1、Designation: D1498 14Standard Test Method forOxidation-Reduction Potential of Water1This standard is issued under the fixed designation D1498; 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 p
2、arentheses 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 apparatus and procedure forthe electrometric measurement of oxidation-reduction potential(ORP) in water. It does not d
3、eal with the manner in which thesolutions are prepared, the theoretical interpretation of theoxidation-reduction potential, or the establishment of a stan-dard oxidation-reduction potential for any given system. Thetest method described has been designed for the routine andprocess measurement of oxi
4、dation-reduction potential.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 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
5、 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:2D1129 Terminology Relating to WaterD1193 Specification for Reagent WaterD2777 Practice for Determination of Precision and
6、 Bias ofApplicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed Conduits3. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D1129.3.2 Definitions of Terms Specific to This Standard:3.2.1 oxidation-reduction poten
7、tial, nthe electromotiveforce, Em, developed between a noble metal electrode and astandard reference electrode.3.2.1.1 DiscussionThis oxidation-reduction potential(ORP) is related to the solution composition by:Em5 Eo12.3RTnFlogAox/Ared#where:Em= ORP,Eo= constant that depends on the choice of refer-
8、ence electrodes,F = Faraday constant,R = gas constant,T = absolute temperature, C + 273.15,n = number of electrons involved in processreaction, andAoxand Ared= activities of the reactants in the process.4. Summary of Test Method4.1 This is a test method designed to measure the ORPwhich is defined as
9、 the electromotive force between a noblemetal electrode and a reference electrode when immersed in asolution. The test method describes the equipment available tomake the measurement, the standardization of the equipmentand the procedure to measure ORP. The ORP electrodes areinert and measure the ra
10、tio of the activities of the oxidized tothe reduced species present.5. Significance and Use5.1 Various applications include monitoring thechlorination/dechlorination process of water, recognition ofoxidants/reductants present in wastewater, monitoring thecycle chemistry in power plants, and controll
11、ing the processingof cyanide and chrome waste in metal plating baths.5.2 The measurement of ORP has been found to be useful inthe evaluation of soils, for evaluating treatment design data atsites contaminated with certain chemicals, and in evaluatingsolid wastes.6. Interferences6.1 The ORP electrode
12、s reliably measure ORP in nearly allaqueous solutions and in general are not subject to solutioninterference from color, turbidity, colloidal matter, and sus-pended matter.6.2 The ORP of an aqueous solution is sensitive to changein temperature of the solution, but temperature correction is1This test
13、 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 Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved Feb. 1
14、5, 2014. Published March 2014. Originallyapproved in 1957. Last previous edition approved in 2008 as D1498 08. DOI:10.1520/D1498-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume inform
15、ation, 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 States1rarely done due to its minimal effect and complex reactions.Temperature corrections are usually applied only when i
16、t isdesired to relate the ORP to the activity of an ion in thesolutions.6.3 The ORP of an aqueous solution is almost alwayssensitive to pH variations even to reactions that do not appearto involve hydrogen or hydroxyl ions. The ORPgenerally tendsto increase with an increase in hydrogen ions and to d
17、ecreasewith an increase in hydroxyl ions during such reactions.6.4 Reproducible oxidation-reduction potentials cannot beobtained for chemical systems that are not reversible. Mostnatural and ground waters do not contain reversible systems, ormay contain systems that are shifted by the presence of ai
18、r. Themeasurement of end point potential in oxidation-reductiontitration is sometimes of this type.6.5 If the metallic portion of the ORP electrode is sponge-like, materials absorbed from solutions may not be washedaway, even by repeated rinsings. In such cases, the electrodemay exhibit a memory eff
19、ect, particularly if it is desired todetect a relatively low concentration of a particular speciesimmediately after a measurement has been made in a relativelyconcentrated solution. A brightly polished metal electrodesurface is required for accurate measurements.6.6 The ORP resulting from interactio
20、ns among severalchemical systems present in mixed solutions may not beassignable to any single chemical.7. Apparatus7.1 MeterMost laboratory pH meters can be used formeasurements of ORP by substitution of an appropriate set ofelectrodes and meter scale. Readability to 1 mV is adequate.The choice wil
21、l depend on the accuracy desired in thedetermination.7.1.1 Most process pH meters can be used for measurementof ORP by substitution of an appropriate set of electrodes andmeter scale. These instruments are generally much morerugged than those which are used for very accurate measure-ments in the lab
