1、Designation: D5128 09Standard 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.
2、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
3、 Table 2) over the pH range of 3 to 11(see Fig. 1), under field operating conditions, utilizing a sealed,non-refillable, reference electrode. pH measurements of waterof low conductivity are problematical for conventional pHelectrodes, methods, and related measurement apparatus.1.2 This test method i
4、ncludes 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
5、.1.3 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
6、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 Continual On-Line Monitoring Systemsfor Wat
7、er AnalysisD4453 Practice for Handling of Ultra-Pure Water SamplesD5464 Test Method for pH Measurement of Water of LowConductivity3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 liquid junction potentiala dc potential that appearsat the point of contact between the reference e
8、lectrodes saltbridge and the sample solution. Ideally this potential is nearzero, and is stable. However, in low conductivity water it1This 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 De
9、posits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved Oct. 1, 2009. Published October 2009. Originallyapproved in 1990. Last previous edition approved in 2005 as D5128 90 (2005).DOI: 10.1520/D5128-09.2For referenced
10、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.TABLE 1 Calculated Conductivity and pH Values at 25C of LowConcentrations of
11、NaOH in Pure WaterANOTE 1This table tabulates the theoretical conductivity and pHvalues of low levels of NaOH in pure water as calculated from availablethermodynamic data.NOTE 2To illustrate the high sensitivity of the sample pH at these lowconcentrations to contaminants, the last column lists error
12、s that wouldresult if the sample were contaminated with an additional 1 mg/L throughsample or equipment handling errors.SampleConcentration,mg/LSampleConductivity,S/cmSamplepHD pH Error from Addi-tional 1 mg/L NaOHContaminate0.001 0.055 7.05 D 2.350.010 0.082 7.45 D 1.950.100 0.625 8.40 D 1.031.0 6.
13、229 9.40 D 0.308.0 49.830 10.30 D 0.05AData courtesy of Ref (13). This data developed from algorithms originallypublished in Ref (14).TABLE 2 Calculated Conductivity and pH Values at 25C of LowConcentrations of HCl in Pure WaterANOTE 1This table tabulates the theoretical conductivity and pHvalues of
14、 low levels of HCl in pure water as calculated from availablethermodynamic dataNOTE 2To illustrate the high sensitivity of the sample pH at these lowconcentrations to contaminants, the last column lists errors that wouldresult if the sample were contaminated with an additional 1 mg/L throughsample o
15、r equipment handling errors.SampleConcentration,mg/LSampleConductivity,S/cmSamplepHD pH Error from Addi-tional 1 mg/L HCl Con-taminate0.001 0.060 6.94 D2.380.010 0.134 6.51 D 1.950.100 1.166 5.56 D 1.031.0 11.645 4.56 D 0.308.0 93.163 3.66 D 0.05AData courtesy of Ref (13). This data developed from a
16、lgorithms originallypublished in Ref (14).1Copyright. (C) ASTM International, 100 Barr Harbor Dr., PO box C-700 West Conshohocken, Pennsylvania 19428-2959, United StatesCopyright by ASTM Intl (all rights reserved); Thu May 3 22:37:50 EDT 2012Downloaded/printed byGuo Dehua (CNIS) pursuant to License
17、Agreement. No further reproductions authorized.becomes larger by an unknown amount, and is a zero offset.Aslong as it remains stable its effect can be minimized by “grabsample” calibration (1).33.1.2 streaming potentialthe static electrical charge that isinduced by the movement of a low ionic streng
18、th solutionhaving a high electrical resistivity or low electrical conductiv-ity (such as pure water), across relatively non-conductivesurfaces such as the pH measurement electrodes glass mem-brane or other non-conductive wetted materials found inflowing sample streams.3.2 Definitions: For definition
19、s of other terms used in thistest method, refer to Terminology D1129 and Practice D3864.4. Summary of Test Method4.1 pH is measured by a pair of electrodes contained in anall stainless steel flow cell. The pH measurement half cell isconstructed of a glass membrane suitable for continuousservice in l
20、ow conductivity water. Many modern pH electrodesare available that perform well in this service. However, thebulb impedance should be kept low to minimize the effects of“streaming potential” (see 3.1.2). The reference half cell issealed (requiring no electrolyte replenishment) and is con-structed in
21、 such a manner that the salt bridge, while makingdiffusion contact to the sample, resists significant dilution forperiods up to several months on continuous operation.4.2 This test method describes the apparatus and proceduresto be used for the continuous on-line pH measurement of lowconductivity wa
