1、Designation: D 1498 08Standard 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 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 t
5、his 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:2D 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 2777 Practice for Determination of Precisi
6、on and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed Conduits3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 oxidation-reduction potentialthe electromotiveforce, Em, developed between a noble metal electrode and astanda
7、rd reference electrode. This oxidation-reduction poten-tial (ORP) is related to the solution composition by:Em5 Eo1 2.3RTnFlogAox/ 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 o
8、f electrons involved in processreaction, andAoxand Ared= activities of the reactants in the process.3.2 For definitions of other 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 electromot
9、ive 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 ratio of the acti
10、vities of the oxidized tothe reduced species present.5. Significance and Use5.1 Various applications include monitoring thechlordination/dechlorination process of water, recgonition ofoxidants/reductants present in wastewater, monitoring thecycle chemistry in power plants, and controlling the proces
11、singof 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 electrodes reliably mea
12、sure ORP in nearly allaqueous solutions and in general are not subject to solutioninterference from color, turbidity, colloidal matter, and sus-pended matter.1This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling
13、Water andWater-Formed Deposits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved May 1, 2008. Published May 2008. Originallyapproved in 1957. Last previous edition approved in 2007 as D 1498 07.2For referenced ASTM stan
14、dards, 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 Barr Harbor Drive, PO Box C700, West Conshohocken,
15、PA 19428-2959, United States.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.Temperature corrections are usually applied only when it isdesired to relate the ORP to the a
16、ctivity 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 decreasewith an increase in hydroxyl io
17、ns 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 air. Themeasurement of end point potenti
18、al 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 effect, particularly if it is desired tod
19、etect 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 interactions among severalchemical systems prese
20、nt 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 will depend on the accuracy desired in th
21、edetermination.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 laboratory. Usually, these more rugged in
22、stru-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 application. Typical factors which may beconsid
23、ered 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 cable is required to transmit the signal.7
24、.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 the reference electrode is of the flow
25、ingjunction 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 be met by maintaining the inside soluti
26、on 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 procedure for checking sensitivity des
27、cribed 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 test solution. The construction of the
28、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 AssemblyA conventional electrode holderor support can be employed for laboratory measurements.Many differe
29、nt styles of electrode holders are suitable forvarious process applications such as measurements in an opentank, process pipe line, pressure vessel, or a high pressuresample line.8. Reagents and Materials8.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indica
30、ted, 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 its usewithout lessening the accuracy
31、of the determination.8.2 Purity of WaterReferences to water shall be under-stood to mean water that meets or exceeds the quantitativespecifications for type I or II reagent water conforming toSpecification D 1193, Section 1.1.8.3 Aqua RegiaMix 1 volume of concentrated nitric acid(HNO3, sp gr 1.42) w
32、ith 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 saltsavailable from the National Institute of Standards and Tech-nology specifically for the prepara
33、tion of standard buffersolutions. The NIST includes numbers and drying procedures.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 L
34、aboratoryChemicals, BDH Ltd., Poole, 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 StandardSampleDesignationAB
35、uffer 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 necessaryAThe buffer salts listed can be purchased from the Office of Standard ReferenceMaterials, National Insti
36、tute of Standards and Technology (NIST), Gaithersburg,MD 20899.D 1498 0828.4.1 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 pota
37、ssium 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 about5gofpotassium dichromate (K2Cr2O7) in 500 mL of concentratedsulfuric acid (H2SO4, sp gr 1.84).8.6 DetergentUse any commercially
38、 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-Ferric ReferenceSolution4Dissolve 39.21 g of ferrous ammonium sulfate(Fe(NH4)2-(SO4)26H2O), 48.22 g of ferric ammonium
39、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 an assayconfirming them to be within 1% of the nominal composition.The solution should be stored in a closed glass or p
40、lasticcontainer.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 Qui
41、nhydrone 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 quinhydrone is used in eachsolution so that solid crystals are always present. Thesereference solutions are stable for on
42、ly 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/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 pot
43、assiumhydroxide (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 provides the potentialof the platinum electrode for various reference electrodes atvarious temperatures in the standard Iodid
44、e/Triiodide solution.9. Sampling9.1 Collect the samples in accordance with PracticesD 3370.10. 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 wa
45、ter between measurements. 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 toothpa
46、ste.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 mixture atroom temperature for several minutes, then rinse with dilutehydrochloric acid, and then thoroughly rinse with
47、 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 longerthan the time specified. In these cleaning operations, particularcare must be exercised to protect glass-metal seals
48、 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 connection. Thereading should be less than 60.5 mV.11.2 Checking the Response of the Electrode to StandardRedox Solutions
49、(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 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 assem
