1、Designation: D 7126 06Standard Test Method forOn-Line Colorimetric Measurement of Silica1This standard is issued under the fixed designation D 7126; 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the on-line determination ofsoluble silica in water by colorimetric analysis using themolybdenum blue method, also
3、known as the heteropoly bluemethod.1.2 This test method is applicable for silica determination inwater with silica concentrations within 0.5 - 5000 ppb (g/L).1.3 This test method covers the determination of solublesilica SiO2(silicon dioxide) or silicates in water. Soluble silicacompounds are consid
4、ered molybdate reactive silica. This testmethod does not cover the determination of colloidal orpolymeric silica, which is considered non-molybdate reactivesilica.1.4 This test method does not cover the laboratory or grabsample measurement of silica in water. Refer to Test MethodD 859.1.5 This stand
5、ard does not purport to address all of thesafety concerns, if any, associated 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.2. Referenced Documents2.1 AS
6、TM Standards:2D 859 Standard Test Method for Silica in WaterD 1066 Standard Practice for Sampling SteamD 1129 Terminology Relating to WaterD 1192 Specification for Equipment for Sampling Waterand Steam in Closed ConduitsD 1193 Specification for Reagent WaterD 2777 Practice for Determination of preci
7、sion and Bias ofApplicable Methods of Committee D-19 on WaterD 3370 Standard Practices for Sampling Water from ClosedConduitsD 3864 Guide for Continual On-Line Monitoring systemsfor Water AnalysisD 5540 Practice for Flow Control and Temperature Controlfor On-Line Water Sampling and Analysis3. Termin
8、ology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D 1129 and Practice D 3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 heteropoly compounda compound in which groupsof different elements are joined together by metal-metal bonds.3.2.2 metalloidan
9、 element which has properties that areintermediate between those of a metal and a nonmetal.3.2.3 photodetectora device for detecting and measuringthe intensity of radiant energy.4. Summary of Test Method4.1 This test method describes the analysis of soluble silicaby analyzing a sample from a continu
10、ous stream. This testmethod is based on the colorimetric determination of solublesilica by the formation and reduction of molybdosilicic acid.Reduced molybdosilicic acid forms a molybdenum blue com-plex. The optical absorbance of this complex is typicallymeasured at 815 6 10 nm. The absorbance is di
11、rectly propor-tional to the concentration of silica in the sample.4.2 This on-line test method requires reagents which areadded sequentially with separate reaction periods. Each reac-tion must be allowed to go to completion before the nextreagent is added.5. Significance and Use5.1 Silicon (Si), a m
12、etalloid, is the second most abundantelement in the earths crust. Various forms of silica (silicondioxide SiO2) are found in quartz, sand and rocks. Thedegradation of these rocks results in silica found in naturalwaters. Silica in natural waters can be found as ionic silica,silicates, colloidal or s
13、uspended particles.5.2 Elevated temperatures and pressure can cause silica inwater to vaporize and form deposits or scale. Scale deposits ofsilica will coat boilers and turbine blades used in power plants.The presence of silica scale affects the ability of metals totransfer heat. Silica needs to be
14、removed when deionized wateris used as a rinse for manufacturing wafers in the semiconduc-tor industry.1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling of Water andWater-Formed Deposits, Analysis of Water fo
15、r Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved March 1, 2006. Published March 2006.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards
16、 volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5.3 Silica is commonly removed by demineralization usinganion exchange resins, distillation, reverse
17、 osmosis or precipi-tation in a lime softening process. The on-line measurement ofsilica is the preferred method to laboratory analyses forindustries trying to obtain and monitor ultra-pure water. Sincesilica is one of the first species to breakthrough anion exchangeresins, on-line silica monitoring
18、 is frequently used to determinethe need for regeneration of an anion or mixed resin bed.6. Interferences6.1 Colored and turbid samples and reagents may interferein the colorimetric detection of silica.At low levels (ppb range)samples can be filtered to eliminate particles and suspendedsolids in sol
19、utions, however filtration is not recommended,because of the potential to introduce silica contamination. Ablank (zero) absorption can be performed prior to the finalabsorption measurement to reduce interferences from turbidityand color.6.2 Phosphate interferes in the colorimetric detection ofsilica
20、 by reacting with the molybdate compound used in thefirst reagent to form molybdophosphoric acid. Molybdophos-phoric acid can be destroyed by the addition of tartaric, citricor oxalic acid after the molybdosilicic acid reaction hascompleted.6.3 Sample temperature is a potential interferant in thedet
21、ermination of silica by this method. If the on-line instrumentallows sufficient time for the sample and the reagents toequilibrate to a constant temperature, sample temperature willnot interfere. At controlled, higher temperatures, the reactionsequence time period (typically 10-15 min) can be decrea
22、sed to5-10 min.6.4 Glassware (which constains silica) will interfere andcause erroneously high results. Therefore, plastic or othernon-silica containing materials should be used for reagents,calibration standards and instrument components.6.5 If a batch style instrument will be used for on-lineanaly
23、sis, adequate rinsing in-between samples is necessary toavoid silica contamination from previous samples.7. Apparatus7.1 Process instrument7.1.1 A spectrophotometer or colorimeter capable of accu-rately measuring absorbance values at 815 6 10 nm isnecessary. The instrument should be capable of analy
24、zing asample from an on-line process water stream by addition of thenecessary reagents for the heteropoly blue method. The instru-ment should also be capable of adding a calibration standardautomatically for recommended calibration of the on-lineinstrument. The instrument may provide alarms, relays,
