ASTM D859-00
(Test Method)Standard Test Method for Silica in Water
Standard Test Method for Silica in Water
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 liter concentration range while the nonmolybdate-reactive silica, if present at all, is in the microgram per liter concentration range.
1.6 Former Test Method A (Gravimetric--Total Silica) was discontinued. Refer to Appendix X1 for historical information.
General Information
Relations
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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 Standard
Designation:D 859–00
Standard Test Method for
Silica in Water
This standard is issued under the fixed designation D 859; 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 (e) 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
2.1 ASTM Standards:
solutions may not apply to waters of different matrices.
D 1066 Practice for Sampling Steam
1.2 This test method is a colorimetric method that deter-
D 1129 Terminology Relating to Water
minesmolybdate-reactivesilica.Itisapplicabletomostwaters,
D 1193 Specification for Reagent Water
but some waters may require filtration and dilution to remove
D 2777 Practice for Determination of Precision and Bias of
interferences from color and turbidity. This test method is
Applicable Methods of Committee D-19 on Water
useful for concentrations as low as 20 µg/L.
D 3370 Practices for Sampling Water from Closed Con-
1.3 This test method covers the photometric determination
duits
of molybdate-reactive silica in water. Due to the complexity of
D 4841 Practice for Estimation of Holding Time for Water
silica chemistry, the form of silica measured is defined by the
Samples Containing Organic and Inorganic Constituents
analytical method as molybdate-reactive silica. Those forms of
D 5810 StandardGuideforSpikingintoAqueousSamples
silica that are molybdate-reactive include dissolved simple
D 5847 Standard Practice for the Writing Quality Control
silicates, monomeric silica and silicic acid, and an undeter-
Specifications for StandardTest Methods forWaterAnaly-
mined fraction of polymeric silica.
sis
1.4 The useful range of this test method is from 20 to 1000
E 60 Practice for Photometric and Spectrophotometric
µg/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L at
Methods for Chemical Analysis of Metals
the lower wavelength (640 nm). It is particularly applicable to
E 275 Practice for Describing and Measuring Performance
treated industrial waters. It may be applied to natural waters
of Ultraviolet, Visible, and Near Infrared Spectrophotom-
and wastewaters following filtration or dilution, or both. For
eters
seawater or brines, this test method is applicable only if
matched matrix standards or standard addition techniques are
3. Terminology
employed.
3.1 Definitions—For definitions of terms used in this test
1.5 This standard does not purport to address all of the
method, refer to Terminology D 1129.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Summary of Test Method
priate safety and health practices and determine the applica-
4.1 This test method is based on the reaction of the soluble
bility of regulatory limitations prior to use.
silica with molybdate ion to form a greenish-yellow complex,
NOTE 1—For many natural waters, a measurement of molybdate-
which in turn is converted to a blue complex by reduction with
reactive silica by this test method provides a close approximation of total
1-amino-2-naphthol-1-sulfonic acid.
silica, and, in practice, the colorimetric method is frequently substituted
for other more time-consuming techniques. This is acceptable when, as
5. Significance and Use
frequently occurs, the molybdate-reactive silica is in the milligram per
5.1 Silicon comprises about 28 % of the lithosphere and is,
litreconcentrationrangewhilethenonmolybdate-reactivesilica,ifpresent
next to oxygen, the most abundant element. It is found as the
at all, is in the microgram per litre concentration range.
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
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.
Current edition approved June 10, 2000. Published September 2000. Originally Annual Book of ASTM Standards, Vol 11.01.
published as D 859 – 45 T. Last previous edition D 859 – 94. Annual Book of ASTM Standards, Vol 03.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D859–00
and is the characteristic element of all important rocks except 8. Reagents and Materials
the carbonates. It is the skeletal material of diatoms but is not
NOTE 3—Store all reagents to be used in this test method in polyeth-
known to play a significant role in the structure of processes of
ylene or other suitable plastic bottles.
higher life forms.
