1、Designation: D7126 15Standard Test Method forOn-Line Colorimetric Measurement of Silica1This standard is issued under the fixed designation D7126; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number
2、in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 kno
3、wn as the heteropoly bluemethod.1.2 This test method is applicable for silica determination inwater with silica concentrations within 0.55000 ppb (g/L).1.3 This test method covers the determination of solublesilica SiO2(silicon dioxide) or silicates in water. Soluble silicacompounds are considered m
4、olybdate 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 MethodD859.1.5 The values stated
5、 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 theresponsibility of the user of this standard to establish appro-priate safety and h
6、ealth practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D859 Test Method for Silica in WaterD1066 Practice for Sampling SteamD1129 Terminology Relating to WaterD1193 Specification for Reagent WaterD2777 Practice for Determina
7、tion of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD3864 Guide for On-Line Monitoring Systems for WaterAnalysisD5540 Practice for Flow Control and Temperature Controlfor On-Line Water Sampling and Analysis3. Terminolog
8、y3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D1129 and Practice D3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 heteropoly compound, na compound in whichgroups of different elements are joined together by metal-metalbonds.3.2.2 metalloid, nan
9、element which has properties that areintermediate between those of a metal and a nonmetal.3.2.3 photodetector, na device for detecting and measur-ing the intensity of radiant energy.4. Summary of Test Method4.1 This test method describes the analysis of soluble silicaby analyzing a sample from a con
10、tinuous 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 i
11、s directly 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),
12、 a metalloid, 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
13、 or suspended particles.5.2 Elevated temperatures and pressure can cause silica inwater to vaporize and form deposits or scale. Scale deposits of1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling Water andWate
14、r-Formed Deposits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of WaterCurrent edition approved July 15, 2015. Published August 2015. Originallyapproved in 2006. Last previous edition approved in 2010 as D7126 10. DOI:10.1520/D7126-15.2For referenc
15、ed 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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Con
16、shohocken, PA 19428-2959. United States1silica 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 removed when deionized wateris used as a rinse for manufacturing wafers in the semiconduc-tor indust
17、ry.5.3 Silica is commonly removed by demineralization usinganion exchange resins, distillation, reverse 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.
18、Sincesilica is one of the first species to breakthrough anion exchangeresins, on-line silica monitoring 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 si
19、lica.At low levels (ppb range)samples can be filtered to eliminate particles and suspendedsolids in solutions, 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 reduc
20、e interferences from turbidityand color.6.2 Phosphate interferes in the colorimetric detection ofsilica 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 th
21、e molybdosilicic acid reaction hascompleted.6.3 Sample temperature is a potential interferant in thedetermination 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. A
22、t controlled, higher temperatures, the reactionsequence time period (typically 1015 min) can be decreasedto 510 min.6.4 Glassware (which contains silica) will interfere andcause erroneously high results. Therefore, containers of plasticor other non-silica containing materials should be used forreage
23、nts, calibration standards, and instrument components.6.5 If a batch style instrument will be used for on-lineanalysis, adequate rinsing in-between samples is necessary toavoid silica contamination from previous samples.7. Apparatus7.1 Process Instrument:7.1.1 A spectrophotometer or colorimeter capa
24、ble of accu-rately measuring absorbance values at 815 6 10 nm isnecessary. The instrument should be capable of analyzing 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 st
25、andardautomatically for recommended calibration of the on-lineinstrument. The instrument may provide alarms, relays, iso-lated analog, and digital outputs.8. Reagents8.1 Purity of WaterReferences to water that is used forreagent preparation, rinsing or dilution shall be understood tomean water that
26、conforms to the quantitative specifications ofType II reagent water of Specification D1193.8.2 Silica Solution, StandardTo prepare a silica standardsolution, dissolve ACS reagent grade or better sodium meta-silicate (Na2SiO39H2O) and dilute with deionized water tovolume, or use commercially prepared