22、ter sample streams. The type of pH sensorassembly and pH instrument interface module are described indetail. The requirements for sample stream manifolds for theconditioning of sample pressure and flow rate are defined, andarrangements for this associated equipment are illustrated.Guidelines for the
23、 proper installation and calibration of the pHsensor and associated sample manifold are discussed alongwith the precautions that must be considered concerningsample contamination and representative sampling for calibra-tion purposes.4.3 The apparatus and procedures described in this testmethod are i
24、ntended to be used with most state-of-the-art,process-grade, pH analyzer/transmitter instruments currently inuse or available from the major manufacturers of such instru-mentation.5. Significance and Use5.1 pH measurements are typically made in solutions thatcontain relatively large amounts of acid,
25、 base, or dissolvedsalts. Under these conditions, pH determinations may be madequickly and precisely. Continuous on-line pH measurements inwater samples of low conductivity are more difficult (4, 5).These low ionic strength solutions are susceptible to contami-nation from the atmosphere, sample stre
26、am hardware, and thepH electrodes. Variations in the constituent concentration oflow conductivity waters cause liquid junction potential shifts(see 3.1.1) resulting in pH measurement errors. The aggressivenature and the high electrical resistance of pure and ultra-pure,low conductivity waters may de
27、grade the pH measurementelectrodes resulting in unstable and drifting pH output signals.5.2 It is essential to make on-line pH measurements of lowconductivity water as accurately as possible to determine theproper control of pH adjustment chemicals, the effectiveness ofdemineralizer equipment, the e
28、vent and nature of impuritycontamination of the water, and information pertaining to theoverall status of the pure water system.6. Interferences6.1 Sample systems for high purity, low conductivity watersare especially sensitive to contamination from atmosphericgases (especially carbon dioxide, see A
29、ppendix X1 and Table3) from collection of “crud” (insoluble deposits of iron oxideand other by-products of metallic corrosion that are presentthroughout the system) in sample lines, from exposure to high3The boldface numbers given in parentheses refer to a list of references at theend of this standa
30、rd.FIG. 1 Restrictions Imposed by the Conductivity pH RelationshipTABLE 3 Calculated pH and Conductivity Values at 25C ofWater Solutions Containing Only Ammonia and Carbon DioxideAAmmoniamg/LCarbon Dioxide0 mg/LCarbon Dioxide0.2 mg/LpH ShiftCaused by0.2 mg/LCO2Contaminationof SampleS/cm pH S/cm pH0
31、0.056 7.00 0.508 5.89 D 1.11 pH0.12 1.462 8.73 1.006 8.18 D 0.55 pH0.51 4.308 9.20 4.014 9.09 D 0.11 pH0.85 6.036 9.34 5.788 9.26 D 0.08 pH1.19 7.467 9.44 7.246 9.38 D 0.06 pHAData extracted from Ref (15).D5128 092Copyright by ASTM Intl (all rights reserved); Thu May 3 22:37:50 EDT 2012Downloaded/pr
32、inted byGuo Dehua (CNIS) pursuant to License Agreement. No further reproductions authorized.ionic strength calibration buffers, from incorrect sample systeminstallation techniques, and from excessive KCl leakage fromthe pH reference half-cell. Refer to Practice D4453 and Refs(2) and (3).6.2 Streamin
33、g potentials that are developed in flowing, lowconductivity water sample streams, and which are dynamic innature, will add to the potential (millivolt) generated by the pHglass measurement half cell in proportion to the H+and OHactivities. This resultant pH error appears as a noisy anddrifting pH si
34、gnal from the pH sensor. These effects areminimized by using a conductive flow cell and, in some cases,a symmetrical combination measurement/reference electrode(6).6.3 Liquid junction potentials, that are most evident in lowconductivity waters, shift the potential of the pH reference halfcell result
35、ing in both short and long-term pH measurementerrors. The instability of liquid junction potentials dependsupon reference half-cell design, electrical conductivity of thesample water, time, and sampling conditions such as flow rateand pressure. Exposure of the pH sensor electrodes to pHcalibration b
36、uffer solutions, that have a higher ionic strengththan the pure water sample stream, causes significant instabil-ity in liquid junction potentials resulting in pH measurementerrors. This pH measurement error is caused by the shifting ofthe pH electrodes from one ionic strength solution to another.6.