8.1 Purity of Reagents—Reagent grade chemicals shall be
5.2 Silica is only slightly soluble in water. The presence of
used in all tests. Unless otherwise indicated, it is intended that
most silica in natural waters comes from the gradual degrada-
all reagents shall conform to the specifications of the Commit-
tion of silica-containing minerals.The type and composition of
tee onAnalytical Reagents of theAmerican Chemical Society,
the silica-containing minerals in contact with the water and the
where such specifications are available. Other grades may be
pH of the water are the primary factors controlling both the
used, provided it is first ascertained that the reagent is of
solubility and the form of silica in the resulting solution. Silica
sufficiently high purity to permit its use without lessening the
may exist in suspended particles, as a colloid, or in solution. It
accuracy of the determination.
may be monomeric or polymeric. In solution it can exist as
8.2 Purity of Water— Unless otherwise indicated, refer-
silicic acid or silicate ion, depending upon pH. The silica
ences to water shall be understood to mean reagent water
content of natural waters is commonly in the 5 to 25 mg/L
conforming to Specification D 1193, Type II. In addition, the
range, although concentrations over 100 mg/L occur in some
water shall be made silica-free by distillation or demineraliza-
areas.
tion and determined as such in accordance with the method of
5.3 Silica concentration is an important consideration in
test being used. The collecting apparatus and storage contain-
some industrial installations such as steam generation and
ers for the reagent water must be polyethylene or other suitable
cooling water systems. Under certain conditions, silica forms
plastic.
troublesome silica and silicate scales, particularly on high-
8.3 Amino-Naphthol-Sulfonic Acid-Solution—Dissolve 0.5
pressure steam turbine blades. In cooling water systems, silica
g of 1-amino-2-naphthol-4-sulfonic acid in 50 mLof a solution
forms deposits when solubility limits are exceeded. In contrast,
containing 1 g of sodium sulfite (Na SO ). After dissolving,
2 3
silica may be added as a treatment chemical in some systems,
add the solution to 100 mL of a solution containing 30 g of
for example, in corrosion control. Silica removal is commonly
sodium hydrogen sulfite (NaHSO ). Make up to 200 mL and
accomplished by ion exchange, distillation, reverse osmosis, or
store in a dark, plastic bottle. Shelf life of this reagent may be
by precipitation, usually with magnesium compounds in a hot
extended by refrigeration. Solution should be adjusted to room
or cold lime softening process.
temperature, 25 6 5°C, before use. Discard when the color
darkens or a precipitate forms.
6. Interferences
8.4 Ammonium Molybdate Solution (75 g/L) (Note 4)—
6.1 Color and turbidity will interfere if not removed by
Dissolve 7.5 g of ammonium molybdate ((NH ) Mo -
4 6 7
filtration or dilution.
O ·4H O) in 100 mL of water.
24 2
6.2 The only specific substance known to interfere in the
NOTE 4—Batch to batch variations in ammonium molybdate have been
color reaction is phosphate. Phosphate interference is elimi-
found to affect results at low concentrations (below 0.1 mg/L). High
nated by the addition of oxalic acid.
blanks, nonlinear calibration curves, and poor reproducibility have been
6.3 Ahighdissolvedsaltsconcentration,suchasinseawater
observed with some batches of this compound. When working with low
or brine samples, can affect color development. This can be
concentrations of silica, a batch of ammonium molybdate known to
compensated for by preparing standards in a matrix similar to
produce reasonable blanks, linearity, and reproducibility should be set
that of samples or by using a standard additions technique.
aside for this purpose.
6.4 Strong oxidizing and reducing agents that may be found
8.5 Hydrochloric Acid (1 + 1)—Mix 1 volume of concen-
in some industrial waste waters may interfere in the reduction
trated hydrochloric acid (HCl, sp gr 1.19) with 1 volume of
step of the reaction. Such waste waters may also contain
water.
organic compounds that may interfere in the color formation.
8.6 OxalicAcid Solution(100g/L)—Dissolve10gofoxalic
acid (H C O ·2H O) in 100 mL of water.
2 2 4 2
7. Apparatus
8.7 Silica Solution, Standard (1 mL = 0.1 mg SiO )—
7.1 Spectrophotometer or Filter Photometer (see Note 2)—
Dissolve 0.473 g of sodium metasilicate (Na SiO ·9H O) in
2 3 2
To obtain maximum sensitivity and reproducibility, a spectro-
water and dilute to 1 L. Check the concentration of this
photometer suitable for measurements at 815 nm is required.
solution gravimetrically.
