1、Designation: D 6569 05Standard 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 number in parentheses
2、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 continuous determination ofpH of water by electrometric measurement using the glass, theantimony or the ion-selective field-effe
3、ct 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 This standard does n
4、ot 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 Documents2.1 ASTM Standar
5、ds:2D 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 1293 Test Methods for pH of WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Methods of Committee D-19 on WaterD 3370 Standard Practices for Sampling Water from ClosedConduitsD 3864 Guide for Con
6、tinual On-Line Monitoring Systemsfor Water AnalysisD 5128 Test Method for On-Line pH Measurement of Waterof Low Conductivity3. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D 1129, Method D 1293 andPractice D 3864.3.2 Definitions of Terms Specific t
7、o 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 solution. Ideally this potential isnear zero and is stable. However, in samples with extreme pHit becomes larger by an unknown amount and
8、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 assemblytypically includes a temperature compensator to compensatefor the varying output of the measuring electrode due totemperature.4.2 The sensor sig
9、nals are processed with an industrial pHanalyzer/transmitter.4.3 The equipment is calibrated with standard pH buffersolutions encompassing or in close proximity to the anticipatedpH measurement range.5. Significance and Use5.1 pH is a measure of the hydrogen ion activity in water. Itis a major param
10、eter affecting the corrosivity and scalingproperties of water, biological life in water and many applica-tions of chemical process control. It is therefore important inwater purification, use and waste treatment before release tothe environment.5.2 On-line pH measurement is preferred over laboratory
11、measurement to obtain real time, continuous values for auto-matic control and monitoring purposes.6. Interferences6.1 Pressure and temperature variations may force processsample into the liquid junction of non-flowing junction refer-ence electrodes and cause changes in the junction potential.Estimat
12、es 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 the sample. Strong acids,bases and extremely high and low ionic strength samplesdevelop liquid junction potentials different from typical
13、cali-brating buffer solutions.(2) Where these conditions exist, themost stable junction potential is obtained using a flowing1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 for Sampling of Water andWater-Formed Deposit
14、s, Surveillance of Water, and Flow Measurement of Water.Current edition approved June 1, 2005. Published July 2005. Originally approvedin 2000. Last previous edition approved in 2000 as D 6569 00.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at
15、 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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700
16、, West Conshohocken, PA 19428-2959, United States.junction reference electrodeone that requires refilling withelectrolyte solution. However, providing positive flow ofelectrolyte through the reference junction places limitations onthe sample pressure that can be tolerated. Follow manufactur-ers reco
17、mmendations.6.3 pH reference electrodes must not be allowed to dry.Electrolyte salts can crystallize in the liquid junction andproduce a high liquid junction impedance. Subsequent pHmeasurements could be noisy, drifting or off-scale. When pHsensors are not in use, they should be typically stored wet
18、 permanufacturers instructions.6.4 There are several temperature effects on pH measure-ment. The pH electrode signal is described by the Nernstequation with its output proportional to the absolute tempera-ture times the pH deviation from the isopotential pointusually 7 pH for glass electrodes. Compe
19、nsation for this effectmay be accomplished automatically with a temperature sensorintegral to the combination pH probe and an algorithm in theinstrument. Alternatively, some instruments may be set manu-ally for a fixed temperature when a temperature signal is notavailable. Errors caused by deviation
20、s from the manual settingmay be calculated from the following (for a conventional glasselectrode system with 7 pH isopotential point).Glass Electrode pH error 5pH 7! 3 T Tf!Tf 1 273(1)where:pH = uncorrected process pHT = process temperature (C)Tf = temperature setting of fixed compensation (C)Other
21、types of electrodes, (antimony, ISFET) have differentisopotential points and therefore different corrections. Consultthe manufacturer.6.5 Solution temperature effects may be caused by changesin the sample, such as ionization of constituents, off-gassing,and precipitation, which occur with changes in
22、 temperature.These are generally small for many samples over moderatetemperature ranges. In waste streams with variation in compo-sition, such effects are usually not predictable. However, forsamples with uniform or predictable composition with tem-perature changes 5C, one may determine the effect f
23、or thesamples being measured and make the correction on allmeasurements. The pH to be reported is referenced to 25Cunless another temperature is specified. Some process instru-ments have built-in solution temperature compensation whichallows entry of a user-defined linear temperature coefficientinto
24、 instrument memory for on-line correction of this effect.The temperature of the solution measured for pH should bemonitored and recorded since this information may be criticalto understanding the base state of the solution.NOTE 1For regulatory monitoring, correction for solution tempera-ture effects
25、 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 reference half-cells.This is not compensated by any instrumentation. For thisreason it is advisable to calibrate as near the measuringtem
26、perature 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 rather than the bulk sample. Flat surfaceelectrodes and high sample flow velocity have been found toprovide some self-cleaning effects. Cl
