1、Designation: D5464 16Standard Test Method forpH Measurement of Water of Low Conductivity1This standard is issued under the fixed designation D5464; 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 is applicable to determine the pH ofwater samples with a conductivity of 2 to 100 S/cm over thepH range of 3 to 11 and is fr
3、equently used in power generationlow conductivity samples. pH measurements of water of lowconductivity are problematic. Specifically, this test methodavoids contamination of the sample with atmospheric gasesand prevents volatile components of the sample from escaping.This test method provides for pH
4、 electrodes and apparatus thataddress additional considerations discussed in Annex A2. Thistest method also minimizes problems associated with thesamples pH temperature coefficient when the operator usesthis test method to calibrate an on-line pH monitor or controller(see Appendix X1).1.2 This test
5、method covers the measurement of pH in waterof low conductivity with a lower limit of 2.0 S/cm, utilizinga static grab-sample procedure where it is not practicable totake a real-time flowing sample.NOTE 1Test Method D5128 for on-line measurement is preferred overthis method whenever possible. Test M
6、ethod D5128 is not subject to thelimited conductivity range, temperature interferences, potential KClcontamination, and time limitations found with this method.1.3 For on-line measurements in water with conductivity of100 S/cm and higher, see Test Method D6569.1.4 For laboratory measurements in wate
7、r with conductivityof 100 S/cm and higher, see Test Method D1293.1.5 The values stated in SI units are to be regarded asstandard.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
8、 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 P
9、recision and Bias ofApplicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD4453 Practice for Handling of High Purity Water SamplesD5128 Test Method for On-Line pH Measurement of Waterof Low ConductivityD6569 Test Method for On-Line Measurement of pH3
10、. Terminology3.1 Definitions of TermsFor definitions of terms used inthis test method, refer to Terminology D1129.3.2 Definitions:3.3 Definitions of Terms Specific to This Standard:3.3.1 liquid-junction potential, na dc potential that ap-pears at the point of contact between the reference electrodes
11、salt bridge (also known as reference junction or diaphragm)and the sample solution.3.3.1.1 DiscussionIdeally, this potential is near zero and isstable. However, in low-conductivity water this potential maychange from its value in buffer solution by an unknownamount, and is a zero offset (1).34. Reag
12、ents4.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended that1These test methods are under the jurisdiction of ASTM Committee D19 onWater and are the direct responsibility of Subcommittee D19.03 on Sampling Waterand Water-Formed Deposits
13、, Analysis of Water for Power Generation and ProcessUse, On-Line Water Analysis, and Surveillance of WaterCurrent edition approved June 1, 2016. Published June 2016. Originallyapproved in 1993. Last previous edition approved in 2011 as D5464 11. DOI:10.1520/D5464-16.2For referenced ASTM standards, v
14、isit 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 in parentheses refer to the list of references at the end ofthis standar
15、d.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1all reagents conform to the specifications of the Committee onAnalytical Reagents of the American Chemical Society wheresuch specifications are available.4Other grades may be used,prov
16、ided it is first ascertained that the reagent is of sufficientlyhigh purity to permit its use without lessening the accuracy ofthe determination.4.2 Purity of WaterReferences to water that is used forreagent preparation, rinsing or dilution shall be understood tomean water that conforms to the quant
17、itative specifications ofType II reagent water of Specification D1193.4.3 Commercial Buffer Solutions Commercially availableprepared buffers traceable to NIST standards should be ad-equate to perform the calibration procedures in 10.1 10.4.The exact pH of the buffer will change with temperature andt
18、his pH versus temperature data will be provided by thepurveyor of the specific buffer. Refer to Test Methods D1293for the preparation of reference buffer solutions if desired.4.4 Buffer ACommercially available 7.0 pH buffer.4.5 Buffer BCommercially available 4.0 pH buffer.4.6 Buffer CCommercially av
19、ailable 9.0 or 10.0 pH buffer.5. Summary of Test Method5.1 The pH meter and associated electrodes are first stan-dardized with two calibration pH buffer solutions.5.2 Agrab sample of high purity water is taken by means ofrinsing and filling two narrow mouth bottles at the samplepoint. Once each cont
