1、Designation: D 1498 07Standard Test Method forOxidation-Reduction Potential of Water1This standard is issued under the fixed designation D 1498; 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
2、 parentheses 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 apparatus and procedure forthe electrometric measurement of oxidation-reduction potential(ORP) in water. It does no
3、t deal 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
4、oxidation-reduction potential.1.2 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 applica-bility of regulatory limitation
5、s prior to use.2. Referenced Documents2.1 ASTM Standards:2D 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed Conduits3. Te
6、rminology3.1 Definitions of Terms Specific to This Standard:3.1.1 oxidation-reduction potentialthe electromotiveforce, Em, developed between a noble metal electrode and astandard reference electrode. This oxidation-reduction poten-tial (ORP) is related to the solution composition by:Em5 Eo1 2.3RTnFl
7、ogAox/ Ared#where:Em= ORP,Eo= constant that depends on the choice ofreference 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.3.2 For definitions of o
8、ther items used in this test method,refer to Terminology D 1129.4. Summary of Test Method4.1 This is a test method designed to measure the ORPwhich is defined as the electromotive force between a noblemetal electrode and a reference electrode when immersed in asolution. The test method describes the
9、 equipment available tomake the measurement, the standardization of the equipmentand the procedure to measure ORP. The ORP electrodes areinert and measure the ratio of the activities of the oxidized tothe reduced species present.5. Significance and Use5.1 Various applications include monitoring thec
10、hlordination/dechlorination process of water, recgonition ofoxidants/reductants present in wastewater, monitoring thecycle chemistry in power plants, and controlling the processingof cyanide and chrome waste in metal plating baths.5.2 The measurement of ORP has been found to be useful inthe evaluati
11、on of soils, for evaluating treatment design data atsites contaminated with certain chemicals, and in evaluatingsolid wastes.6. Interferences6.1 The ORP electrodes reliably measure ORP in nearly allaqueous solutions and in general are not subject to solutioninterference from color, turbidity, colloi
12、dal matter, and sus-pended matter.6.2 The ORP of an aqueous solution is sensitive to changein temperature of the solution, but temperature correction israrely done due to its minimal effect and complex reactions.1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the dir
13、ect 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 WaterCurrent edition approved June 15, 2007. Published July 2007. Originallyapproved in 1957. Last previous approve
14、d in 2000 as D 1498 00.2For referenced ASTM standards, 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.1Copyright ASTM International, 100 Bar
15、r Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Temperature corrections are usually applied only when it 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
16、that do not appearto involve hydrogen or hydroxyl ions. The ORPgenerally tendsto increase with an increase in hydrogen ions and to decreasewith an increase in hydroxyl ions during such reactions.6.4 Reproducible oxidation-reduction potentials cannot beobtained for chemical systems that are not rever
17、sible. Mostnatural and ground waters do not contain reversible systems, ormay contain systems that are shifted by the presence of air. 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, mate
18、rials absorbed from solutions may not be washedaway, even by repeated rinsings. In such cases, the electrodemay exhibit a memory effect, particularly if it is desired todetect a relatively low concentration of a particular speciesimmediately after a measurement has been made in a relativelyconcentra
19、ted solution. A brightly polished metal electrodesurface is required for accurate measurements.6.6 The ORP resulting from interactions among severalchemical systems present in mixed solutions may not beassignable to any single chemical.7. Apparatus7.1 MeterMost laboratory pH meters can be used forme
20、asurements of ORP by substitution of an appropriate set ofelectrodes and meter scale. Readability to 1 mV is adequate.The choice will 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
21、andmeter scale. These instruments are generally much morerugged than those which are used for very accurate measure-ments in the laboratory. Usually, these more rugged instru-ments produce results that are somewhat less accurate andprecise than those obtained from laboratory instruments. Thechoice o
22、f process ORP analyzer is generally based on howclosely the characteristics of the analyzer match the require-ments of the application. Typical factors which may beconsidered include, for example, the types of signals which theanalyzer can produce to drive external devices, and the spanranges availa
23、ble.7.1.2 For remote ORPmeasurements the potential generatedcan be transmitted to an external indicating meter. Specialshielded cable 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 satu
24、rated calomel electrode is used, some potassiumchloride crystals shall be contained in the saturated potassiumchloride solution. If the reference electrode is of the flowingjunction type, a slow outward flow of the reference-electrodesolution is desired. To achieve this, the solution pressure inside
25、the liquid junction should be somewhat in excess of thatoutside the junction. In nonpressurized applications this re-quirement can be met by maintaining the inside solution levelhigher than the outside solution level. If the reference electrodeis of the nonflowing junction type, these outward flow a
