1、Designation: D859 10An American National StandarddoStandard Test Method forSilica in Water1This standard is issued under the fixed designation D859; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A numbe
2、r in parentheses indicates the year of last reapproval. Asuperscript 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*1.1 This test method covers the determination of silica inwate
3、r and waste water; however, the analyst should recognizethat the precision and accuracy statements for reagent watersolutions may not apply to waters of different matrices.1.2 This test method is a colorimetric method that deter-mines molybdate-reactive silica. It is applicable to most waters,but so
4、me waters may require filtration and dilution to removeinterferences from color and turbidity. This test method isuseful for concentrations as low as 20 g/L.1.3 This test method covers the photometric determinationof molybdate-reactive silica in water. Due to the complexity ofsilica chemistry, the f
5、orm of silica measured is defined by theanalytical method as molybdate-reactive silica. Those forms ofsilica that are molybdate-reactive include dissolved simplesilicates, monomeric silica and silicic acid, and an undeter-mined fraction of polymeric silica.1.4 The useful range of this test method is
6、 from 20 to 1000g/L at the higher wavelength (815 nm) and 0.1 to 5 mg/L atthe lower wavelength (640 nm). It is particularly applicable totreated industrial waters. It may be applied to natural watersand wastewaters following filtration or dilution, or both. Forseawater or brines, this test method is
7、 applicable only ifmatched matrix standards or standard addition techniques areemployed.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associa
8、ted with its use. It is theresponsibility 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.NOTE 1For many natural waters, a measurement of molybdate-reactive silica by this test method provides
9、a close approximation of totalsilica, and, in practice, the colorimetric method is frequently substitutedfor other more time-consuming techniques. This is acceptable when, asfrequently occurs, the molybdate-reactive silica is in the milligram perlitre concentration range while the nonmolybdate-react
10、ive silica, if presentat all, is in the microgram per litre concentration range.1.7 Former Test Method A (GravimetricTotal Silica) wasdiscontinued. Refer to Appendix X1 for historical information.2. Referenced Documents2.1 ASTM Standards:2D1066 Practice for Sampling SteamD1129 Terminology Relating t
11、o WaterD1193 Specification for Reagent WaterD2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD4841 Practice for Estimation of Holding Time for WaterSamples Containing Organic and Inorganic
12、 ConstituentsD5810 Guide for Spiking into Aqueous SamplesD5847 Practice for Writing Quality Control Specificationsfor Standard Test Methods for Water AnalysisE60 Practice for Analysis of Metals, Ores, and RelatedMaterials by Molecular Absorption SpectrometryE275 Practice for Describing and Measuring
13、 Performanceof Ultraviolet and Visible Spectrophotometers3. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D1129.4. Summary of Test Method4.1 This test method is based on the reaction of the solublesilica with molybdate ion to form a greenish-yellow
14、complex,which in turn is converted to a blue complex by reduction with1-amino-2-naphthol-1-sulfonic acid.5. Significance and Use5.1 Silicon comprises about 28 % of the lithosphere and is,next to oxygen, the most abundant element. It is found as theoxide in crystalline forms, as in quartz; combined w
15、ith other1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.05 on Inorganic Constituentsin Water.Current edition approved June 15, 2010. Published July 2010. Originallyapproved in 1945. Last previous edition approved in 2005
16、 as D859 05. DOI:10.1520/D0859-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes sect
17、ion appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.oxides and metals in a variety of silicates; and in amorphousforms. Silicon is the most abundant element in igneous rocksand is the characteristic
18、 element of all important rocks exceptthe carbonates. It is the skeletal material of diatoms but is notknown to play a significant role in the structure of processes ofhigher life forms.5.2 Silica is only slightly soluble in water. The presence ofmost silica in natural waters comes from the gradual
19、degrada-tion of silica-containing minerals. The type and composition ofthe silica-containing minerals in contact with the water and thepH of the water are the primary factors controlling both thesolubility and the form of silica in the resulting solution. Silicamay exist in suspended particles, as a
