ASTM E70-2007 Standard Test Method for pH of Aqueous Solutions With the Glass Electrode《带玻璃电极水溶液的pH值测量用标准试验方法》.pdf

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1、Designation: E 70 07Standard Test Method forpH of Aqueous Solutions With the Glass Electrode1This standard is issued under the fixed designation E 70; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last revision.Anumbe

2、r in parentheses indicates the year of last reapproval.Asuperscriptepsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope*1.1 This test method specifies the apparatus and proceduresfor

3、the electrometric measurement of pH values of aqueoussolutions with the glass electrode. It does not deal with themanner in which the solutions are prepared. pH measurementsof good precision can be made in aqueous solutions containinghigh concentrations of electrolytes or water-soluble organiccompou

4、nds, or both. It should be understood, however, that pHmeasurements in such solutions are only a semiquantitativeindication of hydrogen ion concentration or activity. Themeasured pH will yield an accurate result for these quantitiesonly when the composition of the medium matches approxi-mately that

5、of the standard reference solutions. In general, thistest method will not give an accurate measure of hydrogen ionactivity unless the pH lies between 2 and 12 and the concen-tration of neither electrolytes nor nonelectrolytes exceeds 0.1mol/L (M).1.2 The values stated in SI units are to be regarded

6、asstandard. The values in parentheses are for information only.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 standard to establish appro-priate safety and health practices and determine the applic

7、a-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 1193 Specification for Reagent WaterE 180 Practice for Determining the Precision of ASTMMethods for Analysis and Testing of Industrial and Spe-cialty ChemicalsE 691 Practice for Conducting an Interlaboratory

8、 Study toDetermine the Precision of a Test Method3. Terminology3.1 Definition:3.1.1 pHdefined formally as the negative logarithm to thebase 10 of the conventional hydrogen ion activity. See Appen-dix X1.3.2 Definitions of Terms Specific to This Standard:3.2.1 For the purpose of this test method, the

9、 term “meter”shall apply to the instrument used for the measurement ofpotential (either in millivolts or in terms of pH units), the term“electrodes” to the glass electrode and the reference electrode,and the term “assembly” to the combination of the meter andthe electrodes. The performance of the me

10、ter shall be differ-entiated from that of the electrodes.4. Significance and Use4.1 pH is, within the limits described in 1.1, an accuratemeasurement of the hydrogen ion concentration and thus iswidely used for the characterization of aqueous solutions.4.2 pH measurement is one of the main process c

11、ontrolvariables in the chemical industry and has a prominent place inpollution control.5. Apparatus5.1 pH metersMany excellent pH meters are availablefrom commercial sources. To some extent, the choice of meterwill depend on the desired precision of measurement. Themeter may operate on a null-detect

12、ion principle or may utilizedigital readout or a direct deflection meter with a large scale.Power may be supplied by batteries or a-c operation may beprovided. The maximum grid current drawn from the glasselectrode during measurement shall not exceed 2 3 1012A.Automatic or manual adjustment shall al

13、low for changes in1This test method is under the jurisdiction of ASTM Committee E15 onIndustrial and Specialty Chemicals and is the direct responsibility of SubcommitteeE15.01 on General Standards.Current edition approved April 1, 2007. Published May 2007. Originallyapproved in 1952. Last previous e

14、dition approved in 2002 as E 70 97 (2002).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.1*A Summary of Chan

15、ges section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.F/(RT ln 10) when the temperature of the assembly is altered.For referee work, or in case of dispute, meters capable ofdiscriminating chan

16、ges of pH to 0.01 unit (0.6 mV) or less shallbe used.5.2 Reference Electrodes and Glass Electrodes:5.2.1 The saturated calomel electrode and the 3.5 mol/L(M)calomel electrode are suitable as reference electrodes in pHassemblies (Note 1). If the saturated electrode is used, a fewcrystals of solid pot

17、assium chloride shall be present in thechamber surrounding the electrode element at each tempera-ture. The design of the electrode shall permit a fresh liquidjunction between the solution of potassium chloride and thebuffer or test solution to be formed for each test and shall allowtraces of solutio

18、n to be readily removed by washing.NOTE 1Other reference electrodes of constant potential may be used,provided no difficulty is experienced in standardizing the assembly asdescribed in Section 8.5.2.2 The silver-silver chloride electrode also is used widelyas a reference electrode.5.2.3 Commercial g

19、lass electrodes are designed for certainspecific ranges of pH and temperature; consequently, the pHand temperature of the test solutions shall be considered inselecting the glass electrode for use. The pH response shallconform with the requirements set forth in Section 7. The leadsshall be shielded

20、from the effects of body capacitance.5.2.4 If the assembly is in intermittent use, the ends of theelectrodes shall be immersed in distilled water between mea-surements. The high-alkalinity type of glass electrode shall bestored in the borax buffer solution. For prolonged storage, glasselectrodes may