27、eaning 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 measuring electrode surfaces from particlesin the sample can shorten sensor life. Where abrasive particlesare present, the flow velocity p
28、ast the electrode surface shouldbe controlled low enough to minimize abrasion and providesatisfactory electrode life yet high enough to prevent particlesfrom accumulating 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 s
29、mall 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 depending on the pH, temperature, sodiumconcentration and sensor glass formulation. Some manufactur-ers have characterized the alkaline or sodium
30、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 interfere with themeasurement, if present at pH below 5, they attack silica,greatly shortening the life of glass and ISFET electrodes.6.11 Antim
31、ony electrode measurements are subject to majorinterferences from oxidizing or reducing species, non-linearity,irregular temperature characteristics and the physical conditionof the electrode surface. However, the antimony electrode canwithstand hydrofluoric acid which other electrodes cannot andthi
32、s application is its primary use. The typical useful range ofthe antimony electrode is 3-9 pH. Performance is veryapplication-dependent and should be carefully evaluated.6.12 Electrical noise induced on the pH sensor-to-instrument cable can cause erratic and offset readings. RoutepH signal cables se
33、parately from AC power and switchingcircuit wiring.6.13 Electrical insulation leakage in electrode connectorsand cable or cracking of a glass electrode membrane can causethe high impedance pH signal to be attenuated or completelylost. This results in a dead response where the measurementsystem will
34、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 minimizethe distance high impedance signals must be transportedahelp in minimizing noise interference in 6.12 as well.6.14 Ground loop inte
35、rference can occur if the pH measur-ing circuit is not galvanically isolated from earth ground,except for the electrodes themselves. Such interference cangive an offset or off-scale reading when measuring in agrounded process installation but will give satisfactory re-sponse in grab samples or calib
36、ration solutions that are notgrounded. Sources of ground loops include improper wiring ofsensor cables, lack of isolation of analog or digital outputsignals from the measuring circuit, or a leaking sensor bodywhich allows electrical contact of the sample to a part of themeasuring circuit beyond the
37、external electrode surfaces.D6569052Remove output wiring, check sensor wiring and observereadings to locate the cause of grounding problems.6.15 Measurements on samples with conductivity less than100 S/cm are vulnerable to streaming potentials, large junc-tion potentials and other difficulties and a
38、re beyond the scopeof this method. Use Method D 5128.7. Apparatus7.1 Process instrument7.1.1 The measuring system shall use a high impedancepreamplifier, preferably located near the electrode but may becontained within the instrument, capable of measuring the highimpedance pH sensor voltage. When lo
39、cated near the electrode,the preamplifier shall be sealed against moisture intrusion. Aglass pH electrode measuring circuit must have at least 105Megohm input impedance to preserve the signal. Some mea-suring circuits use a differential input and solution groundwhich can tolerate a higher reference
40、junction impedance andreduce liquid junction potential errors.7.1.2 The instrument shall provide indication, alarms, re-lays, isolated analog outputs and digital outputs as needed forthe application. Where output signal isolation from the mea-surement circuit is not provided within the instrument, t
41、hesignal must pass through an external signal isolator beforeconnection to a grounded computer, data acquisition or controlsystem. This will prevent 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 whic
42、h check the impedance of theglass electrode, reference electrode or both to assure theirintegrity.7.2 Process electrodesAlthough measuring and referenceelectrodes and the temperature compensator are describedindividually below, they may also be constructed into a singleprobe housing, frequently call
43、ed a combination electrode. Thedifferent types of measuring electrodes and reference elec-trodes below are options: only one measuring electrode andone 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 sh
44、all have arepeatable response as given in D 1293. It shall have pH,temperature and pressure ratings suitable for the processconditions. It shall be conditioned in the process sample for atleast 30 minutes or as recommended by the manufacturerbefore accurate readings can be taken.7.2.2 ISFET measurin
45、gThe ISFET measuring electrodealong with its unique measuring circuit shall give responseequivalent to a glass electrode measuring system. (ISFETelectrodes typically require an adapter circuit to be compatiblewith glass electrode measuring instruments.)7.2.3 Antimony measuringThe antimony measuring
46、elec-trode shall be pure polished antimony metal that has beenconditioned by soaking in water to produce an oxide layer,according to manufacturers 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 gl
47、ass measuring electrode, ifused, to cancel the temperature effects of the half-cells. It shallcontain sufficient electrolyte with gelling 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 amoun
48、t of it diffuses through the junction into thesample. The only opening of the electrode is its interface withthe process through its liquid junctiona small passage ofporous ceramic, polymer, wood, fiber, ground glass surfaces orother material that allows electrical continuity with the samplewhile li
49、miting 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 systems use another pH glass membrane within thereference electrode in place of a second junction. In that case,the intermediate electrolyte is a concentrated pH buff