20、 colloid, or in solution. Itmay be monomeric or polymeric. In solution it can exist assilicic acid or silicate ion, depending upon pH. The silicacontent of natural waters is commonly in the 5 to 25 mg/Lrange, although concentrations over 100 mg/L occur in someareas.5.3 Silica concentration is an imp
21、ortant consideration insome industrial installations such as steam generation andcooling water systems. Under certain conditions, silica formstroublesome silica and silicate scales, particularly on high-pressure steam turbine blades. In cooling water systems, silicaforms deposits when solubility lim
22、its are exceeded. In contrast,silica may be added as a treatment chemical in some systems,for example, in corrosion control. Silica removal is commonlyaccomplished by ion exchange, distillation, reverse osmosis, orby precipitation, usually with magnesium compounds in a hotor cold lime softening proc
23、ess.6. Interferences6.1 Color and turbidity will interfere if not removed byfiltration or dilution.6.2 The only specific substance known to interfere in thecolor reaction is phosphate. Phosphate interference is elimi-nated by the addition of oxalic acid.6.3 Ahigh dissolved salts concentration, such
24、as in seawateror brine samples, can affect color development. This can becompensated for by preparing standards in a matrix similar tothat of samples or by using a standard additions technique.6.4 Strong oxidizing and reducing agents that may be foundin some industrial waste waters may interfere in
25、the reductionstep of the reaction. Such waste waters may also containorganic compounds that may interfere in the color formation.7. Apparatus7.1 Spectrophotometer or Filter Photometer (see Note 2)To obtain maximum sensitivity and reproducibility, a spectro-photometer suitable for measurements at 815
26、 nm is required.Measurements may be made at 640 nm with a spectrophotom-eter, or 640 to 700 nm with a filter photometer if less sensitivityis preferred. Precision and bias information on this test method(see Section 13) is based on data obtained at 815 nm. A directreading spectrophotometer or filter
27、 photometer may be used.NOTE 2Photometers and photometric practices shall conform toPractice E60. Spectrophotometers shall conform to Practice E275.7.2 Sample Cells The cell size to be used depends on therange covered and the particular instrument used. The higherconcentration range should be attain
28、able with 10-mm pathlength cells. Longer path length cells (40 to 50 mm) arerecommended for concentrations below 0.1 mg/L.8. Reagents and MaterialsNOTE 3Store all reagents to be used in this test method in polyeth-ylene or other suitable plastic bottles.8.1 Purity of ReagentsReagent grade chemicals
29、shall beused in all tests. Unless otherwise indicated, it is intended thatall 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 beused, provided it is first ascertained that
30、 the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the determination.8.2 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean reagent water conformingto Specification D1193, Type I. In addition, the water shall bemade si
31、lica-free by distillation or demineralization and deter-mined as such in accordance with the method of test beingused. The collecting apparatus and storage containers for thereagent water must be polyethylene or other suitable plastic.Type II water was specified at the time of round robin testingof
32、this test method.8.3 Amino-Naphthol-Sulfonic Acid-SolutionDissolve 0.5g of 1-amino-2-naphthol-4-sulfonic acid in 50 mL of a solutioncontaining 1 g of sodium sulfite (Na2SO3). After dissolving,add the solution to 100 mL of a solution containing 30 g ofsodium hydrogen sulfite (NaHSO3). Make up to 200
33、mL withwater and store in a dark, plastic bottle. Shelf life of thisreagent may be extended by refrigeration. Solution should beadjusted to room temperature, 25 6 5 C, before use. Discardwhen the color darkens or a precipitate forms.8.4 Ammonium Molybdate Solution (75 g/L) (Note 4)Dissolve 7.5 g of
34、ammonium molybdate (NH4)6Mo7O244H2O) in 100 mL of water.NOTE 4Batch to batch variations in ammonium molybdate have beenfound to affect results at low concentrations (below 0.1 mg/L). Highblanks, nonlinear calibration curves, and poor reproducibility have beenobserved with some batches of this compou
35、nd. When working with lowconcentrations of silica, a batch of ammonium molybdate known toproduce reasonable blanks, linearity, and reproducibility should be setaside for this purpose.8.5 Hydrochloric Acid (1 + 1)Mix 1 volume of concen-trated hydrochloric acid (HCl, sp gr 1.19) with 1 volume ofwater.