20、ainer is filled to the top with a represen-tative sample excluding any air, the containers are capped andthe samples are transported quickly to a laboratory for analysis.5.3 pH measurement of the sample is made with high puritywater pH calibration apparatus comprised of pH and referenceelectrodes, a
21、nd automatic temperature compensator (if used).The first container is used to rinse the sensors and begintemperature equilibration and the second container is used formeasurement.5.4 A trace amount of KCl electrolyte enters calibrationbuffer solutions and samples via the controlled leakage rate ofth
22、e reference electrode liquid junction (diaphragm) to stabilizethe liquid junction potential. Excessive KCl introduction fromthe electrode liquid junction into low ionic strength sampleswill increase solution conductivity, and may alter solution pH,and should be avoided.5.5 Temperature must be measur
23、ed and both Solution Tem-perature Coefficient (STC) and Nernst electrode effectscompensated, either manually to the measured value or auto-matically by the pH meter. See Appendix X1 for a discussionof temperature effects.6. Significance and Use6.1 pH measurement of low conductivity water is frequent
24、lyapplied to power plant water and condensed steam samples forcorrosion and scale prevention. It is sometimes used in purewater treatment systems between multiple pass membranes tooptimize performance.6.2 High purity water is highly unbuffered and smallamounts of contamination can change the pH sign
25、ificantly.Specifically, high purity water rapidly absorbs CO2gas fromthe atmosphere, which lowers the pH of the sample. Thesample container and accompanying pH measurement tech-nique minimize exposure of the high purity water sample to theatmosphere.6.3 The high purity water sample may contain volat
26、ile tracecomponents that will dissipate from the sample if exposed tothe atmosphere. The sample container used in this test methodwill prevent these losses.6.4 High purity water has a significant solution temperaturecoefficient. For greatest accuracy the sample to be measuredshould be close to the t
27、emperature of the sample stream andappropriate compensation should be applied.6.5 When the preferred Test Method D5128, which requiresa real-time, flowing sample, cannot be utilized for practicalreasons such as physical plant layout, unacceptable loss ofwater, location of on-line equipment sample po
28、ints, or avail-ability of dedicated test equipment, this method offers a viablealternative. The most significant difference between the twotest methods is that Test Method D5128 obtains a real-time pHmeasurement from a flowing sample and this method obtains atime delayed pH measurement from a static
29、 grab sample.6.6 pH measurements of low conductivity water are alwayssubject to interferences (7.1 7.5) and Test Method D5128 ismore effective in eliminating these interferences especiallywith regard to contamination. This static grab sample methodis more prone to contamination and temperature-induc
30、ed errorsbecause of the time lag between the sampling in the plant andsample pH reading which is taken in the laboratory.7. Interferences7.1 High purity, low conductivity samples are especiallysensitive to contamination from atmospheric gases, fromsample containers, from sample handling techniques a
31、nd fromexcessive electrolyte (KCl) contamination from reference elec-trode or sample preparation such as a KCl “dosing” technique.Refer to Practice D4453 and ASTM STP 823 (2) for discussionsof sample handling and avoidance of sample contamination.7.2 High purity water will rapidly absorb CO2from the
32、atmosphere and this will lower the pH of the sample at a ratedepending on the buffer capacity of the sample, the surfacearea of the sample exposed to air, movement of the sample, andthe concentration of CO2at the surface of the sample whichmay increase if the operator exhales over the container duri
33、ngsampling or measurement. See Appendix X3, Table X3.1, andFig. X3.1.4Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For Suggestions on the testing of reagents notlisted by the American Chemical Society, see Annual Standards for LaboratoryChemi
34、cals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,MD.D5464 1627.3 The temperature stability of the sample and how closelythe samples temperature matches the sample streams tem-perature will have a dire
