Designation:D 859–05
An American National Standarddo
Standard Test Method for
Silica in Water 1
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
1.1This 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.
1.2This test method is a colorimetric method that deter-mines 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 20µg/L.
1.3This 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 undeter-mined fraction of polymeric silica.
1.4The useful range of this test method is from 20to 1000µg/L at the higher wavelength (815nm)and 0.1to 5mg/L at the lower wavelength (640nm).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 technique
s are employed.
1.5This standard does not purport to address all of the safety concerns,if any,associated with its use.It is the of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.
N OTE 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.
1.6Former Test Method A (Gravimetric—Total Silica)was discontinued.Refer to Appendix X1for historical information.
2.Referenced Documents 2.1ASTM Standards:2
D 1066Practice for Sampling Steam D 1129Terminology Relating to Water D 1193Specification for Reagent Water
D 2777Practice for Determination of Precision and Bias of Applicable Methods of Committee D19on Water
D 3370Practices for Sampling Water from Closed Conduits D 4841Practice for Estimation of Holding Time for Water Samples Containing Organic and Inorganic Constituents D 5810Standard Guide for Spiking into Aqueous Samples D 5847Standard Practice for Writing Quality Control Specifications for Standard Test Methods for Water Analy-sis
E 60Practice for Analysis of Metals,Ores,and Related Materials by Molecular Absorption Spectrometry
E 275Practice for Describing and Measuring Performance of Ultraviolet,Visible,and Near Infrared Spectrophotom-eters 3.Terminology
3.1Definitions —For definitions of terms used in this test method,refer to Terminology D 1129.
4.Summary of Test Method
4.1This test method is based on the reaction of the soluble silica with molybdate ion to form a greenish-yellow complex,which in turn is converted to a blue complex by reduction with 1-amino-2-naphthol-1-sulfonic acid.
5.Significance and Use
5.1Silicon 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
1
This test method is under the jurisdiction of ASTM Committee D19on Water and is the direct responsibility of Subcommittee D19.05on Inorganic Constituents in Water.
Current edition approved Feb.1,2005.Published February 2005.Originally approved in 1945.Last previous edition approved in 2000as D 859–00.
2
For referenced ASTM standards,visit the ASTM website,,or contact ASTM Customer Service at For Annual Book of ASTM Standards volume information,refer to the standard’s Document Summary page on the ASTM website.
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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.
5.2Silica is only slightly soluble in water.The presence of most silica in natural waters comes from the gradual degrada-tion 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 5to 25mg/L range,although concentrations over 100mg/L occur in some areas.
5.3Silica 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.
6.Interferences
6.1Color and turbidity will interfere if not removed by filtration or dilution.
6.2The only specific substance known to interfere in the color reaction is phosphate.Phosphate interference is elimi-nated by the addition of oxalic acid.
6.3A high dissolved salts concentration,such as in seawater 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.6.4Strong oxidizing and reducing agents that may be found in some industrial waste waters may interfere in the reduction step of the reaction.Such waste waters may also contain organic compounds that may interfere in the color formation.
7.Apparatus
7.1Spectrophotometer or Filter Photometer (see Note 2)—To obtain maximum sensitivity and reproducibility,a spectro-photometer suitable for measurements at 815nm is required.Measurements may be made at 640nm with a spectrophotom-eter,or 640to 700nm with a filter photometer if less sensi
tivity is preferred.Precision and bias information on this test method (see Section 13)is based on data obtained at 815nm.A direct reading spectrophotometer or filter photometer may be used.
N OTE 2—Photometers and photometric practices shall conform to Practice E 60.Spectrophotometers shall conform to Practice E 275.
7.2Sample Cells —The cell size to be used depends on the range covered and the particular instrument used.The higher concentration range should be attainable with 10-mm path
length cells.Longer path length cells (40to 50mm)are recommended for concentrations below 0.1mg/L.8.Reagents and Materials
N OTE 3—Store all reagents to be used in this test method in polyeth-ylene or other suitable plastic bottles.
