ASTM E928-2003 Standard Test Method for Determination of Purity by Differential Scanning Calorimetry《用差示扫描量热法测定纯度的标准试验方法》.pdf

1、Designation: E 928 03Standard Test Method forPurity by Differential Scanning Calorimetry1This standard is issued under the fixed designation E 928; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number

2、 in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This method describes the determination of purity ofmaterials greater than 98.5 mole percent purity using differen-tial scanning calorimetry

3、 and the vant Hoff equation.1.2 This test method is applicable to thermally stablecompounds with well-defined melting temperatures.1.3 Determination of purity by this test method is onlyapplicable when the impurity dissolves in the melt and isinsoluble in the crystal.1.4 Computer- or electronic-base

4、d instruments, techniques,or data treatments equivalent to this test method may also beused.NOTE 1Since all data treatments are not equivalent, it is the respon-sibility of the user to verify equivalency prior to use.1.5 SI values are the standard.1.6 This standard does not purport to address all of

5、 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 limitations prior to use.1.7 There is no ISO method equivalent to this method.2. Referenced

6、Documents2.1 ASTM Standards:E 473 Terminology Relating to Thermal Analysis2E 793 Test Method for Enthalpies of Fusion and Crystalli-zation by Differential Scanning Calorimetry2E 967 Practice for Temperature Calibration of DifferentialScanning Calorimeters and Differential Thermal Analyz-ers2E 968 Pr

7、actice for Heat Flow Calibration of DifferentialScanning Calorimeters2E 1970 Practice for Statistical Treatment of Thermoanalyti-cal Data23. Terminology3.1 DefinitionsThe definitions relating to thermal analysisappearing in Terminology E 473 shall be considered applicableto this method.4. Summary of

8、 Method4.1 This method is based upon the vant Hoff equation3:Ts5 To2 RTo2x! / HF! (1)where:Ts= specimen temperature, KTo= melting temperature of 100 % pure material, KR = gas constant (= 8.314 J mol1K1),x = mole fraction of impurity,H = heat of fusion, J mol1, andF = fraction melted.4.2 This method

9、consists of melting the test specimen that issubjected to a temperature-controlled program while recordingthe heat flow into the specimen as a function of temperature.The resulting melting endotherm area is measured to yield theenthalpy of fusion, H. The melting endotherm area is thenpartitioned int

10、o a series of fractional areas (about ten, compris-ing the first 10 to 50 % of the total area). The fractional area,divided by the total area, yields the fraction melted, F. Eachfractional area is assigned a temperature, Ts.4.3 Eq 1 has the form of Y = mX +b where Y = Ts,X=1/F,m=(RTo2x)/H, and b = T

11、o. A plot of Y versus X shouldproduce a straight line with slope m and intercept b.4.4 In practice, however, the resultant plot of Tsversus 1 /Fis seldom a straight line. To linearize the plot, an incrementalamount of area is added to the the total area and to eachfractional area to produce a revise

12、d value for F. The process ofincremental addition of area is continued until a straight line isobtained.F 5 Apart1 c! / Atotal1 c! (2)where:1This test method is under the jurisdiction of ASTM Committee E37 on ThermalMeasurements and is the direct responsibility of Subcommittee E37.01 on TestMethods

13、and Recommended Practices.Current edition approved March 10, 2003. Published July 2003. Originallypublished as E 928 83. Last previous edition E 928 96.2Annual Book of ASTM Standards, Vol 14.02.3Brennan, W. P., DiVito, M. P., Fynas, R. L., Gray, A. P., “An Overview of theCalorimetric Purity Measurem

14、ent”, in Purity Determinations by Thermal Methods,R. L. Blaine and C. K. Schoff (Eds.), Special Technical Publication 838, AmericanSocieity for Testing and Materials, West Conshohocken, PA 1984, pp.5-15.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-29

15、59, United States.Apart= area of fraction melted, mJAtotal= total area, mJ andc = incremental area, mJ.NOTE 2The best fit straight line may be determined by the leastsquares method. See Practice E 1970.)4.5 The values of mole fraction impurity x and meltingtemperature of the 100 % pure material Toar

16、e determined fromthe slope m and intercept b of the resultant straight line. Thisis Method A.4.6 An alternative form of the vant Hoff equation is givenby4:Apart52c 1 Toc 2 RTo2x m / M# / Ts1 ToApart/ Ts(3)where:m = mass of the sample, mg, andM = molecular weight, g mol1.4.7 Eq 3 has the form of Y =

17、a W+b X+g Z where Y =Apart, a =c,W=1,b =TocRTo2x m/M,X=1/Ts,g = To, andZ=Apartt/ Ts. Eq 3 may be evaluated by multiplelinear regression and x and Todetermined form the resultantvalues of a, b and g. This is Method B.5. Significance and Use5.1 The melting temperature range of a compound broadensas th

18、e impurity level rises. This phenomenon is describedapproximately by the vant Hoff equation for melting pointdepressions. Measuring and recording the instantaneous heatflow into the specimen as a function of temperature during sucha melting process is a practical way for the generation of datasuitab

19、le for analysis by the vant Hoff equation.5.2 The results obtained include: sample purity (expressedas mole percent); enthalpy of fusion (expressed as joules permole); and the melting temperature (expressed in Kelvin) ofthe pure form of the major component.5.3 Generally, the repeatability of this te

20、st method decreasesas the purity level decreases. This test method is ordinarilyconsidered unreliable when the purity level of the majorcomponent of the mixture is less than 98.5 mol % or when theincremental enthalpy correction (c) exceeds 20 % of theoriginal detected enthalpy of fusion.5.4 This met