25、 iso-lated analog and digital outputs.8. Reagents8.1 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean reagent water that meetsthe purity specifications of Type I or Type II water, presentedin D 1193.8.2 Silica Solution, StandardTo prepare a silica standardsol
26、ution, dissolve ACS reagent grade or better sodium meta-silicate (Na2SiO39H2O) and dilute with deionized water tovolume, or use commercially prepared standards. Refer to theinstrument manufacturers manual for the correct standardconcentration to use. Standards and samples should not befrozen, which
27、can result in lower soluble silica values. (1)Standards should be prepared at room temperature and storedat 4C to maintain maximum stability.8.3 Silica ReagentsThe colorimetric analysis described inthis method requires at least 3 reagents for the on-linedetermination of silica. The reagents necessar
28、y for this methodare specific for the on-line instrument, colorimeter or spectro-photometer used. Refer to the instrument manufacturers rec-ommended reagents. It is essential that the reagents used arematched to the analyzer. Either use reagents provided orprescribed by the manufacturer or validate
29、the results obtainedfrom any other reagent formulation. This validation mustinclude validation of the calibration data.NOTE 1All reagents and calibration standards used in this test methodshould be stored in polyethylene, plastic or other non-silica containingbottles.NOTE 2The first reagent used is
30、an acidified molybdate reagent whichreacts with silica in the sample to form molybdosilicic acid. If phosphateis present in the sample, molybdophosphoric acid is also formed. The firstreaction requires time to ensure all of the silica in the sample has time toreact with the acidified molybdate reage
31、nt. The reaction time depends onthe concentration of the reagent and sample temperature. The user shoulduse the reaction time and temperature suggested by the reagent and/orinstrument manufacturer.The second reagent destroys the molybdophosphoric acid formed in thefirst step. Phosphate interferes in
32、 the measurement of silica because thereduced product absorbs at the same wavelength as that of the reductionproduct of molybdosilicic acid.The third reagent reduces the molybdosilicic acid formed in the firststep. The reduced molybdenum complex forms the heteropoly bluecompound and absorption of th
33、is molybdenum (heteropoly) blue complexis measured at 815 6 10 nm.9. Sampling9.1 For sampling, refer to standards Practice D 1066, Speci-fication D 1192, Practices D 3370, and Practice D 5540,asapplicable.10. Calibration10.1 The instrument should be calibrated using a silicastandard solution prepare
34、d as described in section 8.2 or byusing commercial standards. The standard silica solutionshould be used as a calibration standard according to theinstrument manufacturers instructions for calibration. If usingvalidated reagents other than those provided or prescribed bythe instrument manufacturer
35、be sure to regenerate the calibra-tion curve with these reagents.11. Procedure11.1 The instrument should be calibrated according tosection 10.1.11.2 Prepare and connect the reagents according to manu-facturers instructions. Determine required sample flow asspecified by the instrument manufacturer an
36、d measure flow rateif necessary. Switch the analyzer to automatic analysis modefor continuous on-line silica measurement.D712606211.3 Sample should be collected from a flowing stream anddiverted into or through a reaction cell/vessel. Refer to Fig. 1.The volume of sample inside the reaction cell sho
37、uld encom-pass the light path from the emitting light source. Each reagentused in the analysis should be added into the reaction cell by apump or liquid metering device. The appropriate reaction timerequired after each reagent addition should be allowed for bythe instrument.11.4 The light source fro
38、m the on-line instrument shouldtravel through the sample to a photodetector. In some instru-ments, the light is directed through the sample to a reflector.The reflected light then travels back through the sample to thephotodetector to be measured at 815 6 10 nm. Some instru-ments also take a second
39、reference wavelength measurement ata wavelength out side the absorbtion band for silica (approxi-mately 450 nm) An analog signal from the photodetector ispassed to a microprocessor in the electronics assembly to becalculated as absorbance.12. Report12.1 Report readings to the nearest 0.1 ppb (g/L) S
40、ilica.13. Quality Control13.1 Instrument calibration must be performed according tothe manufacturers schedule and instructions.13.2 Instrument calibration and blank readings must bechecked whenever reagents or components in the optical pathare changed. A calibration verification should be carried ou
41、t ona sample of known silica concentration at a concentrationdifferent than that used for the instrument calibration.13.3 The user should confirm that the unit is giving properresponse using the sample matrix and operating under theenvironmental extremes of interest.14. Precision and Bias14.1 Neithe
42、r precision nor bias data can be obtained for thistest method from a collaborative study designed in accordancewith the requirements of Practice D 2777 since this test methodis a continuous determination. This inability to obtain precisionand bias data for continuous determinations is recognized and
43、stated in the scope of Practice D 2777.14.2 The precision and bias of this method is expected to besimilar to or better than the static method for silica determina-tion (Practice D 859) which uses similar chemistries. Theon-line method is expected to improve the precision byminimizing contamination,
44、 operator involvement, and expo-sure to air, which can contain silica.15. Keywords15.1 colorimetric; on-line; process measurement; silicaREFERENCES(1) Clescari, L., Greenberg, A., Eaton, A. (Eds). (1998). Pp. 4-117-4-123.Standard Methods for the Examination of Water and Wastewater,20thEdition. Washi
45、ngton, D.C.: American Public Health Association.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights,
46、 and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for rev
47、ision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you
48、 shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).FIG. 1 Fluidics System of an On-Line Silica InstrumentD7126063