8.1Purity of Reagents —Reagent grade chemicals shall be used in all tests.Unless otherwise indicated,it is intended that all reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.3Other grades may be used,provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
8.2Purity of Water —Unless otherwise indicated,refer-ences to water shall be understood to mean reagent water conforming to Specification D 1193,Type I.In addition,the water shall be made silica-free by distillation or demineraliza-tion and determined as such in accordance with the method of test being used.The collecting apparatus and storage contain-ers for the reagent water must be polyethylene or other suitable plastic.Type II water was specified at the time of round robin testing of this test method.
8.3Amino-Naphthol-Sulfonic Acid-Solution —Dissolve 0.5g of 1-amino-2-naphthol-4-sulfonic acid in 50mL of a solution containing 1g of sodium sulfite (Na 2SO 3).After dissolving,add the solution to 100mL of a solution containing 30g of sodium hydrogen sulfite (NaHSO 3).Make up to 200mL with water and store in a dark,plastic bottle.Shelf life of this reagent may be extended by refrigeration.Solution should be adjusted to room temperature,2565°C,before use.Discard when the color darkens or a precipitate forms.
8.4Ammonium Molybdate Solution (75g/L)(Note 4)—Dissolve 7.5g of ammonium molybdate ((NH 4)6Mo 7-O 24·4H 2O)in 100mL of water.
N OTE 4—Batch to batch variations in ammonium molybdate have been found to affect results at low concentrations (below 0.1mg/L).High blanks,nonlinear calibration curves,and poor reproducibility have
been observed with some batches of this compound.When working with low concentrations of silica,a batch of ammonium molybdate known to produce reasonable blanks,linearity,and reproducibility should be set aside for this purpose.
8.5Hydrochloric Acid (1+1)—Mix 1volume of concen-trated hydrochloric acid (HCl,sp gr 1.19)with 1volume of water.
8.6Oxalic Acid Solution (100g/L)—Dissolve 10g of oxalic acid (H 2C 2O 4·2H 2O)in 100mL of water.
3
Reagent Chemicals,American Chemical Society Specifications ,American Chemical Society,Washington,DC.For suggestions on the testing of reagents not listed by the American Chemical Society,see Analar Standards for Laboratory Chemicals ,BDH Ltd.,Poole,Dorset,U.K.,and the United States Pharmacopeia and National Formulary ,U.S.Pharmaceutical Convention,Inc.(USPC),Rockville,
MD.
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8.7Silica Solution,Standard (1mL =0.1mg SiO 2)—Dis-solve 0.473g of sodium metasilicate (Na 2SiO 3·9H 2O)in water and dilute to 1L.Check the concentration of this solution gravimetrically.4
N OTE 5—This solution may require filtration to remove fine particulate matter containing silica.This filt
ration,if needed,should precede stan-dardization gravimetrically.4This step was not included as a requirement in the collaborative tests from which precision and bias determined.
9.Sampling
9.1Collect the samples in accordance with Practice D 1066or Practices D 3370,as applicable.
9.2Use plastic or stainless steel sample bottles,provided with rubber or plastic stoppers.
9.3If the water being sampled is at elevated temperature,cool to less than 35°C but do not freeze .
9.4The holding time for the samples may be calculated in accordance with Practice D 4841.
10.Calibration and Standardization
10.1Prepare a series of at least four standards covering the desired concentration range by proper dilution of the standard silica solution (see 8.7).Treat 50.0-mL aliquots of the stan-dards in accordance with 11.1-11.3.Prepare a blank using a 50.0-mL aliquot of water that has been similarly treated.
10.2For standards in the 20to 1000µg/L range,set the spectrophotometer at 815nm and read the absor
bance of each standard against the reagent blank.For standards in the 0.1to 5mg/L range,set the spectrophotometer at 640nm (filter photometer 640to 700nm).
