1、Designation:D712606 Designation: D7126 10Standard 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
2、 revision. A number 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 of soluble silica in water by colorimetric analysis using the molybdenum
3、blue method, also known as the heteropoly blue method.1.2 This test method is applicable for silica determination in water with silica concentrations within 0.5 - 5000 ppb (g/L).1.3 This test method covers the determination of soluble silica SiO2(silicon dioxide) or silicates in water. Soluble silic
4、acompounds are considered molybdate reactive silica. This test method does not cover the determination of colloidal or polymericsilica, which is considered non-molybdate reactive silica.1.4 This test method does not cover the laboratory or grab sample measurement of silica in water. Refer to Test Me
5、thod D859.1.51.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard t
6、o establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D859 Test Method for Silica in WaterD1066 Practice for Sampling SteamD1129 Terminology Relating to Water D1192Guide for Equipment for S
7、ampling Water and Steam in Closed ConduitsD1193 Specification for Reagent WaterD2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD3864 Guide for Continual On-Line Monitoring Systems for Wa
8、ter AnalysisD5540 Practice for Flow Control and Temperature Control for On-Line Water Sampling and Analysis3. Terminology3.1 Definitions For definitions of terms used in this test method, refer to Terminology D1129 and Practice D3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 heteropol
9、y compoundheteropoly compound, na compound in which groups of different elements are joined together bymetal-metal bonds.3.2.2 metalloidmetalloid, nan element which has properties that are intermediate between those of a metal and a nonmetal.3.2.3 photodetectorphotodetector, na device for detecting
10、and measuring the intensity of radiant energy.4. Summary of Test Method4.1 This test method describes the analysis of soluble silica by analyzing a sample from a continuous stream. This test methodis based on the colorimetric determination of soluble silica by the formation and reduction of molybdos
11、ilicic acid. Reducedmolybdosilicic acid forms a molybdenum blue complex. The optical absorbance of this complex is typically measured at 815 610 nm. The absorbance is directly proportional to the concentration of silica in the sample.4.2 This on-line test method requires reagents which are added seq
12、uentially with separate reaction periods. Each reaction mustbe allowed to go to completion before the next reagent is added.1This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.03 on Sampling of Water andWater-Formed Deposits
13、, Analysis of Water for Power Generation and Process Use, On-Line Water Analysis, and Surveillance of Water.WaterCurrent edition approved MarchDec. 1, 2006.2010. Published March 2011. Originally approved in 2006 as D712606. DOI: 10.1520/D7126-106.2For referenced ASTM standards, visit the ASTM websit
14、e, www.astm.org, or contact ASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.1This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indicatio
15、n of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be consid
16、ered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Significance and Use5.1 Silicon (Si), a metalloid, is the second most abundant element in the earths crust. Various forms of silica (silicon dioxideSiO2) ar
17、e found in quartz, sand and rocks. The degradation of these rocks results in silica found in natural waters. Silica in naturalwaters can be found as ionic silica, silicates, colloidal or suspended particles.5.2 Elevated temperatures and pressure can cause silica in water to vaporize and form deposit
18、s or scale. Scale deposits of silicawill coat boilers and turbine blades used in power plants. The presence of silica scale affects the ability of metals to transfer heat.Silica needs to be removed when deionized water is used as a rinse for manufacturing wafers in the semiconductor industry.5.3 Sil
19、ica is commonly removed by demineralization using anion exchange resins, distillation, reverse osmosis or precipitationin a lime softening process. The on-line measurement of silica is the preferred method to laboratory analyses for industries tryingto obtain and monitor ultra-pure water. Since sili
20、ca is one of the first species to breakthrough anion exchange resins, on-line silicamonitoring is frequently used to determine the need for regeneration of an anion or mixed resin bed.6. Interferences6.1 Colored and turbid samples and reagents may interfere in the colorimetric detection of silica. A
21、t low levels (ppb range)samples can be filtered to eliminate particles and suspended solids in solutions, however filtration is not recommended, becauseof the potential to introduce silica contamination. A blank (zero) absorption can be performed prior to the final absorptionmeasurement to reduce in
22、terferences from turbidity and color.6.2 Phosphate interferes in the colorimetric detection of silica by reacting with the molybdate compound used in the first reagentto form molybdophosphoric acid. Molybdophosphoric acid can be destroyed by the addition of tartaric, citric or oxalic acid afterthe m
23、olybdosilicic acid reaction has completed.6.3 Sample temperature is a potential interferant in the determination of silica by this method. If the on-line instrument allowssufficient time for the sample and the reagents to equilibrate to a constant temperature, sample temperature will not interfere.
