ASTM D6569-2005(2009) 895 Standard Test Method for On-Line Measurement of pH1《pH值在线测量的标准试验方法》.pdf

1、Designation: D 6569 05 (Reapproved 2009)Standard Test Method forOn-Line Measurement of pH1This standard is issued under the fixed designation D 6569; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A numb

2、er 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 continuous determination ofpH of water by electrometric measurement using the glass, theantimony or the ion-sel

3、ective field-effect transistor (ISFET)electrode as the sensor.1.2 This test method does not cover measurement ofsamples with less than 100 S/cm conductivity. Refer to TestMethod D 5128.1.3 This test method does not cover laboratory or grabsample measurement of pH. Refer to Test Method D 1293.1.4 The

4、 values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.5 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-priat

5、e 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 WaterD 1193 Specification for Reagent WaterD 1293 Test Methods for pH of WaterD 2777 Practice for Determination of Precision

6、 and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed ConduitsD 3864 Guide for Continual On-Line Monitoring Systemsfor Water AnalysisD 5128 Test Method for On-Line pH Measurement of Waterof Low Conductivity3. Terminology3.1 DefinitionsFor defini

7、tions of terms used in this testmethod, refer to Terminology D 1129, Test Method D 1293 andGuide D 3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 liquid junction potentialthe dc potential which ap-pears at the point of contact between the reference electrodessalt bridge and the sample

8、 solution. Ideally this potential isnear zero and is stable. However, in samples with extreme pHit becomes larger by an unknown amount and is a zero offset.4. Summary of Test Method4.1 pH is measured as a voltage between measuring elec-trode and reference electrode elements. The sensor assemblytypic

9、ally includes a temperature compensator to compensatefor the varying output of the measuring electrode due totemperature.4.2 The sensor signals are processed with an industrial pHanalyzer/transmitter.4.3 The equipment is calibrated with standard pH buffersolutions encompassing or in close proximity

10、to the anticipatedpH measurement range.5. Significance and Use5.1 pH is a measure of the hydrogen ion activity in water. Itis a major parameter affecting the corrosivity and scalingproperties of water, biological life in water and many applica-tions of chemical process control. It is therefore impor

11、tant inwater purification, use and waste treatment before release tothe environment.5.2 On-line pH measurement is preferred over laboratorymeasurement to obtain real time, continuous values for auto-matic control and monitoring purposes.6. Interferences6.1 Pressure and temperature variations may for

12、ce processsample into the liquid junction of non-flowing junction refer-ence electrodes and cause changes in the junction potential.Estimates of 0.2 to 0.5 pH errors from this source have beencited. (1)36.2 Liquid junction potentials at the reference electrode canvary depending on the composition of

13、 the sample. Strong acids,bases and extremely high and low ionic strength samplesdevelop liquid junction potentials different from typical cali-brating buffer solutions.(2) Where these conditions exist, the1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct re

14、sponsibility of Subcommittee D19.03 for Sampling of Water andWater-Formed Deposits, Surveillance of Water, and Flow Measurement of Water.Current edition approved May 1, 2009. Published June 2009. Originallyapproved in 2000. Last previous edition approved in 2005 as D 6569 05.2For referenced ASTM sta

15、ndards, 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

16、 this standard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.most stable junction potential is obtained using a flowingjunction reference electrodeone that requires refilling withelectrolyte solution. However, providing positive fl

17、ow ofelectrolyte through the reference junction places limitations onthe sample pressure that can be tolerated. Follow manufactur-ers recommendations.6.3 pH reference electrodes must not be allowed to dry.Electrolyte salts can crystallize in the liquid junction andproduce a high liquid junction impe

18、dance. Subsequent pHmeasurements could be noisy, drifting or off-scale. When pHsensors are not in use, they should be typically stored wet permanufacturers instructions.6.4 There are several temperature effects on pH measure-ment. The pH electrode signal is described by the Nernstequation with its o

19、utput proportional to the absolute tempera-ture times the pH deviation from the isopotential pointusually 7 pH for glass electrodes. Compensation for this effectmay be accomplished automatically with a temperature sensorintegral to the combination pH probe and an algorithm in theinstrument. Alternat