26、ndpressurization considerations shall not apply. The referenceelectrode and junction shall perform satisfactorily as requiredin the procedure for checking sensitivity described in 11.2.7.3 Oxidation-Reduction ElectrodeA noble metal is usedin the construction of oxidation-reduction electrodes. The mo
27、stcommon metals employed are platinum and gold; silver israrely used. It is important to select a metal that is not attackedby the test 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 wi
28、th the testsolution should be approximately 1 cm2.7.4 Electrode AssemblyA conventional electrode holderor support can be employed for laboratory measurements.Many different styles of electrode holders are suitable forvarious process applications such as measurements in an opentank, process pipe line
29、, pressure vessel, or a high pressuresample line.8. Reagents and Materials8.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to the specifications of the Commit-tee onAnalytical Reagents of theAmerican Ch
30、emical Society.3Other grades may be used, provided it is first ascertained thatthe reagent is of sufficiently high purity to permit its usewithout lessening the accuracy of the determination.8.2 Purity of WaterUnless otherwise indicated, referenceto water shall be understood to mean reagent water co
31、nformingto Specification D 1193, Type I or II.8.3 Aqua RegiaMix 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
32、 1 lists the buffer saltsavailable from the National Institute of Standards and Tech-nology specifically for the preparation of standard buffersolutions. 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
33、phthalate(KHC8H4O4) in water and dilute to 1 L.3Reagent Chemicals, American Chemical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole
34、, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,MD.TABLE 1 National Institute of Standards and Technology (NIST)Materials for Reference Buffer SolutionsNIST StandardSampleDesignationABuffer Salt Drying Procedure186-II-e
35、 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 necessaryAThe buffer salts listed can be purchased from the Office of Standard ReferenceMaterials, National Institute of Standards and Technology (N
36、IST), Gaithersburg,MD 20899.D 1498 0728.4.2 Phosphate Reference Buffer Solution (pHs= 6.86 at25C)Dissolve 3.39 g of potassium dihydrogen 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 about5gofpot
37、assium dichromate (K2Cr2O7) in 500 mL of concentratedsulfuric acid (H2SO4, sp gr 1.84).8.6 DetergentUse any commercially available “low-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
38、-Ferric ReferenceSolution4Dissolve 39.21 g of ferrous ammonium sulfate(Fe(NH4)2-(SO4)26H2O), 48.22 g of ferric ammonium sulfate(FeNH4(SO4)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
39、 an assayconfirming them to be within 1% of the nominal composition.The solution should be stored in a closed glass or plasticcontainer.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 p
40、latinum electrode forvarious reference electrodes at 25C in the standard ferrous-ferric solution.8.9 Redox Reference Quinhydrone SolutionsMix 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 q
41、uinhydrone is used in eachsolution so that solid crystals are always present. Thesereference solutions are stable for only 8 h. Table 3 lists thenominal millivolt redox readings for the quinhydrone referencesolutions at temperatures of 20C, 25C, and 30C.8.10 Redox Standard Solution; Iodide/Triiodide
42、Dissolve664.04 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 (KOH) in water and dilut to 1 L. Mix solution. Thissolution is stable at least one year. Solution can be stored in aclosed glass or plastic container. Table 4 provi
43、des the potentialof the platinum electrode for various reference electrodes atvarious temperatures in the standard Iodide/Triiodide solution.9. Sampling9.1 Collect the samples in accordance with PracticesD 3370.10. Preparation10.1 Electrode TreatmentCondition and maintain ORPelectrodes as recommende
44、d by the manufacturer. If the assem-bly is in intermittent use, the immersible ends of the electrodeshould be kept in water between measurements. Cover thejunctions and fill-holes of reference electrodes to reduceevaporation during prolonged storage.10.2 ORP Electrode CleaningIt is desirable to clea
45、n theelectrode daily. Remove foreign matter by a preliminarytreatment with a detergent or mild abrasive, such as toothpaste.If this is insufficient, use 1 + 1 nitric acid. Rinse the electrodein water several times. An alternative cleaning procedure is toimmerse the electrode in chromic acid cleaning
46、 mixture atroom temperature for several minutes, then rinse with dilutehydrochloric acid, and then thoroughly rinse with water. Ifthese 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 lo
47、ngerthan the time specified. In these cleaning operations, particularcare must be exercised to protect glass-metal seals from suddenchanges of temperature, which might crack them.11. Standarization11.1 Turn on meter according to manufacturers instructions.Check zero on meter by shorting the input co
48、nnection. Thereading should be less than 60.5 mV.11.2 Checking the Response of the Electrode to StandardRedox Solutions (see 8.8, 8.9 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 fromsections 8.8, 8.9 and
49、8.10 to check the response of theelectrode. Fill the sample container with fresh redox standardsolution and immerse the electrodes. The reading should bewithin 30 mV of the value expected for the standard solution.Repeat the measurement with fresh solution. The secondreading should not differ from the first by more than 10 mV.12. Procedure12.1 After the electrode/meter assembly has been standard-ized as described in 11, wash the electrodes with three changesof water or by means of a flowing stream from a wash bottle.Place the sample in a clean glass b
