ASTM D7126-15(2023)

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.
Designation: D7126 − 15 (Reapproved 2023)
Standard Test Method for
On-Line Colorimetric Measurement of Silica
This standard is issued under the fixed designation D7126; 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.
1. Scope 2. Referenced Documents
1.1 This test method covers the on-line determination of 2.1 ASTM Standards:
soluble silica in water by colorimetric analysis using the D859 Test Method for Silica in Water
molybdenum blue method, also known as the heteropoly blue D1066 Practice for Sampling Steam
method. D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
1.2 This test method is applicable for silica determination in
D2777 Practice for Determination of Precision and Bias of
water with silica concentrations within 0.5 ppb to 5000 ppb
Applicable Test Methods of Committee D19 on Water
(μg/L).
D3370 Practices for Sampling Water from Flowing Process
1.3 This test method covers the determination of soluble
Streams
silica SiO (silicon dioxide) or silicates in water. Soluble silica
D3864 Guide for On-Line Monitoring Systems for Water
compounds are considered molybdate reactive silica. This test
Analysis
method does not cover the determination of colloidal or
D5540 Practice for Flow Control and Temperature Control
polymeric silica, which is considered non-molybdate reactive
for On-Line Water Sampling and Analysis
silica.
3. Terminology
1.4 This test method does not cover the laboratory or grab
sample measurement of silica in water. Refer to Test Method
3.1 Definitions—For definitions of terms used in this test
D859.
method, refer to Terminology D1129 and Practice D3864.
3.2 Definitions of Terms Specific to This Standard:
1.5 The values stated in SI units are to be regarded as
3.2.1 heteropoly compound, n—a compound in which
standard. No other units of measurement are included in this
groups of different elements are joined together by metal-metal
standard.
bonds.
1.6 This standard does not purport to address all of the
3.2.2 metalloid, n—an element which has properties that are
safety concerns, if any, associated with its use. It is the
intermediate between those of a metal and a nonmetal.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.2.3 photodetector, n—a device for detecting and measur-
mine the applicability of regulatory limitations prior to use.
ing the intensity of radiant energy.
1.7 This international standard was developed in accor-
dance with internationally recognized principles on standard-
4. Summary of Test Method
ization established in the Decision on Principles for the
4.1 This test method describes the analysis of soluble silica
Development of International Standards, Guides and Recom-
by analyzing a sample from a continuous stream. This test
mendations issued by the World Trade Organization Technical
method is based on the colorimetric determination of soluble
Barriers to Trade (TBT) Committee.
silica by the formation and reduction of molybdosilicic acid.
Reduced molybdosilicic acid forms a molybdenum blue com-
plex. The optical absorbance of this complex is typically
This test method is under the jurisdiction of ASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use,
On-Line Water Analysis, and Surveillance of Water For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved April 1, 2023. Published April 2023. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2006. Last previous edition approved in 2015 as D7126 – 15. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D7126-15R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7126 − 15 (2023)
measured at 815 nm 6 10 nm. The absorbance is directly 6.5 If a batch style instrument will be used for on-line
proportional to the concentration of silica in the sample. analysis, adequate rinsing in-between samples is necessary to
avoid silica contamination from previous samples.
4.2 This on-line test method requires reagents which are
added sequentially with separate reaction periods. Each reac-
7. Apparatus
tion must be allowed to go to completion before the next
7.1 Process Instrument:
reagent is added.
7.1.1 A spectrophotometer or colorimeter capable of accu-
rately measuring absorbance values at 815 nm 6 10 nm is
5. Significance and Use
necessary. The instrument should be capable of analyzing a
5.1 Silicon (Si), a metalloid, is the second most abundant
sample from an on-line process water stream by addition of the
element in the earth’s crust. Various forms of silica (silicon
necessary reagents for the heteropoly blue method. The instru-
dioxide SiO ) are found in quartz, sand, and rocks. The
ment should also be capable of adding a calibration standard
degradation of these rocks results in silica found in natural
automatically for recommended calibration of the on-line
waters. Silica in natural waters can be found as ionic silica,
instrument. The instrument may provide alarms, relays, iso-
silicates, colloidal or suspended particles.
lated analog, and digital outputs.
5.2 Elevated temperatures and pressure can cause silica in
8. Reagents
water to vaporize and form deposits or scale. Scale deposits of
silica will coat boilers and turbine blades used in power plants.
8.1 Purity of Water—References to water that is used for
The presence of silica scale affects the ability of metals to
reagent preparation, rinsing or dilution shall be understood to
transfer heat. Silica needs to be removed when deionized water
mean water that conforms to the quantitative specifications of
is used as a rinse for manufacturing wafers in the semiconduc-
Type II reagent water of Specification D1193.
tor industry.
8.2 Silica Solution, Standard—To prepare a silica standard
5.3 Silica is commonly removed by demineralization using
solution, dissolve ACS reagent grade or better sodium meta-
anion exchange resins, distillation, reverse osmosis or precipi-
silicate (Na SiO •9H O) and dilute with deionized water to
2 3 2
tation in a lime softening process. The on-line measurement of
volume, or use commercially prepared standards. Refer to the
silica is the preferred method to laboratory analyses for
instrument manufacturers manual for the correct standard
industries trying to obtain and monitor ultra-pure water. Since
concentration to use. Standards and samples should not be
silica is one of the first species to breakthrough anion exchange
frozen, which can result in lower soluble silica values.
resins, on-line silica monitoring is frequently used to determine
Standards should be prepared at room temperature and stored
the need for regeneration of an anion or mixed resin bed.
at 4 °C to maintain maximum stability.
8.3 Silica Reagents—The colorimetric analysis described in
6. Interferences
this method requires at least 3 reagents for the on-line
6.1 Colored and turbid samples and reagents may interfere
determination of silica. The reagents necessary for this method
in the colorimetric detection of silica. At low levels (ppb range)
are specific for the on-line instrument, colorimeter or spectro-
samples can be filtered to eliminate part
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