20、ively, some instruments may be set manu-ally for a fixed temperature when a temperature signal is notavailable. Errors caused by deviations from the manual settingmay be calculated from the following (for a conventional glasselectrode system with 7 pH isopotential point).Glass Electrode pH error 5pH

21、 7! 3 T Tf!Tf 1 273(1)where:pH = uncorrected process pHT = process temperature (C)Tf = temperature setting of fixed compensation (C)Other types of electrodes, (antimony, ISFET) have differentisopotential points and therefore different corrections. Consultthe manufacturer.6.5 Solution temperature eff

22、ects may be caused by changesin the sample, such as ionization of constituents, off-gassing,and precipitation, which occur with changes in temperature.These are generally small for many samples over moderatetemperature ranges. In waste streams with variation in compo-sition, such effects are usually

23、 not predictable. However, forsamples with uniform or predictable composition with tem-perature changes 5C, one may determine the effect for thesamples being measured and make the correction on allmeasurements. The pH to be reported is referenced to 25Cunless another temperature is specified. Some p

24、rocess instru-ments have built-in solution temperature compensation whichallows entry of a user-defined linear temperature coefficientinto instrument memory for on-line correction of this effect.The temperature of the solution measured for pH should bemonitored and recorded since this information ma

25、y be criticalto understanding the base state of the solution.NOTE 1For regulatory monitoring, correction for solution tempera-ture effects should not be done without consulting the governing authority.6.6 A small temperature influence can occur due to differ-ences in the composition of measuring and

26、 reference half-cells.This is not compensated by any instrumentation. For thisreason it is advisable to calibrate as near the measuringtemperature as possible.6.7 Coating of the measuring electrode may produce a slowor erroneous response since the sensing surface is in contactwith the coating layer

27、rather than the bulk sample. Flat surfaceelectrodes and high sample flow velocity have been found toprovide some self-cleaning effects. Cleaning may be accom-plished manually using solvents, acids, detergents, etc. Clean-ing may be automated by a number of approaches. SeeAppendix X1.6.8 Abrasion of

28、measuring electrode surfaces from particlesin the sample can shorten sensor life. Where abrasive particlesare present, the flow velocity past the electrode surface shouldbe controlled low enough to minimize abrasion and providesatisfactory electrode life yet high enough to prevent particlesfrom accu

29、mulating into a coating as in 6.7.6.9 High pH conditions can produce an alkaline error as theglass pH sensor responds to sodium or other small cations inaddition to hydrogen. This type of error is greater at highertemperatures. The result is always a negative error in the rangeof 0 to -1 pH dependin

30、g on the pH, temperature, sodiumconcentration and sensor glass formulation. Some manufactur-ers have characterized the alkaline or sodium error sufficientlyto closely estimate those errors. Some process ISFET elec-trodes do not experience these errors.6.10 While fluorides in the sample do not interf

31、ere with themeasurement, if present at pH below 5, they attack silica,greatly shortening the life of glass and ISFET electrodes.6.11 Antimony electrode measurements are subject to majorinterferences from oxidizing or reducing species, non-linearity,irregular temperature characteristics and the physi

32、cal conditionof the electrode surface. However, the antimony electrode canwithstand hydrofluoric acid which other electrodes cannot andthis application is its primary use. The typical useful range ofthe antimony electrode is 3-9 pH. Performance is veryapplication-dependent and should be carefully ev

33、aluated.6.12 Electrical noise induced on the pH sensor-to-instrument cable can cause erratic and offset readings. RoutepH signal cables separately from AC power and switchingcircuit wiring.6.13 Electrical insulation leakage in electrode connectorsand cable or cracking of a glass electrode membrane c

34、an causethe high impedance pH signal to be attenuated or completelylost. This results in a dead response where the measurementsystem will not give response away from the calibration point.Keep pH signal cables and connectors clean and dry. Pream-plifiers are normally located close to pH sensors to m

