ASTM_D_7126_-_15.pdf

Designation:D712615Standard 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 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 known 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 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 MethodD859.1.5 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 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 ASTM Standards:2D859 Test Method for Silica in WaterD1066 Practice for Sampling SteamD1129 Terminology Relating to 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 ConduitsD3864 Guide for On-Line Monitoring Systems for WaterAnalysisD5540 Practice for Flow Control and Temperature Controlfor On-Line Water Sampling and Analysis3.Terminology3.1 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 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 continuous 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 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),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 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 andWater-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 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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 silica 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 industry.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 ob