24、Atcontrolled, higher temperatures, the reaction sequence time period (typically 10-15 min) can be decreased to 5-10 min.6.4 Glassware (which constains silica) will interfere and cause erroneously high results. Therefore, plastic or other non-silicacontaining materials should be used for reagents, ca
25、libration standards and instrument components.6.5 If a batch style instrument will be used for on-line analysis, adequate rinsing in-between samples is necessary to avoid silicacontamination from previous samples.7. Apparatus7.1 Process instrument7.1.1 A spectrophotometer or colorimeter capable of a
26、ccurately measuring absorbance values at 815 6 10 nm is necessary. Theinstrument should be capable of analyzing a sample from an on-line process water stream by addition of the necessary reagentsfor the heteropoly blue method. The instrument should also be capable of adding a calibration standard au
27、tomatically forrecommended calibration of the on-line instrument. The instrument may provide alarms, relays, isolated analog and digital outputs.8. Reagents8.1 Purity of WaterUnless otherwise indicated, references to water shall be understood to mean reagent water that meets thepurity specifications
28、 of Type I or Type II water, presented in References to water that is used for reagent preparation, rinsingor dilution shall be understood to mean water that conforms to the quantitative specifications of Type II reagent water ofSpecification D1193.8.2 Silica Solution, StandardTo prepare a silica st
29、andard solution, dissolve ACS reagent grade or better sodium metasilicate(Na2SiO39H2O) and dilute with deionized water to volume, or use commercially prepared standards. Refer to the instrumentmanufacturers manual for the correct standard concentration to use. Standards and samples should not be fro
30、zen, which can resultin lower soluble silica values. (1) Standards should be prepared at room temperature and stored at 4C to maintain maximumstability.8.3 Silica ReagentsThe colorimetric analysis described in this method requires at least 3 reagents for the on-linedetermination of silica. The reage
31、nts necessary for this method are specific for the on-line instrument, colorimeter orspectrophotometer used. Refer to the instrument manufacturers recommended reagents. It is essential that the reagents used arematched to the analyzer. Either use reagents provided or prescribed by the manufacturer o
32、r validate the results obtained from anyother reagent formulation. This validation must include validation of the calibration data.NOTE 1All reagents and calibration standards used in this test method should be stored in polyethylene, plastic or other non-silica containing bottles.NOTE 2The first re
33、agent used is an acidified molybdate reagent which reacts with silica in the sample to form molybdosilicic acid. If phosphate ispresent in the sample, molybdophosphoric acid is also formed. The first reaction requires time to ensure all of the silica in the sample has time to reactwith the acidified
34、 molybdate reagent. The reaction time depends on the concentration of the reagent and sample temperature. The user should use thereaction time and temperature suggested by the reagent and/or instrument manufacturer.The second reagent destroys the molybdophosphoric acid formed in the first step. Phos
35、phate interferes in the measurement of silica because the reducedproduct absorbs at the same wavelength as that of the reduction product of molybdosilicic acid.The third reagent reduces the molybdosilicic acid formed in the first step. The reduced molybdenum complex forms the heteropoly blue compoun
36、d andabsorption of this molybdenum (heteropoly) blue complex is measured at 815 6 10 nm.D7126 1029. Sampling9.1 For sampling, refer to standards Practice D1066, Specification D1192, Practices D3370, and Practice D5540, as applicable.10. Calibration10.1 The instrument should be calibrated using a sil
37、ica standard solution prepared as described in section 8.2 or by usingcommercial standards. The standard silica solution should be used as a calibration standard according to the instrumentmanufacturers instructions for calibration. If using validated reagents other than those provided or prescribed
38、 by the instrumentmanufacturer be sure to regenerate the calibration curve with these reagents.11. Procedure11.1 The instrument should be calibrated according to section 10.1.11.2 Prepare and connect the reagents according to manufacturers instructions. Determine required sample flow as specifiedby
39、the instrument manufacturer and measure flow rate if necessary. Switch the analyzer to automatic analysis mode for continuouson-line silica measurement.11.3 Sample should be collected from a flowing stream and diverted into or through a reaction cell/vessel. Refer to Fig. 1. Thevolume of sample insi
40、de the reaction cell should encompass the light path from the emitting light source. Each reagent used in theanalysis should be added into the reaction cell by a pump or liquid metering device. The appropriate reaction time required aftereach reagent addition should be allowed for by the instrument.
41、11.4 The light source from the on-line instrument should travel through the sample to a photodetector. In some instruments, thelight is directed through the sample to a reflector. The reflected light then travels back through the sample to the photodetector tobe measured at 815 6 10 nm. Some instrum
42、ents also take a second reference wavelength measurement at a wavelength out sidethe absorbption band for silica (approximately 450 nm) An analog signal from the photodetector is passed to a microprocessor inthe electronics assembly to be calculated as absorbance.12. Report12.1 Report readings to th
43、e nearest 0.1 ppb (g/L) Silica.13. Quality Control13.1 Instrument calibration must be performed according to the manufacturers schedule and instructions.13.2 Instrument calibration and blank readings must be checked whenever reagents or components in the optical path arechanged.Acalibration verifica
44、tion should be carried out on a sample of known silica concentration at a concentration different thanthat used for the instrument calibration.13.3 The user should confirm that the unit is giving proper response using the sample matrix and operating under theenvironmental extremes of interest.14. Pr
45、ecision and Bias14.1 Neither precision nor bias data can be obtained for this test method from a collaborative study designed in accordance withthe requirements of Practice D2777 since this test method is a continuous determination. This inability to obtain precision and biasFIG. 1 Fluidics System o
46、f an On-Line Silica InstrumentD7126 103data for continuous determinations is recognized and stated in the scope of Practice D2777.14.2 The precision and bias of this method is expected to be similar to or better than the static method for silica determination(Practice D859), which uses similar chemi
47、stries. The on-line method is expected to improve the precision by minimizingcontamination, operator involvement, and exposure 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
48、-117-4-123. Clescari, L., Greenberg, A., Eaton, A., Rice, E. (Eds). (2005). Pp. 4-117-4-123.Standard Methods for the Examination of Water and Wastewater, 20,21thEdition. Washington, D.C.: American Public Health Association.ASTM International takes no position respecting the validity of any patent ri
49、ghts 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, 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 revision of this standard or for additional standardsand should be addressed to ASTM Internationa