27、 standards. Refer to theinstrument manufacturers manual for the correct standardconcentration to use. Standards and samples should not befrozen, which can result in lower soluble silica values.3Standards should be prepared at room temperature and storedat 4C to maintain maximum stability.8.3 Silica
28、ReagentsThe colorimetric analysis described inthis method requires at least 3 reagents for the on-linedetermination of silica. The reagents necessary for this methodare specific for the on-line instrument, colorimeter or spectro-photometer used. Refer to the instrument manufacturers rec-ommended rea
29、gents. It is essential that the reagents used arematched to the analyzer. Either use reagents provided orprescribed by the manufacturer or validate the results obtainedfrom any other reagent formulation. This validation mustinclude validation of the calibration data.NOTE 1All reagents and calibratio
30、n standards used in this test methodshould be stored in polyethylene, plastic or other non-silica containingbottles.NOTE 2The first reagent used is an acidified molybdate reagent whichreacts with silica in the sample to form molybdosilicic acid. If phosphateis present in the sample, molybdophosphori
31、c acid is also formed. The firstreaction requires time to ensure all of the silica in the sample has time toreact with the acidified molybdate reagent. The reaction time depends onthe concentration of the reagent and sample temperature. The user shoulduse the reaction time and temperature suggested
32、by the reagent orinstrument manufacturer, or both.The second reagent destroys the molybdophosphoric acid formed in thefirst step. Phosphate interferes in the measurement of silica because thereduced product absorbs at the same wavelength as that of the reductionproduct of molybdosilicic acid.The thi
33、rd reagent reduces the molybdosilicic acid formed in the firststep. The reduced molybdenum complex forms the heteropoly bluecompound and absorption of this molybdenum (heteropoly) blue complexis measured at 815 6 10 nm.9. Sampling9.1 For sampling, refer to Practices D1066, D3370, andD5540, as applic
34、able.10. Calibration10.1 The instrument should be calibrated using a silicastandard solution prepared as described in 8.2 or by usingcommercial standards. The standard silica solution should beused as a calibration standard according to the instrumentmanufacturers instructions for calibration. If us
35、ing validatedreagents other than those provided or prescribed by the3Clescari, L., Greenberg, A., Eaton, A., and Rice, E., (Eds), Standard Methodsfor the Examination of Water and Wastewater, 21st Edition, Washington, D.C.:American Public Health Association, 2005, pp. 4-1174-123.D7126 152instrument m
36、anufacturer be sure to regenerate the calibrationcurve with these reagents.11. Procedure11.1 The instrument should be calibrated according to 10.1.11.2 Prepare and connect the reagents according to manu-facturers instructions. Determine required sample flow asspecified by the instrument manufacturer
37、 and measure flow rateif necessary. Switch the analyzer to automatic analysis modefor continuous on-line silica measurement.11.3 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 should e
38、ncom-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 from the
39、 on-line instrument shouldtravel through the sample to a photodetector. In someinstruments, the light is directed through the sample to areflector. The reflected light then travels back through thesample to the photodetector to be measured at 815 6 10 nm.Some instruments also take a second reference
40、 wavelengthmeasurement at a wavelength out side the absorption band forsilica (approximately 450 nm) An analog signal from thephotodetector is passed to a microprocessor in the electronicsassembly to be calculated as absorbance.12. Report12.1 Report readings to the nearest 0.1 ppb (g/L) Silica.13. Q
41、uality 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 out ona sampl
42、e 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 Neither precision
43、 nor bias data can be obtained for thistest method from a collaborative study designed in accordancewith the requirements of Practice D2777 since this test methodis a continuous determination. This inability to obtain precisionand bias data for continuous determinations is recognized andstated in th
44、e scope of Practice D2777.14.2 The precision and bias of this method is expected to besimilar to or better than the static method for silica determina-tion (Practice D859), which uses similar chemistries. Theon-line method is expected to improve the precision byminimizing contamination, operator inv
45、olvement, and expo-sure to air, which can contain silica.15. Keywords15.1 colorimetric; on-line; process measurement; silicaASTM 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 exp
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