1、Designation: D5128 14Standard 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. 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 cont
4、inuous 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 ass
5、tandard. 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 a
6、pplica-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
7、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
8、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 nea
9、r 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
10、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
11、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
12、 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 forconditi
13、oning 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 Wat
14、er 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
15、 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.Copyrig
16、ht ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.3 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 measurement
17、s 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 Continuous pH measurement is used to
18、 monitor powerplant cycle chemistry where small amounts of ammonia oramines or both are added to minimize corrosion by hightemperature pure water and steam.5.3 Conventional pH measurements are made in solutionsthat contain relatively large amounts of acid, base, or dissolvedsalts. Under these condit
19、ions, 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 hardware, and thepH electrodes. Variation
20、s in the constituent concentration oflow conductivity waters cause liquid junction potential shifts(see 3.2.1) resulting in pH measurement errors. Special pre-cautions are required.6. Interferences6.1 Sample systems for high purity, low conductivity watersare especially sensitive to contamination fr
21、om atmosphericgases (especially carbon dioxide, see Appendix X1 and Table3), and to accumulation of power plant corrosion particles insample lines that can absorb or desorb contaminants. ExcessiveTABLE 1 Calculated Conductivity and pH Values at 25C of LowConcentrations of NaOH in Pure WaterANOTE 1Th
22、is table tabulates the theoretical conductivity and pH valuesof 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 errors that wouldresult if the s
23、ample were contaminated with an additional 1 mg/L throughsample or equipment handling errors.SampleConcentration,mg/LSampleConductivity,S/cmSamplepH pH Error from Addi-tional 1 mg/L NaOHContaminate0.001 0.055 7.05 2.350.010 0.082 7.45 1.950.100 0.625 8.40 1.031.0 6.229 9.40 0.308.0 49.830 10.30 0.05
24、AData courtesy of Ref (1).BThis data developed from algorithms originallypublished in Ref (2).BThe boldface numbers in parentheses refer to a list of references at the end ofthis standard.TABLE 2 Calculated Conductivity and pH Values at 25C of LowConcentrations of HCl in Pure WaterANOTE 1This table
25、tabulates the theoretical conductivity and pH valuesof 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
26、contaminated with an additional 1 mg/L throughsample or equipment handling errors.SampleConcentration,mg/LSampleConductivity,S/cmSamplepH pH Error from Addi-tional 1 mg/L HCl Con-taminate0.001 0.060 6.94 2.380.010 0.134 6.51 1.950.100 1.166 5.56 1.031.0 11.645 4.56 0.308.0 93.163 3.66 0.05AData cour
27、tesy of Ref (1). This data developed from algorithms originallypublished in Ref (2).FIG. 1 Restrictions Imposed by the Conductivity pH RelationshipTABLE 3 Calculated pH and Conductivity Values at 25C of WaterSolutions Containing Only Ammonia and Carbon DioxideAAmmoniamg/LCarbon Dioxide0 mg/LCarbon D
28、ioxide0.2 mg/LpH ShiftCaused by0.2 mg/LCO2Contaminationof SampleS/cm pH S/cm pH0 0.056 7.00 0.508 5.89 1.11 pH0.12 1.462 8.73 1.006 8.18 0.55 pH0.51 4.308 9.20 4.014 9.09 0.11 pH0.85 6.036 9.34 5.788 9.26 0.08 pH1.19 7.467 9.44 7.246 9.38 0.06 pHAData extracted from Ref (8).D5128 142KCl electrolyte
29、leakage from the pH reference half-cell canalso affect the sample. Refer to Practice D4453 and Refs (6)and (7).6.2 Streaming potentials that are developed by flowing, lowconductivity water across non-conductive surfaces are dynamicin nature and will add to the potential (millivolt) generated bythe p
30、H sensor. This resultant pH error appears as a noisy,flow-sensitive and drifting pH signal. These effects are mini-mized by using a conductive flow cell, low sample flowratesand, in some cases, a symmetrical combination measurement/reference electrode (9).6.3 Reference electrode liquid junction pote
31、ntials are sig-nificant in low conductivity waters and shift the potential of thepH reference half-cell, resulting in both short and long-term pHmeasurement errors. The instability of liquid junction poten-tials depends on reference half-cell design, electrical conduc-tivity of the sample water, tim
32、e, and sampling conditions suchas flow rate and pressure. Generally, reference electrodes withrefillable liquid electrolyte flowing junctions provide morestable junction potential than non-refillable, sealed referenceelectrodes. Exposure of the pH electrodes to pH calibrationbuffer solutions, that h
