1、Designation: D8031 16Standard Test Method forAcid Number of Terephthalic Acid (TA) by AutomaticPotentiometric Titration1This standard is issued under the fixed designation D8031; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、year of last revision. A number in parentheses 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 determination of acidnumber in terephthalic acid (TA) by automatic potentiometrict
3、itration. As the index of TA purity, acid number is usuallywithin 674 to 676 mg KOH/g.1.2 In determining the conformance of the test results usingthis method, results shall be rounded off in accordance with therounding-off method of Practice E29.1.3 The values stated in SI units are to be regarded a
4、sstandard. No other units of measurement are included in thisstandard.1.4 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
5、 applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D664 Test Method for Acid Number of Petroleum Productsby Potentiometric TitrationD1193 Specification for Reagent WaterD4790 Terminology of Aromatic Hydrocarbons and RelatedChemicalsD6809 Guide for Quali
6、ty Control and Quality AssuranceProcedures for Aromatic Hydrocarbons and Related Ma-terialsE29 Practice for Using Significant Digits in Test Data toDetermine Conformance with SpecificationsE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE300 Practice for Sampling Industrial
7、ChemicalsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method2.2 Other Document:3OSHA Regulations, 29 CFR paragraphs 1910.1000 and1910.12003. Terminology3.1 Definitions:3.1.1 acid number, nthe quantity of base, expressed asmilligrams of potassium hydroxide
8、 per gram of sample that isrequired to titrate a sample in a specified solvent to a specifiedend point.4. Summary of Test Method4.1 ATA sample is dissolved in pyridine, diluted with waterand titrated potentiometrically with standard sodium hydroxidesolution, using a glass indicating electrode and a
9、referenceelectrode or a combination electrode. The meter readings areplotted automatically against the respective volumes of titrat-ing solution and the end points are taken only at well-definedinflections in the resulting curve. The acid number is calculatedas milligrams of KOH per gram of TA sampl
10、e. Its theoreticalvalue of TA sample is 675.5 mg KOH/g.5. Significance and Use5.1 An estimate of TA purity can be determined by titratingwith KOH. As an index of TA purity, the acid number can beused as a guide in the quality control of TA production.6. Apparatus6.1 Analytical Balance.6.2 Automatic
11、Potentiometric TitratorAutomatic titrationsystems capable of adding fixed increments of titrant at fixedtime intervals (monotonic) or variable titrant increments withelectrode stability between increment additions (dynamic) withendpoint seeking capabilities as prescribed in the method. As aminimum,
12、the apparatus shall meet the performance andspecification requirements as warranted by the manufacturer.1This test method is under the jurisdiction of ASTM Committee D16 onAromatic Hydrocarbons and Related Chemicals and is the direct responsibility ofSubcommittee D16.02 on Oxygenated Aromatics.Curre
13、nt edition approved March 1, 2016. Published June 2016. DOI: 10.1520/D8031-16.2For 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 on
14、the ASTM website.3Available from U.S. Government Printing Office, Superintendent ofDocuments, 732 N. Capitol St., NW, Washington, DC 20401-0001, http:/www.access.gpo.gov.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16.2.1 Amonotonic
15、 or dynamic mode of titrant shall be used.During the titration, the speed and volume of the addition mayvary depending on the rate of change of the system. Therecommended maximum volume increment is 0.5 mL and therecommended minimum volume increment is 0.05 mL.6.2.2 Buret, capable of delivering titr
16、ant in 0.05 mL orlarger increments. The buret tip should be able to deliver titrantdirectly into the titration vessel without exposure to thesurrounding air. Burets with total capacities of 5 mL, 10 mLand 20 mL are typically available from commercial manufac-turers of auto titrators and can be used
17、if their specificationsmeet the criteria of 6.2.1 and 6.2.2.6.2.3 Titration Stand, suitable for supporting the electrode,stirrer and buret tip.6.3 Sensing Electrode, standard pH, suitable for nonaqueoustitrations.6.4 Reference Electrode, silver/silver chloride (Ag/AgCl)reference electrode, filled wi
18、th 1M to 3M LiCl in ethanol.7. Reagents7.1 Purity of ReagentsUnless otherwise indicated, it isintended that all reagents shall conform to the reagent gradespecification of the Analytical Reagents of the AmericanChemical Society,4where such specifications are available.Other grades may be used, provi
19、ded it is first ascertained thatthe reagent is of sufficient high purity to permit its use withoutlessening the performance or accuracy of the determination.Reagent chemicals shall be used for all tests.7.2 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean Typ