35、ct effect on accuracy of the pHdetermination since temperature compensation is not perfect.7.4 If pH is to be referenced to 25C as required by mostspecifications, temperature compensation must be provided forboth the Nernst response of the electrode output (provided inmost pH meters) and solution io
36、nization effects (provided onlywith some on-line pH meters or by calculation with labmeters). For a discussion of temperature effects on pH mea-surements of high purity water see Appendix X1.7.5 The reference junction potential can vary with ionicstrength of the sample and provide an undetectable ze
37、ro offsetbetween the high ionic strength of the buffer solution and thelow ionic strength of the sample. A flowing junction referenceelectrode (one which requires periodic refilling with electrolytesolution or that has internal electrolyte pressurization or both)minimizes this effect.8. Apparatus8.1
38、 pH Meter, capable of reading to 0.01 pH. The metershould preferably have automatic temperature compensationfor the Nernst response of pH electrodes which provides theconversion of the electrode millivolt signal to the pH value atthe measured temperature. Some on-line meters also providean input for
39、 a solution temperature coefficient for ionizationeffects of the particular sample. This allows direct readout ofpH referenced to 25C. Otherwise a calculation is required.8.2 Sample ContainersTwo clean, narrow-mouth 250 to500 mL bottles with cap are required. The mouth diametershould be the minimum
40、necessary to allow insertion of theelectrode(s), and temperature compensator or thermometer. A3-hole stopper may be used to hold these sensors. Thecontainer and cap minimize exposure to atmospheric gases.8.3 Combination pH ElectrodeA probe incorporating themeasuring, reference and temperature compen
41、sator functionsin a single unit is recommended for its ease of insertion into avery narrow mouth sample container. Each function shouldconform to the characteristics in 8.4 8.6. Where this is notavailable, individual electrodes and compensator (8.4 8.6)may be used with a 3-hole stopper to hold them
42、and seal thecontainer during measurement.8.4 pH Glass ElectrodeThe pH response of the glasselectrode shall conform to the requirements set forth in 12.1through 12.5 ofTest Methods D1293. New glass electrodes andthose that have been stored dry shall be conditioned andmaintained as recommended by the
43、manufacturer.8.5 Reference ElectrodeDouble junction design, having arefillable flowing junction with a positive electrolyte leakagerate not to exceed 10 L/h. A sealed reference electrode issuitable only if it is internally pressurized to force an electro-lyte flow outward. Prepare and maintain the r
44、eference electrodeaccording to the manufacturers instructions.8.6 Temperature CompensatorSee paragraph 10.4 in TestMethods D1293. The automatic temperature compensator mustfit into the sample container to measure the temperature of thewater within the container.8.7 Temperature IndicatorA direct temp
45、erature indicatingdevice must be used to measure sample water temperaturewithin the sample container if an automatic temperaturecompensator is not used.8.8 Equipment for this test method should be dedicated forhigh purity water use only.9. Sampling and Sample Handling9.1 The sample containers should
46、 be rinsed three timesbefore use, with sample or reagent water.9.2 Static grab samples are taken via clean tubing attachedto the sample take-off point. The other end of the tubing isinserted to the bottom of the sample container. The sampleflows through the tube, into the bottom of the container, an
47、dthen over the top to trough or sink drain.After a 5-min flush ofthe container with the sample water, the tubing is removed andthe cap immediately installed, excluding any air. This seals thecontainer and isolates the sample from the atmosphere. Thesecond container is filled and capped in the same w
48、ay. In thelab, the first container is used to rinse the electrodes andtemperature device and the second container is used formeasurement.10. Calibration10.1 Turn on the pH meter and allow it to warm upaccording to the manufacturers instructions. If an on-linemeter is used that has a solution tempera
49、ture coefficient setting,be sure it is turned off or is set to 0 pH/C for calibration inbuffer solutions. Conventional Nernst temperature compensa-tion should be active at all times and requires no setting.10.2 Remove the electrode(s) and temperature compensator(if used) from storage. Check the reference electrode for properelectrolyte level as recommended by the manufacturer.10.3 Calibrate the electrode(s) and pH meter at two pointsaccording to manufacturers instructions.Also, refer to Section12 of Test Methods D1293 for guidelines on the cali