33、ave a higher ionic strength than the purewater sample stream, causes significant change in the liquidjunction potential of sealed reference electrodes from what it isin a low conductivity sample, resulting in pH measurementerrors that appear immediately after calibration in buffersolutions.6.3.1 Liq
34、uid junction potentials must be stable to make areliable calibration of the system. Sealed reference electrodesthat have been exposed to the much higher ionic strength ofbuffer solutions require considerable rinse time to establish astable liquid junction potential in high purity water. To deter-min
35、e the pH electrodes suitability in low conductivity water,a comparative low conductivity water sample calibration, oron-line calibration with low conductivity standards similar tothe samples being addressed should be performed, as describedin 9.5.6.3.2 The severity of the error resulting from a liqu
36、idjunction 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 measur-ing 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.Table 4 an
37、d Fig. X1.1 provide a correlation between pH andspecific conductivity of dilute ammonia.6.4 Temperature compensation of pH in low conductivitywater is more significant and more involved than in conven-tional measurements.6.4.1 All pH measurements must compensate for the chang-ing output of the elect
38、rode with temperature. This effect isrepresented by a changing slope with units of millivolts per pH.This slope is proportional to absolute temperature according tothe Nernst equation.6.4.2 In addition, pH measurements in low conductivitywater in the power industry must compensate for the change oft
39、he dissociation of water and ammonia or amines with tem-perature to report pH at 25C. This is typically set into theinstrument by the user with a solution temperature coefficientin units of pH per C. Most process pH instrumentation for usein power plants has a setting for solution temperature compen
40、-sation which the user must enter to activate this compensation.Laboratory instrumentation typically does not have this capa-bility (5, 9, 10, 11, 12, 13).6.4.3 Further discussion of the temperature effects on pHmeasurements is presented in Annex A1.6.5 The flow rate to the pH electrodes and related
41、 apparatusmust be controlled in order to obtain repeatable results. Adiscussion of the flow sensitivity is presented in Annex A2.7. Apparatus7.1 A complete high purity water pH sensor assembly isrequired. The pH flow cell and connecting tubing should beconstructed of stainless steel and be earth gro
42、unded. The sensorassembly design shall provide shielding to prevent noisepick-up and shall minimize air entrapment and corrosionparticle accumulation. Sample discharge shall be near the topof the flow cell to purge any air bubbles rapidly and shall godownward to an open drain to prevent any back pre
43、ssure on theelectrode(s).7.1.1 A single probe combination measuring and referenceelectrode with integral temperature compensator enables use ofa very small volume flow cell which creates a high sample flowvelocity that prevents power plant sample corrosion particlesfrom accumulating.7.1.2 Where sepa
44、rate measuring, reference and temperaturecompensator probes are used, a larger volume flow cell isnecessary and the design should enable convenient periodiccleanout of accumulated corrosion particles.7.2 Electrodes should have half-cells that quickly equili-brate to each other and the sample tempera
45、ture. Refer to Ref(6).7.3 Electrodes should be suitable for continuous service inlow conductivity water (14).7.4 Changes in liquid junction potential (3) with time andeventual degradation of the reference half-cell caused byTABLE 4 pH versus Specific Conductivity At 25CANOTE 1This table tabulates th
46、e 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 2.54
47、0.30 0.62 9.02 2.910.35 0.72 9.07 3.250.40 0.82 9.11 3.570.45 0.93 9.15 3.880.50 1.03 9.18 4.171.00 2.06 9.38 6.581.50 3.09 9.49 8.472.00 4.11 9.56 10.08AData courtesy of Ref (1). This data developed from algorithms originallypublished in Ref (2).D5128 143diffusion of low ionic strength sample water
48、 into the high ionicstrength electrolyte of the half-cell, must be avoided in order tomaintain an accurate and stable pH measurement. To achievethis, a liquid-electrolyte, flowing-junction reference system ispreferred. A sealed reference half-cell (requiring no electrolytereplenishment) that is pres
49、surized or otherwise constructed insuch a manner that the salt bridge, while making diffusioncontact to the sample, resists significant dilution for periods upto several months of continuous operation in low conductivitywater measurements may also be used.7.5 Asample stream conditioning manifold with capabilitiessimilar to Fig. 2 shall be used immediately upstream of the pHsensor. The manifold provides proper sample stream pressureand flow rate control after primary sample cooling and pressureregulation. This manifold shall als