22、oratory. Usually, these more rugged instru-ments produce results that are somewhat less accurate andprecise than those obtained from laboratory instruments. Thechoice of process ORP analyzer is generally based on howclosely the characteristics of the analyzer match the require-ments of the applicati
23、on. Typical factors which may beconsidered include, for example, the types of signals which theanalyzer can produce to drive external devices, and the spanranges available.7.1.2 For remote ORPmeasurements the potential generatedcan be transmitted to an external indicating meter. Specialshielded cabl
24、e is required to transmit the signal.7.2 Reference ElectrodeA calomel, silver-silver chloride,or other reference electrode of constant potential shall be used.If a saturated calomel electrode is used, some potassiumchloride crystals shall be contained in the saturated potassiumchloride solution. If
25、the reference electrode is of the flowingjunction type, a slow outward flow of the reference-electrodesolution is desired. To achieve this, the solution pressure insidethe liquid junction should be somewhat in excess of thatoutside the junction. In nonpressurized applications this re-quirement can b
26、e met by maintaining the inside solution levelhigher than the outside solution level. If the reference electrodeis of the nonflowing junction type, these outward flow andpressurization considerations shall not apply. The referenceelectrode and junction shall perform satisfactorily as requiredin the
27、procedure for checking sensitivity described in 11.2.7.3 Oxidation-Reduction ElectrodeA noble metal is usedin the construction of oxidation-reduction electrodes. The mostcommon metals employed are platinum and gold; silver israrely used. It is important to select a metal that is not attackedby the t
28、est solution. The construction of the electrode shall besuch that only the noble metal comes in contact with the testsolution. The area of the noble metal in contact with the testsolution should be approximately 1 cm2.7.4 Electrode AssemblyAconventional electrode holder orsupport can be employed for
29、 laboratory measurements. Manydifferent styles of electrode holders are suitable for variousprocess applications such as measurements in an open tank,process pipe line, pressure vessel, or a high pressure sampleline.8. Reagents and Materials8.1 Purity of ReagentsReagent grade chemicals shall beused
30、in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to the specifications of the Commit-tee onAnalytical Reagents of theAmerican Chemical Society.3Other grades may be used, provided it is first ascertained thatthe reagent is of sufficiently high purity to permit i
31、ts usewithout lessening the accuracy of the determination.8.2 Purity of WaterReferences to water that is used forreagent preparation, rinsing, or dilution shall be understood tomean water that conforms to the quantitative specifications fortype I or II reagent water of Specification D1193.8.3 Aqua R
32、egiaMix 1 volume of concentrated nitric acid(HNO3, sp gr 1.42) with 3 volumes of concentrated hydrochlo-ric acid (HCl, sp gr 1.18). It is recommended that only enoughsolution be prepared for immediate requirements.8.4 Buffer Standard SaltsTable 1 lists the buffer salts3Reagent Chemicals, American Ch
33、emical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see the United States Pharmacopeia andNational Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,MD.TABLE 1 National Instit
34、ute of Standards and Technology (NIST)Materials for Reference Buffer SolutionsNIST StandardSampleDesignationABuffer Salt Drying Procedure186-II-e disodium hydrogen phosphate 2 h in oven at 130C186-I-e potassium dihydrogen phosphate 2 h in oven at 130C185-g potassium hydrogen phthalate drying not nec
35、essaryAThe buffer salts listed can be purchased from the Office of Standard ReferenceMaterials, National Institute of Standards and Technology (NIST), Gaithersburg,MD 20899.D1498 142available from the National Institute of Standards and Tech-nology specifically for the preparation of standard buffer
36、solutions. The NIST includes numbers and drying procedures.8.4.1 Phthalate Reference Buffer Solution (pHs= 4.00 at25C)Dissolve 10.12 g of potassium hydrogen phthalate(KHC8H4O4) in water and dilute to 1 L.8.4.2 Phosphate Reference Buffer Solution (pHs= 6.86 at25C)Dissolve 3.39 g of potassium dihydrog
37、en phosphate(KH2PO4) and 3.53 g of anhydrous disodium hydrogen phos-phate (Na2HPO4) in water and dilute to 1 L.8.5 Chromic Acid Cleaning SolutionDissolve about5gofpotassium dichromate (K2Cr2O7) in 500 mL of concentratedsulfuric acid (H2SO4, sp gr 1.84).8.6 DetergentUse any commercially available “lo
38、w-suds”liquid or solid detergent.8.7 Nitric Acid (1+1)Mix equal volumes of concentratednitric acid (HNO3, sp gr 1.42) and water.8.8 Redox Standard Solution; Ferrous-Ferric ReferenceSolution4Dissolve 39.21 g of ferrous ammonium sulfate(Fe(NH4)2-(SO4)26H2O), 48.22 g of ferric ammonium sulfate(FeNH4(SO
39、4)212H2O) and 56.2 mL of sulfuric acid (H2SO4,sp gr 1.84) in water and dilute to 1 L. It is necessary to preparethe solution using reagent grade chemicals that have an assayconfirming them to be within 1% of the nominal composition.The solution should be stored in a closed glass or plasticcontainer.
