0
115 112.2 −2.4 no
381.8 10.0 1.7
39 38.6 − 1.0 no
941.8 30.0 5.9
NOTE 4—Batch to batch variations in ammonium molybdate have been
10.2 For standards in the 20 to 1000 µg/L range, set the
found to affect results at low concentrations (below 0.1 mg/L). High
blanks, nonlinear calibration curves, and poor reproducibility have been
spectrophotometer at 815 nm and read the absorbance of each
observed with some batches of this compound. When working with low
standard against the reagent blank. For standards in the 0.1 to
concentrations of silica, a batch of ammonium molybdate known to
5 mg/L range, set the spectrophotometer at 640 nm (filter
produce reasonable blanks, linearity, and reproducibility should be set
photometer 640 to 700 nm).
aside for this purpose.
10.3 Read directly in concentration if this capability is
8.5 Hydrochloric Acid (1+1)—Mix 1 volume of concen-
provided with the spectrophotometer or filter photometer
trated hydrochloric acid (HCl, sp gr 1.19) with 1 volume of
instrument or prepare a calibration curve
...