20、e III of SpecificationD1193. Boil the water gently for 5 to 10 min to remove anyCO2and cool the water to room temperature.7.3 Pyridine(WarningFlammable and harmful ifinhaled, swallowed or absorbed through the skin.).7.4 Oxalic Acid (standard reagent, H2C2O42H2O)(WarningHarmful if absorbed through th
21、e skin.).7.5 Sodium Hydroxide Solution (0.5 M)Weigh 20 g ofsodium hydroxide (NaOH) (WarningHighly corrosive to allbody tissue.) in a beaker. Add 100 mL water (boiling gently for5 to 10 min and cooling to room temperature, free of CO2)todissolve and cool the solution. Then quantitatively transfer the
22、solution into a 1000 mL volumetric flask and dilute to volumewith the above water. The NaOH solution should be stored ina plastic bottle and stopper must be stressed.NOTE 1After standardization, the NaOH solution should not be storedin a glass container because it will be slowly neutralized from exp
23、osure toa glass container. It will also “cement” a glass stopper into a glasscontainer.8. Hazards8.1 Consult current federal regulations, suppliers SafetyData Sheets, and local regulations for all materials used in thistest method.9. Sampling, Test Specimens, and Test Units9.1 Use only representativ
24、e samples obtained as describedin Practice E300, unless otherwise specified.10. Preparation of Apparatus10.1 Adjust the apparatus in accordance with the manufac-turers instructions.10.2 Preparation of Electrodes:10.2.1 When theAg/AgCl reference electrode is used for thetitration and it contains an e
25、lectrolyte which is not 1M to 3MLiCl in ethanol, replace the electrolyte. Drain the electrolytefrom the electrode, wash away all the salt (if present) withwater and then rinse with ethanol. Rinse several times with theLiCl electrolyte solution. Finally, replace the sleeve and fill theelectrode with
26、the LiCl electrolyte to the filling hole. Whenrefitting the sleeve, ensure that there will be a free flow ofelectrolyte into the system. A combination electrode shall beprepared in the same manner. The electrolyte in a combinationelectrode can be removed with the aid of a vacuum suction.10.3 Testing
27、 of ElectrodesTest the meter-electrode com-bination when first put into use, or when new electrodes areinstalled, and retest at intervals thereafter. Rinse the electrodeswith solvent then with water. Dip them into a pH 4 aqueousbuffer solution. Read the mV value after stirring one minute.Remove the
28、electrodes and rinse with water. Dip the electrodesin a pH 7 aqueous buffer. Read the mV value after stirring oneminute. Calculate the mV difference. A good electrode systemwill have a difference of at least 158 mV (20 to 25C). If thedifference is less than 158 mV, lift the sleeve of the electrodean
29、d ensure electrolyte flow. Repeat the measurements. If thedifference is still less than 158 mV, clean or replace theelectrode(s).10.3.1 When the sensing electrode and the reference elec-trode are separate, one pair of electrodes shall be considered asone unit. If one or the other is changed, it shal
30、l be consideredas a different pair and shall be re-tested.10.4 Maintenance and Storage of Electrodes:10.4.1 Follow the manufacturers instructions for storageand use of the electrode.10.4.2 Prior to each titration soak the prepared electrode inwater for at least 2 min. Rinse the electrode with deioni
31、zedwater immediately before use. The glass membrane needs to berehydrated after titration on non-aqueous material.10.4.3 When not in use, immerse the lower half of thecombination electrode in LiCl electrolyte. Do not allow elec-trodes to remain immersed in a titrated sample for anyappreciable period
32、 of time between titrations. While theelectrodes are not extremely fragile, handle them carefully atall times.11. Standardization of Titrant11.1 Weigh, to the nearest 0.0001 g, 0.7 to 0.8 g of oxalicacid, into a 250-mL titration beaker.4Reagent Chemicals, American Chemical Society Specifications, Am
33、ericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (U
34、SPC), Rockville,MD.D8031 16211.2 Add 100 mL of CO2-free water to the beaker. Swirl thebeaker periodically until the sample is entirely dissolved. Add20 mL of pyridine to the beaker.11.3 Immerse the electrode assembly into the sample solu-tion. Start the stirrer at a rate to ensure rapid and efficien
35、tmixing without causing undue turbulence and splashing.11.4 Titrate using the standard NaOH solution. The meterreadings are plotted automatically against the respective vol-umes of titrating solution and the end points are taken only atwell-defined inflections in the resulting curve. Record theamoun
36、t of titrant required.11.5 Perform a blank titration by repeating the above stepswithout adding oxalic acid.11.6 Calculate the molarity of the NaOH solution as fol-lows:C 5m 3 1000M 3 V 2 V0!(1)where:C = molarity of NaOH solution, mol/L,V = NaOH solution required for titration of the oxalic acid(11.
