1、Designation: D 5128 90 (Reapproved 2005)Standard Test Method forOn-Line pH Measurement of Water of Low Conductivity1This standard is issued under the fixed designation D 5128; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the yea
2、r of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 100
3、S/cm (see Table 1 and 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
4、.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.1.3 This standard does not purport to address all of thesafety
5、 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:2D 1129 Terminology Relating to Wat
6、erD 1193 Specification for Reagent WaterD 1293 Test Methods for pH of WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Methods of Committee D19 on WaterD 3864 Guide for Continual On-Line Monitoring Systemsfor Water AnalysisD 4453 Practice for Handling of Ultra-Pure Water Sam
7、ples2.2 ASTM Proposal:P 228 ProposedTest Methods for pH Measurement ofWaterof Low Conductivity33. 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 electrodes saltbridge and the sample
8、 solution. Ideally this potential is near1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling of Water andWater-Formed Deposits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and
9、 Surveillance of Water.Current edition approved Jan. 1, 2005. Published January 2005.2For 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
10、page onthe ASTM website.3Withdrawn.TABLE 1 Calculated Conductivity and pH Values at 25C of LowConcentrations of 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 illustra
11、te 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 or equipment handling errors.SampleConcentration,mg/LSampleConductivity,S/cmSamplepHD pH Error fr
12、om 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 6.229 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 o
13、f LowConcentrations of HCl in Pure WaterANOTE 1This table tabulates the theoretical conductivity and pHvalues of 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 las
14、t column lists errors 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 HCl Con-taminate0.001 0.060 6.94 D2.380.010 0.134 6.51 D 1.950.100 1.1
15、66 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 algorithms originallypublished in Ref (14).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.zero, and is stable. However, i
16、n low conductivity water itbecomes larger by an unknown amount, and is a zero offset.Aslong as it remains stable its effect can be minimized by “grabsample” calibration (1).43.1.2 streaming potentialthe static electrical charge that isinduced by the movement of a low ionic strength solutionhaving a
17、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: DefinitionsFor definitions of othe
18、r termsused in this test method, refer to Terminology D 1129 andPractice D 3864.4. 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 low condu
19、ctivity 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 such a
20、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 water samp
21、le 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 proper
22、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 intended
23、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, base, o
24、r 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 stream hardw
25、are, 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 degrade th
26、e 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 event and
27、 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 Appendix
28、X1 and Table3) from collection of “crud” (insoluble deposits of iron oxide4The boldface numbers given in parentheses refer to a list of references at theend of this standard.FIG. 1 Restrictions Imposed by the Conductivity pH RelationshipTABLE 3 Calculated pH and Conductivity Values at 25C ofWater So
29、lutions 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 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
30、 D 0.08 pH1.19 7.467 9.44 7.246 9.38 D 0.06 pHAData extracted from Ref (15).D 5128 90 (2005)2and other by-products of metallic corrosion that are presentthroughout the system) in sample lines, from exposure to highionic strength calibration buffers, from incorrect sample systeminstallation technique
31、s, and from excessive KCl leakage fromthe pH reference half-cell. Refer to Practice D 4453 and Refs(2) and (3).6.2 Streaming 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
32、measurement half cell in proportion to the H+and OHactivities. This resultant pH error appears as a noisy anddrifting pH signal 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 ju
33、nction potentials, that are most evident in lowconductivity waters, shift the potential of the pH reference halfcell resulting in both short and long-term pH measurementerrors. The instability of liquid junction potentials dependsupon reference half-cell design, electrical conductivity of thesample
34、water, time, and sampling conditions such as flow rateand pressure. Exposure of the pH sensor electrodes to pHcalibration buffer solutions, that have a higher ionic strengththan the pure water sample stream, causes significant instabil-ity in liquid junction potentials resulting in pH measurementerr
35、ors. This pH measurement error is caused by the shifting ofthe pH electrodes from one ionic strength solution to another.6.3.1 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 str
36、engthof buffer solutions will require considerable rinse time toestablish a stable liquid junction potential in high purity water.To determine the pH electrodes suitability in low conductivitywater, a comparative low conductivity water sample calibra-tion, or on-line calibration with low conductivit
37、y standardssimilar to the samples being addressed should be performed, asdescribed in 9.5.6.3.2 The severity of the error resulting 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) foll
38、owed 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-of-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 signific
39、ant in low conductivitywater on the accuracy of the pH measurement (5, 6, 7, 8, 9, 10).Adiscussion of the temperature effects 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. Adiscussio
40、n of the flow sensitivity is presented in Appendix X3.7. Apparatus7.1 A complete high purity water pH sensor assembly isrequired. The pH flow cell, connecting tubing, and electrodehousings should be constructed of stainless steel (316 ispreferred and electropolished 304 is acceptable), and the whole
41、system should be properly grounded. Provisions for the nec-essary shielding to eliminate noise pick-up and for minimizingair 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
42、notleach any contaminates into the sample may be incorporatedinto the sensor assembly where required.NOTE 1The temperature response of the measurement electrodes mayaffect the accuracy and repeatability of the measurement. Electrodes thatquickly equilibrate to each other and the sample temperature m
43、ust beselected for this service. Refer to Practice D 1293, X 1.2 and Ref (2).NOTE 2Continuous exposure of the pH electrode to low ionic strengthsolutions may result in the degradation of the glass membrane portion ofsome pH electrodes (11). Electrodes suitable for continuous service in lowconductivi
44、ty water should be included in the pH sensor assembly.NOTE 3Changes in liquid junction potentials (1) with time andeventual 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
45、to effect an accurate and stable pHmeasurement. A sealed reference half cell (requiring no electrolytereplenishment) that 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 oper
46、ation in lowconductivity water measurements, must be included in the pH sensorassembly. A trace amount of KCl will diffuse 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
47、of the pH sensor. Themanifold will provide proper sample stream pressure and flowrate control secondary to primary sample 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 tha
48、t 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 module4locat
49、ed in a National Electrical Manufac-turersAssociation (NEMA) 4X junction box within 3 m (10 ft)of the on-line pH sensor shall be provided. This interfaceTABLE 4 pH versus Specific Conductivity At 25CANOTE 1This table tabulates the theoretical pH and specific conduc-tivity values of low levels of ammonium hydroxide in reagent water ascalculated from available thermodynamic data.Ammonia,mg/L NH3Ammonium Hy-droxide, mg/LNH4OHpHSpecificConductivity,S/cm0.10 0.21 8.65 1.240.15 0.31 8.79 1.720.20 0.41 8.89 2.150.25 0.51 8.96 2.540.30 0.62 9.02 2.910.35 0.72 9.07
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