10.3Prepare a calibration curve for measurements at 815nm by plotting absorbance versus micrograms SiO 2per litre on linear graph paper.For measurements at 640nm,plot absor-bance versus milligrams SiO 2per litre.A direct reading spectrophotometer or filter photometer may be used.11.Procedure
11.1Transfer quantitatively 50.0mL (or an aliquot diluted to 50mL)of the sample that has been filtered through a 0.45-µm membrane filter,if necessary,to remove turbidity,to a polyethylene or other suitable plastic container and add,in quick succession,1mL of HCl (1+1)and 2mL of the ammonium molybdate solution.Mix well.
11.2After exactly 5min,add 1.5mL of oxalic acid solution and again mix well.
11.3After 1min,add 2mL of amino-naphthol-sulfonic acid solution.Mix well and allow to stand for 10min.
11.4Prepare a reagent blank by treating a 50.0-mL aliquot of water as directed in 11.1-11.3.
11.5Measure the absorbance of the sample at 815nm against the reagent blank (or at 640nm for highe
r concentra-tions).
12.Calculation
12.1Silica concentration in micrograms SiO 2per litre may be read directly from the calibration curve at 815nm prepared in 10.3.For measurements made at 640nm,silica concentra-tion may be read directly in milligrams SiO 2per litre from the calibration curve prepared in 10.3.
13.Precision and Bias 5
13.1The collaborative test of this test method was per-formed using reagent water by six laboratories,two operators each.Each operator made six determinations at each level,for a total of 72determinations at each level.
13.2Precision —The overall and single-operator precision of this test method for measurements at 815nm in reagent water are shown in Table 1.
13.3Bias —Recoveries of known amounts of silica from reagent water are shown in Table 2.
13.4Precision and bias for this test method conform to Practice D 2777–77,which was in place at the ti
me of collaborative testing.Under the allowances made in 1.4of D 2777–98,these precision and bias data do meet existing requirements for interlaboratory studies of Committee D19test methods.
14.Quality Control (QC)
14.1In order to be certain that analytical values obtained using this test method are valid and accurate within the confidence limits of the test,the following QC procedures must be followed when running the test.
14.2Calibration and Calibration Verification
14.2.1When beginning use of this method,an initial Cali-bration Verification Standard (CVS)should be used to verify the calibration standards and acceptable instrument perfor-mance.This verification should be performed on each analysis day.The CVS is a solution of the method analyte of known concentration (mid-calibration range)used to fortify reagent water.If the determined CVS concentrations are not within 615%of the known value,the analyst should reanalyze the
4
Refer to former Test Method A (Gravimetric—Total Silica)last published in the 1988Annual Book of AST
M Standards for complete description of procedure.
5
Supporting data have been filed at ASTM International Headquarters and may be obtained by requesting Research Report RR:D19–147.
TABLE 1Overall (S T )and Single-Operator (S o )Interlaboratory
Precision for Silica
Mean Concentration (X),
µg/L
S T ,µg/L S o ,µg/L 38.6 3.40.8112.2 5.8 1.0381.810.0 1.7941.8
30.0
5.9
TABLE 2Recovery
Amount Added,µg SiO 2/L
Amount Found,µg SiO 2/L
Bias,%Statistically Significant 95%Level
951941.8−1.0no 381381.8+0.2no 115112.2−2.4no 39
38.6
−1.0
no
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CVS.If the value still falls outside acceptable limits,a new calibration curve is required that must be confirmed by a successful CVS before continuing with on-going analyses.14.2.2One CVS should then be run with each sample batch (maximum of 20samples)to verify the previously established calibration curves.If the determined analyte concentrations fall outside acceptable limits (615%)that analyte is judged out of control,and the source of the problem should be identified before continuing with on-going analyses.
14.3Initial Demonstration of Laboratory Capability
14.3.1The laboratory using this test should perform an initial demonstration of laboratory capability.Analyze seven replicates of an Initial Demonstration of Performance (IDP)solution.The IDP solution contains method analytes of known concentration,prepared from a different source to the calibra-tion standards,used to fortify reagent water.Ideally,the IPD solution should be prepared by an independent source from reference materials.The level 2spiking solution used for the precision and bias study is a suitable IDP solution.