40、8.8.1 The ferrous-ferric reference solution is a reasonablystable solution with a measurable oxidation-reduction poten-tial. Table 2 presents the potential of the platinum electrode forvarious reference electrodes at 25C in the standard ferrous-ferric solution.8.9 Redox Reference Quinhydrone Solutio
41、nsMix 1 L ofpH 4 buffer solution, (see 8.4.1), with 10 g of quinhydrone.Mix 1 L of pH 7 buffer solution, (see 8.4.2), with 10 g ofquinhydrone. Be sure that excess quinhydrone is used in eachsolution so that solid crystals are always present. Thesereference solutions are stable for only 8 h. Table 3
42、lists thenominal millivolt redox readings for the quinhydrone referencesolutions at temperatures of 20C, 25C, and 30C.8.10 Redox Standard Solution; Iodide/TriiodideDissolve664.04 g of potassium iodide (KI), 1.751 g of resublimed I2,12.616 g of boric acid (H3BO3), and 20 ml of 1 M potassiumhydroxide
43、(KOH) in water and dilute to 1 L. Mix solution. Thissolution is stable at least one year. Solution can be stored in aclosed glass or plastic container. Table 4 provides the potentialof the platinum electrode for various reference electrodes atvarious temperatures in the standard Iodide/Triiodide sol
44、ution.9. Sampling9.1 Collect the samples in accordance with PracticesD3370.10. Preparation10.1 Electrode TreatmentCondition and maintain ORPelectrodes as recommended by the manufacturer. If the assem-bly is in intermittent use, the immersible ends of the electrodeshould be kept in water between meas
45、urements. Cover thejunctions and fill-holes of reference electrodes to reduceevaporation during prolonged storage.10.2 ORP Electrode CleaningIt is desirable to clean theelectrode daily. Remove foreign matter by a preliminarytreatment with a detergent or mild abrasive, such as toothpaste.If this is i
46、nsufficient, use 1 + 1 nitric acid. Rinse the electrodein water several times. An alternative cleaning procedure is toimmerse the electrode in chromic acid cleaning mixture atroom temperature for several minutes, then rinse with dilutehydrochloric acid, and then thoroughly rinse with water. Ifthese
47、steps are insufficient, immerse the ORP electrode inwarm (70C) aqua regia and allow to stand for 1 min. Thissolution dissolves noble metal and should not be used longerthan the time specified. In these cleaning operations, particularcare must be exercised to protect glass-metal seals from suddenchan
48、ges of temperature, which might crack them.11. Standarization11.1 Turn on meter according to manufacturers instruc-tions. Check zero on meter by shorting the input connection.The reading should be less than 60.5 mV.11.2 Checking the Response of the Electrode to StandardRedox Solutions (see 8.8, 8.9
49、and 8.10)Wash the electrodeswith three changes of water or by means of a flowing streamfrom a wash bottle. Use one or more of the solutions from 8.8,8.9, and 8.10 to check the response of the electrode. Fill thesample container with fresh redox standard solution andimmerse the electrodes. The reading should be within 30 mV ofthe value expected for the standard solution. Repeat themeasurement with fresh solution. The second reading shouldnot differ from the first by more than 10 mV.12. Procedure12.1 After the electrode/meter assembly has been stand