36、8.6 Oxalic Acid Solution (100 g/L)Dissolve 10 g of oxalicacid (H2C2O42H2O) in 100 mL of water.8.7 Silica Solution, Standard (1 mL = 0.1 mg SiO2) Dis-solve 0.473 g of sodium metasilicate (Na2SiO39H2O) in waterand dilute to 1 L. Check the concentration of this solution3Reagent Chemicals, American Chem
37、ical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. P
38、harmaceutical Convention, Inc. (USPC), Rockville,MD.D859 102gravimetrically.4Alternatively, certified silica stock solutionsare commercially available through chemical supply vendorsand may be used.NOTE 5This solution may require filtration to remove fine particulatematter containing silica. This fi
39、ltration, if needed, should precede stan-dardization gravimetrically.4This step was not included as a requirementin the collaborative tests from which precision and bias determined.9. Sampling9.1 Collect the samples in accordance with Practice D1066or Practices D3370, as applicable.9.2 Use plastic o
40、r stainless steel sample bottles, providedwith rubber or plastic stoppers.9.3 If the water being sampled is at elevated temperature,cool to less than 35C but do not freeze.9.4 The holding time for the samples may be calculated inaccordance with Practice D4841.10. Calibration and Standardization10.1
41、Prepare a series of at least four standards covering thedesired concentration range by proper dilution of the standardsilica solution (see 8.7). Treat 50.0-mL aliquots of the stan-dards in accordance with 11.1-11.3. Prepare a blank using a50.0-mL aliquot of water that has been similarly treated.10.2
42、 For standards in the 20 to 1000 g/L range, set thespectrophotometer at 815 nm and read the absorbance of eachstandard against the reagent blank. For standards in the 0.1 to5 mg/L range, set the spectrophotometer at 640 nm (filterphotometer 640 to 700 nm).10.3 Prepare a calibration curve for measure
43、ments at 815nm by plotting absorbance versus micrograms SiO2per litre onlinear graph paper. For measurements at 640 nm, plot absor-bance versus milligrams SiO2per litre. A direct readingspectrophotometer or filter photometer may be used.11. Procedure11.1 Transfer quantitatively 50.0 mL (or an aliquo
44、t dilutedto 50 mL) of the sample that has been filtered through a0.45-m membrane filter, if necessary, to remove turbidity, toa polyethylene or other suitable plastic container and add, inquick succession, 1 mL of HCl (1 + 1) and 2 mL of theammonium molybdate solution. Mix well.11.2 After exactly 5
45、min, add 1.5 mL of oxalic acid solutionand again mix well.11.3 After 1 min, add 2 mL of amino-naphthol-sulfonic acidsolution. Mix well and allow to stand for 10 min.11.4 Prepare a reagent blank by treating a 50.0-mL aliquotof water as directed in 11.1-11.3.11.5 Measure the absorbance of the sample a
46、t 815 nmagainst the reagent blank (or at 640 nm for higher concentra-tions).12. Calculation12.1 Silica concentration in micrograms SiO2per litre maybe read directly from the calibration curve at 815 nm preparedin 10.3. For measurements made at 640 nm, silica concentra-tion may be read directly in mi
47、lligrams SiO2per litre from thecalibration curve prepared in 10.3. A direct reading spectro-photometer or filter photometer may be used.13. Precision and Bias513.1 The collaborative test of this test method was per-formed using reagent water by six laboratories, two operatorseach. Each operator made
48、 six determinations at each level, fora total of 72 determinations at each level.13.2 PrecisionThe overall and single-operator precisionof this test method for measurements at 815 nm in reagentwater are shown in Table 1.13.3 BiasRecoveries of known amounts of silica fromreagent water are shown in Ta
49、ble 2.13.4 Precision and bias for this test method conform toPractice D2777 77, which was in place at the time ofcollaborative testing. Under the allowances made in 1.4 ofD2777 08, these precision and bias data do meet existingrequirements for interlaboratory studies of Committee D19 testmethods.14. Quality Control (QC)14.1 In order to be certain that analytical values obtainedusing this test method are valid and accurate within theconfidence limits of the test, the following QC procedures mustbe followed when running the test.14.2 Calibration and Cal