37、3.1 Liquid junction potentials must be stable so as to makereliable calibration of the system possible. Reference elec-trodes that have been exposed to the much higher ionic strengthof buffer solutions will require considerable rinse time toestablish a stable liquid junction potential in high purity
38、 water.To determine the pH electrodes suitability in low conductivitywater, a comparative low conductivity water sample calibra-tion, or on-line calibration with low conductivity standardssimilar to the samples being addressed should be performed, asdescribed in 9.5.6.3.2 The severity of the error r
39、esulting from a liquidjunction potential shift when the ionic strength of the samplechanges, for example, measuring 1.0 mg/L ammonia(pH = 9.38 and conductivity = 6.58 S/cm) followed by mea-suring 0.1 mg/L ammonia (pH = 8.65 and conductivity = 1.24S/cm) is not known and is a deficiency in the state-o
40、f-the-art.See Table 4.6.4 Temperature stability of the flowing sample stream andpH correlation to the desired 25C reference temperature has adirect effect which is more significant in low conductivitywater on the accuracy of the pH measurement (5, 6, 7, 8, 9, 10).Adiscussion of the temperature effec
41、ts on pH measurements ispresented in Appendix X2.6.5 The flow rate to the pH electrodes and related apparatusmust be controlled in order to obtain repeatable results. Adiscussion of the flow sensitivity is presented in Appendix X3.7. Apparatus7.1 A complete high purity water pH sensor assembly isreq
42、uired. The pH flow cell, connecting tubing, and electrodehousings should be constructed of stainless steel (316 ispreferred and electropolished 304 is acceptable), and the wholesystem should be properly grounded. Provisions for the nec-essary shielding to eliminate noise pick-up and for minimizingai
43、r entrapment and “crud” accumulation shall be furnished inthe flow cell and sensor assembly design. The use of plasticssuch as TFE and PVDF and other wetted materials that will notleach any contaminates into the sample may be incorporatedinto the sensor assembly where required.NOTE 1The temperature
44、response of the measurement electrodes mayaffect the accuracy and repeatability of the measurement. Electrodes thatquickly equilibrate to each other and the sample temperature must beselected for this service. Refer to Practice D1293,X1D1293.2 and Ref(2).NOTE 2Continuous exposure of the pH electrode
45、 to low ionic strengthsolutions may result in the degradation of the glass membrane portion ofsome pH electrodes (11). Electrodes suitable for continuous service in lowconductivity water should be included in the pH sensor assembly.NOTE 3Changes in liquid junction potentials (1) with time andeventua
46、l degradation of the reference half cell caused by diffusion of lowionic strength sample water into the high ionic strength electrolyte of thehalf cell, must be avoided in order to effect an accurate and stable pHmeasurement. A sealed reference half cell (requiring no electrolytereplenishment) that
47、is constructed in such a manner that the salt bridge,while making diffusion contact to the sample, resists significant dilutionfor periods up to several months on continuous operation in lowconductivity water measurements, must be included in the pH sensorassembly. A trace amount of KCl will diffuse
48、 with time into the sample.7.2 A sample stream manifold constructed of all stainlesssteel, PTFE, and glass wetted components as shown in Fig. 2shall be used immediately upstream of the pH sensor. Themanifold will provide proper sample stream pressure and flowrate control secondary to primary sample
49、cooling and pressureregulation. This manifold shall also provide grab sample outletfor proper calibration of the pH sensor. This manifold shall beconstructed in such a manner that when a grab sample is beingtaken for calibration purposes, neither the sample flow rate norpressure shall be permitted to vary at the on-line pH sensorlocation.7.3 When the high purity water pH sensor assembly is notdirectly coupled to the pH analyzer (within 3 m (10 ft.), aninterface module3located in a National Electrical Manufac-turersAssociation (NEMA) 4X junction box within 3 m
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