37、4), mL,V0= NaOH solution required for titration of the blank(11.5), mL,M = 63.04 g/mol, molar mass of the oxalic acid (7.4), andm = mass of oxalic acid titrated, g.12. Procedure12.1 Weigh, to the nearest 0.0001 g, 0.8 to 1.5 g of TAsample, into a 250-mL titration beaker.12.2 Add 20 mL of pyridine wi
38、th swirling the beakerperiodically. Stir the solution gently until the entire solid hasdissolved. Add 100 mL of CO2-free water.12.3 Immerse the electrode assembly, as used inStandardization, into the sample solution. Start the stirrer at arate to ensure rapid and efficient mixing without causing und
39、ueturbulence and splashing.12.4 Titrate using the standard NaOH solution. The meterreadings are plotted automatically against the respective vol-umes of titrating solution and the end points are taken only atwell-defined inflections in the resulting curve. Record theamount of titrant required.12.5 P
40、erform a blank titration by repeating the above stepswithout adding the TA sample.13. Calculation13.1 Calculate the acid number as follows:Acid number, mg of KOH/g = A 2 B! 3 C 3 M# W (2)where:A = NaOH solution required for titration of the TA sample(12.4), mL,B = NaOH solution required for titratio
41、n of the blank (12.5),mL,C = molarity of the NaOH solution (11.6), mol/L,M = 56.11 g/mol, molar mass of the KOH, andW = mass of TA sample titrated, g.14. Report14.1 Report the value of acid number, to the nearest 0.1 unit.14.2 Report the following information in the report:14.2.1 The complete identi
42、fication of the sample tested.14.2.2 Any deviation from the procedure specified (forexample, operating conditions).14.2.3 Results of the test.14.2.4 Any abnormal situations observed during the test.15. Precision and Bias15.1 The precision of this test method is based on anintralaboratory study of Te
43、st Method D8031 conducted in2015. One laboratory tested one TA sample for acid number byautomatic potentiometric titration. Every test result representsan individual determination. The laboratory reported 20 repli-cate results for each analysis/material combination in order toestimate the repeatabil
44、ity limits of the standard. Practice E691was followed for the design and analysis of the repeatabilitydata; the details are given in Research Report RR:D16-1057.515.1.1 Repeatability Limit (r)Two test results obtainedwithin one laboratory shall be judged not equivalent if theydiffer by more than the
45、 “r” value for that material; “r”istheinterval representing the critical difference between two testresults for the same material, obtained by the same operatorusing the same equipment on the same day in the samelaboratory.15.1.1.1 Repeatability limits are listed in Table 1.15.1.2 Reproducibility ha
46、s not been determined and will bedetermined within five years.15.2 BiasAt the time of the study, the test specimenschosen for analysis were not accepted reference materialssuitable for determining the bias for this test method, thereforeno statement on bias is being made.16. Quality Guidelines16.1 L
47、aboratories shall have a quality control system inplace.16.1.1 Confirm the performance of the test instrument ortest method by analyzing a quality control sample followingthe guidelines of standard statistical quality control practices.5Supporting data have been filed at ASTM International Headquart
48、ers and maybe obtained by requesting Research Report RR:D16-1057. ContactASTM CustomerService at serviceastm.org.TABLE 1 Repeatability Limits (TA)AutomaticPotentiometricTitrationAverage(mg KOH/g)XRepeatabilityStandardDeviationSrRepeatabilityLimitracid number 675.1 1.16 3.25D8031 16316.1.2 A quality
49、control sample is a stable material isolatedfrom the production process and representative of the samplebeing analyzed.16.1.3 When QA/QC protocols are already established inthe testing facility, these protocols are acceptable when theyconfirm the validity of test results.16.1.4 When there are no QA/QC protocols established inthe testing facility, use the guidelines described in GuideD6809 or similar statistical quality control practices.17. Keywords17.1 acid number; automatic potentiometric titration;terephthalic acid (TA)ASTM International takes no positi