Measurements may be made at 640 nm with a spectrophotom-
NOTE 5—This solution may require filtration to remove fine particulate
eter,or640to700nmwithafilterphotometeriflesssensitivity
matter containing silica. This filtration, if needed, should precede stan-
is preferred. Precision and bias information on this test method
dardization gravimetrically. This step was not included as a requirement
(see Section 14) is based on data obtained at 815 nm.
NOTE 2—Photometers and photometric practices shall conform to
Reagent Chemicals, American Chemical Society Specifications, American
Practice E 60. Spectrophotometers shall conform to Practice E 275.
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
7.2 Sample Cells— The cell size to be used depends on the
listed by the American Chemical Society, see Analar Standards for Laboratory
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
range covered and the particular instrument used. The higher
and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
concentration range should be attainable with 10-mm path
MD.
length cells. Longer path length cells (40 to 50 mm) are
RefertoformerTestMethodA(Gravimetric—TotalSilica)lastpublishedinthe
recommended for concentrations below 0.1 mg/L. 1988 Annual Book of ASTM Standards for complete description of procedure.
D859–00
in the collaborative tests from which precision and bias determined.
10.4 Laboratory Control Sample
10.4.1 OneLaboratoryControlSample(LCS)shouldberun
9. Sampling
with each sample batch (maximum of 20 samples).The LCS is
9.1 Collect the samples in accordance with Practice D 1066
a solution of method analytes of known concentration added to
or Practices D 3370, as applicable.
a matrix which sufficiently challenges the Test Method. A
9.2 Use plastic or stainless steel sample bottles, provided
synthetic“water”matrixofrelevancetotheuser(e.g.,drinking
with rubber or plastic stoppers.
water or wastewater) spiked with the method analytes at the
9.3 If the water being sampled is at elevated temperature,
level of the IDP solution would be an example of an appropri-
cool to less than 35°C but do not freeze.
ate LCS.
9.4 The holding time for the samples may be calculated in
The analyte recoveries for the LCS should fall within the
accordance with Practice D 4841.
control limits of x 6 3S, where x is the IDP amount and (S) is
the standard deviation of the mean recovery established from
10. Quality Control (QC)
the interlaboratory precision and bias study data at the IDP
10.1 In order to be certain that analytical values obtained
levels, as shown below:
using this test method are valid and accurate within the
Lower Recovery Upper Recovery
confidencelimitsofthetest,thefollowingQCproceduresmust
Analyte LCS Amount Limit Limit
be followed when running the test.
Silica 115 µg/L 112 µg/L 118 µg/L
10.2 Calibration and Calibration Verification
10.2.1 When beginning use of this method, an initial Cali-
10.5 Method Blank
bration Verification Standard (CVS) should be used to verify
10.5.1 A reagent blank should be run when generating the
the calibration standards and acceptable instrument perfor-
initial calibration curves.Ablank should also be run with each
mance. This verification should be performed on each analysis
sample batch (maximum of 20 samples) to check for sample or
day. The CVS is a solution of method analytes of known
system contamination.
concentration (mid-calibration range) used to fortify reagent
10.6 Matrix Spike
water. If the determined CVS concentrations are not within
10.6.1 One Matrix Spike (MS) should be run with each
615% of the known values, the analyst should reanalyze the
sample batch (maximum of 20 samples) to test method
CVS. If the values still fall outside acceptable limits, a new
recovery. The MS should be prepared in accordance with
calibration curve is required which must be confirmed by a
Guide D 5810. Spike a portion of a water (or other) sample
successful CVS before continuing with on-going analyses.
from each batch with the method analytes at the level of the
10.2.2 One CVS should then be run with each sample batch
IDP solution. The % recovery of the spike should fall within
(maximum of 20 samples) to verify the previously established
limits established from the interlaboratory precision and bias
calibrationcurves.Ifthedeterminedanalyteconcentrationsfall
study data (assuming a background level of zero), according to
outside acceptable limits (615%) that analyte is judged out of
Standard D 5847, as shown below:
control, and the source of the problem should be identified
Lower Recovery Upper Recovery
before continuing with on-going analyses.
Analyte MS Amount Limit (%) Limit (%)
10.3 Initial Demonstration of Laboratory Capability
Silica 115 µg/L 82.5% 112.7%
10.3.1 The laboratory using
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.