The mean and standard deviation of the seven values should then be calculated and compared,according to Practice D 5847,to the single operator precision and recovery estab-lished for this Test Method.The upper limit for acceptable precision and the range of acceptable recoveries are detailed below:
Analyte IDP Solution Amount Method S 0Acceptable IDP Precision,n =7Silica
115µg/L
1.0µg/L
#1.7µg/L
Analyte Method Mean Recovery Lower Acceptable IDP Recovery
Upper Acceptable IDP Recovery
Silica
112.2µg/L
97.0µg/L
127.4µg/L
14.4Laboratory Control Sample
14.4.1One Laboratory Control Sample (LCS)should be run with each sample batch (maximum of 20samples).The LCS is a solution of method analytes of known concentration added to a matrix that sufficiently challenges the Test Method.A synthetic “water”matrix of relevance to the user (e.g.,drinking water or wastewater)spiked with the method analyte at the level of the IDP solution would be an example of an appropri-ate LCS.
The analyte recoveries for the LCS should fall within the control limits of x 63S,where x is the IDP amount and (S)is the standard deviation of the mean recovery established from
the interlaboratory precision and bias study data at the IDP levels,as shown below:
Analyte LCS Amount Lower Recovery
Limit
Upper Recovery
Limit
Silica
115µg/L
112µg/L
118µg/L
14.5Method Blank
14.5.1A reagent blank should be run when generating the initial calibration curves.A blank should also be run with each sample batch (maximum of 20samples)to check for sample or system contamination.14.6Matrix Spike
14.6.1One Matrix Spike (MS)should be run with each sample batch (maximum of 20samples)to test method recovery.The MS should be prepared in accordance with Guide D 5810.Spike a portion of a water (or other)sample from each batch with the method analytes at the level of the IDP solution.The %recovery of the spike should fall within limits established from the interlaboratory precision and bias study data (assuming a background level of zero),according to Practice D 5847,as shown below:
Analyte MS Amount Lower Recovery
Limit (%)
Upper Recovery
Limit (%)
Silica
115µg/L
82.5%
112.7%
14.7Duplicate
14.7.1One Matrix Duplicate (MD)should be run with each sample batch (maximum of 20samples)to test method precision.If non-detects are expected in all the samples to be analyzed,a Matrix Spike Duplicate should be run instead.The precision of the duplicate analysis should be compared,accord-ing to Practice D 5847,to the nearest tabulated S 0value established from the interlaboratory precision and bias study data for each analyte.
14.8Independent Reference Material
14.8.1In order to verify the quantitative values produced by the test method,an Independent Reference Material (IRM),submitted to the laboratory as a regular sample (if practical),should be analyzed once per quarter.The concentration of the IRM should be within the scope of the method,as defined in 1.4.The values obtained must fall within the limits specified by the outside source.15.Keywords
15.1colorimetric;silica;
water
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APPENDIXreactive substance
(Nonmandatory Information)
X1.RATIONALE FOR DISCONTINUATION OF TEST METHODS
X1.1Test Method A (Gravimetric—Total Silica)
X1.1.1This test method was discontinued in 1988.This test method may be found in its entirety in the 1988Annual Book of ASTM Standards ,V ol 11.01.
X1.1.2The gravimetric procedures covered by Test Method A are applicable to the determination of total silica present in water and waste water.The lower limit of this method is 5mg of silica.Since the method includes an evaporation step,the applicable concentration range depends upon the volume of sample used in the determination.
X1.1.3Silicon compounds dissolved or suspended in the water are concentrated and precipitated as partially dehydrated
silica by evaporation with hydrochloric acid (HCl).Dehydra-tion is completed by ignition,and the silica is volatilized as silicon tetrafluoride.The residue is weighed before and after volatilization as silicon tetrafluoride to obtain the weight of silica in the original sample.Complex silicate residues that do not yield to this treatment are dissolved by alkali fusion and dehydrated with HCl.
X1.1.4This test method was discontinued because there were insufficient laboratories interested in participating in another collaborative study to obtain the necessary precision and bias as required by Practice D 2777.
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