1、Designation: D859 16Standard 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 number in parentheses indicates the
2、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 U.S. Department of Defense.1. Scope*1.1 This test method covers the determination of silica inwater and waste water; however
3、, 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 some waters may require filt
4、ration 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 form of silica measured is
5、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 from 20 to 1000g/L at the
6、 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 applicable only ifmatched
7、 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, associated with its use. It is th
8、eresponsibility 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 a close approximation of t
9、otalsilica, 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-reactive silica, if presentat a
10、ll, 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 to WaterD1193 Specification
11、 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 ConstituentsD5810 Guide f
12、or 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 SpectrophotometryE275 Practice for Describing and Measuring Performance ofUltraviolet and Visible Spe
13、ctrophotometers3. Terminology3.1 Definitions:3.1.1 For definitions of terms used in this standard, refer toTerminology 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 complex,which in turn is converted to
14、a blue complex by reduction with1-amino-2-naphthol-1-sulfonic acid.1This 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, 2016. Published June 2016. Original
15、lyapproved in 1945. Last previous edition approved in 2010 as D859 10. DOI:10.1520/D0859-16.2For 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 S
16、ummary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15. Significance and Use5.1 Silicon comprises about 28 % of the lithosphere and is,next to
17、oxygen, the most abundant element. It is found as theoxide in crystalline forms, as in quartz; combined with otheroxides and metals in a variety of silicates; and in amorphousforms. Silicon is the most abundant element in igneous rocksand is the characteristic element of all important rocks exceptth
18、e 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 degrada-tion of silica-containing minera
19、ls. 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 colloid, or in solution. Itmay be monom
20、eric 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 important consideration insome industrial i
21、nstallations 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 limits are exceeded. In contrast,silica may
22、 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 process.6. Interferences6.1 Color and turbid
23、ity 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 as in seawateror brine samples, can affe
24、ct 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 the reductionstep of the reaction. Such
25、waste waters may also containorganic compounds that may interfere in the color formation.7. Apparatus7.1 Spectrophotometer or Filter Photometer (see Note2)To obtain maximum sensitivity and reproducibility, aspectrophotometer suitable for measurements at 815 nm isrequired. Measurements may be made at
26、 640 nm with aspectrophotometer, or 640 to 700 nm with a filter photometerif less sensitivity is preferred. Precision and bias informationon this test method (see Section 13) is based on data obtainedat 815 nm. A direct reading spectrophotometer or filter pho-tometer may be used.NOTE 2Photometers an
27、d photometric practices shall conform toPractice E60. Spectrophotometers shall conform to Practice E275.7.2 Sample CellsThe cell size to be used depends on therange covered and the particular instrument used. The higherconcentration range should be attainable with 10-mm pathlength cells. Longer path
28、 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 shall beused in all tests. Unless otherwise i
29、ndicated, 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 the reagent is ofsufficiently high purity to
30、 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 silica-free by distillation or demineralization
31、 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 this test method.8.3 Amino-Naphthol-Sulfonic
32、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 mL withwater and store in a dark, plastic bot
33、tle. 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 ammonium molybdate (NH4)6Mo7O244H2O) in 100 m
34、L 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 compound. When working with lowconcentrations of si
35、lica, a batch of ammonium molybdate known toproduce reasonable blanks, linearity, and reproducibility should be setaside for this purpose.3Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by th
36、e American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,MD.D859 1628.5 Hydrochloric Acid (1+1)Mix 1 volume of concen-trated hydrochloric
37、acid (HCl, sp gr 1.19) with 1 volume ofwater.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)Dissolve 0.473 g of sodium metasilicate (Na2SiO39H2O) inwater and dilute to 1 L. Check the concentration of thi
38、ssolution gravimetrically.4Alternatively, certified silica stocksolutions of appropriate known purity are commercially avail-able through chemical supply vendors and may be used.NOTE 5This solution may require filtration to remove fine particulatematter containing silica. This filtration, if needed,
39、 should precede stan-dardization gravimetrically.4This step was not included as a requirementin the collaborative tests from which precision and bias determined.8.8 Filter PaperPurchase suitable filter paper. Typicallythe filter papers have a pore size of 0.45-m membrane.Material such as fine-textur
40、ed, acid-washed, ashless paper, orglass fiber paper are acceptable. The user must first ascertainthat the filter paper is of sufficient purity to use withoutadversely affecting the bias and precision of the test method.9. Sampling9.1 Collect the samples in accordance with Practice D1066or Practices
41、D3370, as applicable.9.2 Use plastic or 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
42、. Calibration and Standardization10.1 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 wate
43、r that has been similarly treated.10.2 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 R
44、ead directly in concentration if this capability isprovided with the spectrophotometer or filter photometerinstrument or prepare a calibration curve for measurements at815 nm by plotting absorbance versus micrograms SiO2perlitre on linear graph paper. For measurements at 640 nm, plotabsorbance versu
45、s milligrams SiO2per litre.11. Procedure11.1 Transfer quantitatively 50.0 mL (or an aliquot dilutedto 50 mL) of the sample that has been filtered through a0.45-m membrane filter (8.8), if necessary, to removeturbidity, to a polyethylene or other suitable plastic containerand add, in quick succession
46、, 1 mL of HCl (1 + 1) and 2 mL ofthe ammonium molybdate solution. Mix well.11.2 After exactly 5 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 treatin
47、g a 50.0-mL aliquotof water as directed in 11.1 11.3.11.5 Measure the absorbance of the sample at 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 pr
48、eparedin 10.3. For measurements made at 640 nm, silica concentra-tion may be read directly in milligrams 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 meth
49、od was per-formed using reagent water by six laboratories, two operatorseach. Each operator made 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 Table 2.4Refer to former Test MethodA(GravimetricTotal Silica) last published in the1988 Annual Book of ASTM Standards for complete description of procedure.5Sup
