ASTM D6569-2014 4851 Standard Test Method for On-Line Measurement of pH《在线测量pH值的标准试验方法》.pdf
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1、Designation: D6569 14Standard Test Method forOn-Line Measurement of pH1This standard is issued under the fixed designation D6569; 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 in
2、dicates 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-selective field-effect
3、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 D5128.1.3 This test method does not cover laboratory or grabsample measurement of pH. Refer to Test Method D1293.1.4 The values stated in SI u
4、nits 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-priate safety and health pr
5、actices and determine the applica-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 Tes
6、t Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD3864 Guide for On-Line Monitoring Systems for WaterAnalysisD5128 Test Method for On-Line pH Measurement of Waterof Low Conductivity3. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod,
7、 refer to Terminology D1129, Test Method D1293 andGuide D3864.3.2 Definitions of Terms Specific to This Standard:3.2.1 liquid junction potential, nthe dc potential whichappears at the point of contact between the reference elec-trodes salt bridge and the sample solution.3.2.1.1 DiscussionIdeally thi
8、s potential is near zero and isstable. However, in samples with extreme pH it becomes largerby 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 assemblytypically includes a temper
9、ature 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 to the anticipatedpH m
10、easurement 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 important inwater purificat
11、ion, use and waste treatment before release tothe environment.1This 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 Deposits, Surveillance of Water, and Flow Measurement of Water.Current
12、 edition approved Feb. 1, 2014. Published February 2014. Originallyapproved in 2000. Last previous edition approved in 2009 as D6569 05 (2009).DOI: 10.1520/D6569-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Bo
13、ok of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15.2 On-line pH measurement is preferred over laboratorymeasurement to obtain real
14、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.Estimates of 0.2 to 0.5 pH errors
15、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 cali-brating buffer solutio
16、ns (2). Where these conditions exist, themost stable junction potential is obtained using a flowingjunction reference electrodeone that requires refilling withelectrolyte solution. However, providing positive flow ofelectrolyte through the reference junction places limitations onthe sample pressure
17、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 impedance. Subsequent pHmeasurements could be noisy, drifting or off-scale. When pHsensors ar
18、e 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 output proportional to the absolute tempera-ture times the pH deviation from the isopotent
19、ial 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. Alternatively, some instruments may be set manu-ally for a fixed temperature when a temperature s
20、ignal 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 2 7! 3 T 2 Tf!Tf1273(1)where:pH = uncorrected process pH,T = process temperature (C), an
21、dTf = 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 effects may be caused by changesin the sample, such as ionization of constituents,
22、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 incomposition, such effects are usually not predictable. However,for samples with uniform or predictable composition with
23、temperature changes 5C, one may determine the effect forthe samples 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 entr
24、y 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 may be criticalto understanding the base state of the solution.NOTE 1For regulatory mo
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