21、 be allowed to become dry, and referenceelectrodes shall be capped to prevent undue evaporation.NOTE 2New glass electrodes and those that have been stored dry shallbe conditioned as recommended by the manufacturer. Requirements forthe physical dimensions and shape of the electrodes and the compositi

22、onof the internal reference solution are not considered part of this testmethod.6. Reagents and Materials6.1 The pH(S) of six recommended standard solutions atseveral temperatures is listed in Table 1. The buffer solutionsshall be prepared from highly purified materials sold specifi-cally as pH stan

23、dards (Note 3). Potassium hydrogen phthalateand the two phosphate salts shall be dried at 110C for 1 hbefore use, but borax and sodium bicarbonate shall not beheated above room temperature. Potassium dihydrogen citrateshall be dried for1hat80C, and sodium carbonate shall beignited for1hat270C before

24、 use. The standard solutionsshall be prepared as described in 6.4-6.9. They shall bepreserved in bottles of chemically resistant glass or polyethyl-ene and shall be replaced at an age of 6 weeks, or earlier if avisible change should occur in the solution.NOTE 3Six of the buffer salts can be obtained

25、 in the form of standardreference materials from the National Bureau of Standards. These mate-rials are numbered as follows:Buffer Salt SRM No.Potassium hydrogen phthalate 185Potassium dihydrogen phosphate 186IDisodium hydrogen phosphate 186IIBorax 187Sodium bicarbonate 191Sodium carbonate 192The pH

26、(S) values may vary slightly from one lot to another; conse-quently, the values given on the SRM certificate should be used inpreference to those given in Table 2, if slight differences exist.6.2 Commercial standard buffers are available. For the mostexact measurements, the value of the commercial b

27、uffer shouldbe verified using one of the recommended standard buffers inTable 1.6.3 Distilled Water The conductivity of the distilled watershall not exceed 2 3 106scm1. For the preparation of thecitrate, phthalate, and phosphate solutions, the water need notbe freed of dissolved carbon dioxide. The

28、water used for theborax standard and the carbonate standard shall be boiled for15 min or purged with air free of carbon dioxide and shall beprotected with a soda-lime tube or equivalent (Note 4) whileTABLE 1 pH(S) of Standard SolutionsA,BTemperature, C A B C D E F0 3.863 4.003 6.984 7.534 9.464 10.3

29、1710 3.820 3.998 6.923 7.472 9.332 10.17920 3.788 4.002 6.881 7.429 9.225 10.06225 3.776 4.008 6.865 7.413 9.180 10.01230 3.766 4.015 6.853 7.400 9.139 9.96635 3.759 4.024 6.844 7.389 9.102 9.92540 3.753 4.035 6.838 7.380 9.068 9.88950 3.749 4.060 6.833 7.367 9.011 9.82860 . 4.091 6.836 . 8.962 .70

30、. 4.126 6.845 . 8.921 .80 . 4.164 6.859 . 8.885 .90 . 4.205 6.877 . 8.850 .AThe compositions of the standard solutions are:AKH2citrate, m = 0.05 mol kg1BKH phthalate, m = 0.05 mol kg1CKH2PO4, m = 0.025 mol kg1;Na2HPO4, m = 0.025 mol kg1DKH2PO4, m = 0.008695 mol kg1;Na2HPO4, m = 0.03043 mol kg1ENa2B4

31、O7, m = 0.01 mol kg1FNaHCO3, m = 0.025 mol kg1;Na2CO3, m = 0.025 mol kg1where m denotes molality.BFor a discussion of the manner in which these pH(S) values were assigned, see Chapter 4 of the book by Bates, R. G., Determination of pH, Theory and Practice,John Wiley and Sons, Second edition, New Yor

32、k, 1973.E70072cooling and in storage.The pH of the carbon dioxide-free watershall be between 6.6 and 7.5 at 25C. The temperature of thewater used to prepare the standards shall be within 2C of25C. The amounts of the buffer salts given in 5.3 through 5.8are weights in air near sea level determined wi

33、th brass weights.NOTE 4The water used for preparing the standard buffer solutionsshall be Types I or II reagent water in accordance with SpecificationD 1193. Precautions shall be taken to prevent contamination of thedistilled water with traces of the material used for protection againstcarbon dioxid

34、e.6.4 Citrate, Standard Solution A (molality = 0.05 mol/kg;pH(S) = 3.776 at 25C) Dissolve 11.41 g of potassiumdihydrogen citrate in distilled water and dilute to 1 L.6.5 Phthalate, Standard Solution B (molality = 0.05 mol/kg;pH(S) = 4.008 at 25C) Dissolve 10.12 g of potassiumhydrogen phthalate in di

