ASTM_D_5128_-_14.pdf

Designation:D512814Standard Test Method forOn-Line pH Measurement of Water of Low Conductivity1This standard is issued under the fixed designation D5128;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 precise on-line determina-tion of pH in water samples of conductivity lower than 100S/cm(see Table 1 and Table 2)over the pH range of 3 to 11(see Fig.1),under field operating conditions.pH measure-ments of water of low conductivity are problematic forconventional pH electrodes,methods,and related measurementapparatus.1.2 This test method includes the procedures and equipmentrequired for the continuous pH measurement of low conduc-tivity water sample streams including the requirements for thecontrol of sample stream pressure,flow rate,and temperature.For off-line pH measurements in low conductivity samples,refer to Test Method D5464.1.3 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.4 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:2D1129 Terminology Relating to WaterD1193 Specification for Reagent WaterD1293 Test Methods for pH of WaterD2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD3864 Guide for On-Line Monitoring Systems for WaterAnalysisD4453 Practice for Handling of High Purity Water SamplesD5464 Test Method for pH Measurement of Water of LowConductivity3.Terminology3.1 DefinitionsFor definitions of other terms used in thistest method,refer to Terminology D1129 and Practice D3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 liquid junction potential,na dc potential that ap-pears at the point of contact between the reference electrodessalt bridge(sometimes called diaphragm)and the samplesolution.3.2.1.1 DiscussionIdeally this potential is near zero,andis stable.However,in low conductivity water it becomes largerby an unknown amount,and is a zero offset.As long as itremains stable its effect can be minimized by“grab sample”calibration(3).33.2.2 streaming potential,nthe static electrical charge thatis induced by the movement of a low ionic strength solutionhaving a high electrical resistivity or low electrical conductiv-ity(such as pure water),across relatively non-conductivesurfaces such as the pH electrode or other non-conductivewetted materials found in flowing sample streams.4.Summary of Test Method4.1 pH is measured by electrodes contained in an allstainless steel flow cell.The pH measurement half-cell isconstructed of a glass membrane suitable for continuousservice in low conductivity water.The reference half-cell isconstructed in such a manner that the salt bridge uses either aflowing liquid electrolyte,or a pressurized gel electrolyte thatresists significant dilution for periods up to several months ofcontinuous operation.4.2 This test method describes the apparatus and proceduresto be used for the continuous on-line pH measurement of lowconductivity water sample streams.The requirements forconditioning sample pressure and flow rate are defined,andarrangements for this associated equipment are illustrated.1This 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 Water.Current edition approved Jan.1,2014.Published January 2014.Originallyapproved in 1990.Last previous edition approved in 2009 as D5128 09.DOI:10.1520/D5128-14.2For 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.3The boldface numbers given in parentheses refer to a list of references at theend of this standard.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 4.3 The apparatus and procedures described in this testmethod are intended to be used with process-grade,pHanalyzer/transmitter instruments.5.Significance and Use5.1 Continuous pH measurements in low conductivitysamples are sometimes required in pure water treatment usingmultiple pass reverse osmosis processes.Membrane rejectionefficiency for several contaminants depends on pH measure-ment and control between passes where the conductivity is low.5.2 C