35、inimizethe distance high impedance signals must be transportedahelp in minimizing noise interference in 6.12 as well.6.14 Ground loop interference can occur if the pH measur-ing circuit is not galvanically isolated from earth ground,except for the electrodes themselves. Such interference cangive an

36、offset or off-scale reading when measuring in agrounded process installation but will give satisfactory re-sponse in grab samples or calibration solutions that are notgrounded. Sources of ground loops include improper wiring ofsensor cables, lack of isolation of analog or digital outputsignals from

37、the measuring circuit, or a leaking sensor bodywhich allows electrical contact of the sample to a part of themeasuring circuit beyond the external electrode surfaces.D 6569 05 (2009)2Remove output wiring, check sensor wiring and observereadings to locate the cause of grounding problems.6.15 Measurem

38、ents on samples with conductivity less than100 S/cm are vulnerable to streaming potentials, large junc-tion potentials and other difficulties and are beyond the scopeof this method. Use Test Method D 5128.7. Apparatus7.1 Process instrument7.1.1 The measuring system shall use a high impedancepreampli

39、fier, preferably located near the electrode but may becontained within the instrument, capable of measuring the highimpedance pH sensor voltage. When located near the electrode,the preamplifier shall be sealed against moisture intrusion. Aglass pH electrode measuring circuit must have at least 105Me

40、gohm input impedance to preserve the signal. Some mea-suring circuits use a differential input and solution groundwhich can tolerate a higher reference junction impedance andreduce liquid junction potential errors.7.1.2 The instrument shall provide indication, alarms, re-lays, isolated analog output

41、s and digital outputs as needed forthe application. Where output signal isolation from the mea-surement circuit is not provided within the instrument, thesignal must pass through an external signal isolator beforeconnection to a grounded computer, data acquisition or controlsystem. This will prevent

42、 ground loop errors in the measure-ment as described in 6.14.7.1.3 Some instruments provide as a part of their measuringcircuit, sensor diagnostics which check the impedance of theglass electrode, reference electrode or both to assure theirintegrity.7.2 Process electrodesAlthough measuring and refer

43、enceelectrodes and the temperature compensator are describedindividually below, they may also be constructed into a singleprobe housing, frequently called a combination electrode. Thedifferent types of measuring electrodes and reference elec-trodes below are options: only one measuring electrode and

44、one reference electrode are used for measurement.7.2.1 Glass measuringThe pH glass measuring electrodeis by far the most common type of pH sensor. It shall have arepeatable response as given in Test Method D 1293. It shallhave pH, temperature and pressure ratings suitable for theprocess conditions.

45、It shall be conditioned in the processsample for at least 30 minutes or as recommended by themanufacturer before accurate readings can be taken.7.2.2 ISFET measuringThe ISFET measuring electrodealong with its unique measuring circuit shall give responseequivalent to a glass electrode measuring syste

46、m. (ISFETelectrodes typically require an adapter circuit to be compatiblewith glass electrode measuring instruments.)7.2.3 Antimony measuringThe antimony measuring elec-trode shall be pure polished antimony metal that has beenconditioned by soaking in water to produce an oxide layer,according to man

47、ufacturers instructions.7.2.4 Non-flowing Liquid Junction Reference7.2.4.1 The non-flowing reference electrode shall contain anelectrode half-cell similar to the glass measuring electrode, ifused, to cancel the temperature effects of the half-cells. It shallcontain sufficient electrolyte with gellin

48、g agent or other meansto restrict its loss and give acceptable life in the application.Despite the name “non-flowing,” the electrolyte is consumableas a trace amount of it diffuses through the junction into thesample. The only opening of the electrode is its interface withthe process through its liq

49、uid junctiona small passage ofporous ceramic, polymer, wood, fiber, ground glass surfaces orother material that allows electrical continuity with the samplewhile limiting loss of electrolyte. Some non-flowing referenceelectrodes are refillable.7.2.4.2 For fouling processes containing sulfides, or otherspecies that could react with the electrolyte, a second or doubleliquid junction shall be provided as a barrier to contaminationor dilution of the inner electrolyte. A long path between theliquid junction and the inner half-cell is also helpful. Someelectrode