35、stilled water and dilute to 1 L.6.6 Phosphate, Standard Equimolal Solution C (molality ofeach phosphate salt = 0.025 mol/kg; pH(S) = 6.865 at25C)Dissolve 3.388 g of potassium dihydrogen phosphateand 3.533 g of disodium hydrogen phosphate in distilled waterand dilute to 1 L.6.7 Phosphate, Standard So

36、lution D (1 + 3) (molality ofKH2PO4= 0.008695 mol/kg, molality of Na2HPO4= 0.03043mol/kg); pH(S) = 7.413 at 25C)Dissolve 1.179 g of potas-sium dihydrogen phosphate and 4.302 g of disodium hydrogenphosphate in distilled water and dilute to 1 L.6.8 Borax, Standard Solution E (molality = 0.01 mol/kg;pH

37、(S) = 9.180 at 25C)Dissolve 3.80 g of sodium tetrabo-rate decahydrate (borax) in distilled water and dilute to 1 L.6.9 Carbonate, Standard Solution F (molality of each car-bonate salt = 0.025 mol/kg; pH(S) = 10.012 at 25C)Dissolve 2.092 g of sodium bicarbonate and 2.640 g of sodiumcarbonate in disti

38、lled water and dilute to 1 L.7. Performance Tests of Meter and ElectrodesNOTE 5Except for measurements of the highest precision, it willusually be unnecessary to perform the tests described in this section. In theusual pH measurement, the stability of the meter, the accuracy of the scalereading, and

39、 the pH response of the glass electrode over the range of themeasurements are verified by checking the assembly with a series ofstandard buffer solutions.7.1 AssemblyThe assembly shall be judged to be perform-ing satisfactorily if it furnishes, within acceptable limits ofaccuracy, the correct pH val

40、ues for the standard buffer solu-tions listed in Table 2. When the electrodes are immersed in abuffer solution, the measured potential difference shall besubstantially constant, and the cause of any instability shall bedetermined.7.2 MeterThe meter shall be brought to electrical balancein accordance

41、 with the manufacturers instructions. The per-formance shall then be tested by applying a known variablepotential through a resistance of approximately 200 MV to theterminals of the meter, the high-resistance lead being con-nected to the terminal corresponding to the glass electrode. Thesource of po

42、tential may be a precision-type potentiometer witha range of 1100 mV or more and a limit of error not greaterthan 0.1 mV. The 200-MV resistor shall be properly shielded toavoid capacity pickup. Commencing with a value of zero, theapplied potential shall be increased in increments of 100 mV,and the r

43、eadings of the dial of the meter at balance shall benoted. The process shall be extended to cover the entire rangeof the meter. In no case shall the difference between the appliedvoltage and that indicated by the meter differ by more than 1mV per increment of applied voltage.NOTE 6If the cumulative

44、error at the end of the scale exceeds 63mV,a calibration curve for the meter shall be constructed and correctionsapplied to each measurement of electromotive force or pH. Differences ofelectromotive force (volts) are converted to corresponding differences ofpH by multiplying by F/(RT ln 10) (Table X

45、1.1). Inasmuch as the meteris made to read correctly at the pH of the standard, the calibrationcorrection to be applied to a pH measurement is the difference between thescale corrections at the pH of the standard and that of the unknown, withdue regard for sign.7.3 Glass Electrodes The difference of

46、 potential betweenthe glass electrode and the standard hydrogen gas electrodeshall be measured when both electrodes are immersed in thesame portion of various buffer solutions over the pH range inwhich the glass electrode is to be used. For these comparisonsthe cell shall be placed in a water bath t

47、hermostaticallycontrolled to 60.1C near 25C.The solutions used for this testshall be those listed in Section 6. The standards of pH 9.18 andbelow (at 25C) shall be used to test electrodes of thegeneral-purpose type. The borax and carbonate standards shallbe used to test the high-alkalinity type of e

48、lectrode. Thesebuffer solutions shall be supplemented by a 0.1 mol/kg (M)carbonate-free solution of sodium hydroxide, the pH of whichis approximately 12.8 at 25C. The difference of potentialbetween the general-purpose glass electrode and the hydrogenelectrode shall be independent, within 61 mV, of p

49、H changesin the range from 3.8 to 9.18 pH. The difference of potentialbetween the hydrogen electrode and a glass electrode of thehigh-alkalinity type shall be the same, within + 3 mV, at pH12.8 as at pH 9.18.8. Calibration and Standardization8.1 Turn on the instrument, allow to warm up thoroughly,and bring to electrical balance in accordance with the manu-facturers instructions. Wash the glass and reference electrodesand the sample cup three times with distilled water. Allow thewater to drain from the electrodes, but the sample c