21、hod is used for quality control, specificationacceptance, and research.6. Interferences6.1 This method is nonspecific. Many impurities may causethe melting temperature broadening. Thus, it is not useful inidentifying the nature of the impurity or impurities but only thetotal mol percent of impurity

22、present.6.2 The vant Hoff theory assumes the following:6.2.1 The impurities dissolve in the melt of the majorconstituent forming a solution approximately described byideal solution theory;6.2.2 The solubility of the impurity in the solid of the majorconstituent is negligible; and6.2.3 The major cons

23、tituent displays a single well-definedmelting endotherm in the temperature range of interest. Micro-scopic investigations of the melt and the solid may help toestablish whether or not solid or liquid solutions have beenformed.6.2.4 The solute and solvent are close in molecular size.6.3 In some cases

24、 the sample may react with air during thetemperature cycle, causing an incorrect transition to be mea-sured. Where it has been shown that this effect is present,provision shall be made for sealing the specimen and runningthe test under an inert gas blanket. Since some materialsdegrade near the melti

25、ng region, carefully distinguish betweendegradation and transition. See Appendix X1.6.4 Since milligram quantities of sample are used, ensurethat samples are homogeneous and representative.6.5 Sublimation or decomposition will lead to a differentheat consumption and, perhaps, a change in composition

26、 of thespecimen. The specimen holder should be examined after themeasurement for crystals not part of the resolidified melt.7. Apparatus7.1 The essential equipment required to provide the mini-mum instrument capability for this test method includes:7.1.1 Differential Scanning Calorimeter (DSC), cons

27、istingof:7.1.1.1 DSC Test Chamber, composed of a furnace(s) toprovide uniform controlled heating of a specimen and refer-ence to a constant temperature or at a constant rate within theapplicable temperature range of this test method; a temperaturesensor to provide an indication of the specimen tempe

28、rature to60.1 K; a differential sensor to detect a heat flow differencebetween the specimen and reference equivalent to 10 W; anda means of sustaining a test chamber environment of N2at apurge rate of 15 to 50 6 -5 mL/min.7.1.1.2 Temperature Controller, capable of executing aspecific temperature pro

29、gram by operating the furnace(s)between selected temperature limits at a rate of temperaturechange of 0.3 to 0.7 K/min constant to 60.01 K/min.7.1.1.3 Recording Device, to record and display of the heatflow on the Y-axis (ordinate) and temperature on the X-axis(abscissa).7.1.2 Containers, that are i

30、nert to the specimen, and that areof suitable structural shape and integrity for use in the DSC testchamber, made of materials of high thermal conductivity, suchas aluminum.7.2 Planimeter, computer- or electronic-based data treat-ment or other instrumentation to determine area to within 61 % precisi

31、on.7.3 Balance, with a capacity of at least 100 mg capable ofweighing to an accuracy of 0.01 mg.8. Sampling8.1 The test sample (liquid or solid) should be mixed priorto sampling and sampled by removing portions from variousparts of the container. Combine the portions and mix well toprovide a represe

32、ntative sample for the purity determinations.Only 1 to 3 mg is required for each analysis.8.2 Avoid any physical or mechanical treatment of thematerial that will cause chemical changes. For example,4Widman, G., Scherrer, O., “A New Program for DSC Purity Analysis”, Journalof Thermal Analysis, 1991,

33、37, pp. 1957 - 1964.E928032grinding the sample for size reduction often introduces suchchanges as a result of heat generated by friction.9. Calibration9.1 Perfrom any calibrations procedures called for by theinstrument manufacturer as described in the operations manual.9.2 Calibrate the apparatus te

34、mperature signal at the heatingrate to be used in this method (see section 10.8) using ASTMStandard E 967. High purity (99.99 %) indium metal is aconvenient material to use for this purpose.9.3 Calibrate the apparatus heat flow signal at the heatingrate to be used in this method (see section 10.8) u

35、sing ASTMStandard E 968. High purity (99.99 %) indium metal is aconvenient material to use for this purpose.9.4 Determine the leading edge slope (S)inKmW1fromthe heat flow calibration curve obtained in Section 9.3NOTE 3The value of S is negative.10. Procedure10.1 CautionToxic and corrosive effluents

36、 may be re-leased upon heating the material. It is the responsibility of theuser of the standard to take appropriate safety measures.10.2 Wash the empty specimen container in an appropriatesolvent, such as hexane, then heat to 700 K for 1 min.10.3 Cool the specimen container and store in a desiccato

37、runtil ready for use.10.4 Weigh 1 to 3 mg of the sample to an accuracy of 0.01mg in a pre-cleaned specimen container.10.5 Under ambient conditions, hermetically seal the speci-men container so there will be no mass loss during the scan.Minimize the free space between the specimen and the lid toavoid

38、 sublimation onto the lid.NOTE 4If oxidation is suspected, hermetically seal in an inertatmosphere.10.6 Purge the cell with dry nitrogen at a flow rate of 15 to50 mL/min throughout the experiment.10.7 Place the encapsulated specimen in the specimencontainer and heat rapidly up to 25 K below the melt

39、ingtemperature. Allow the instrument temperature to stabilize.10.8 Heat the specimen from the temperature selected in10.7 to completion of the melt at the rate of 0.3 to 0.7 K min1.A minimum of 200 data points should be taken in the meltregion.10.9 Reweigh the specimen after completion of scan, exam

40、-ine contents (see 6.5) and discard. Do not accept data if massloss exceeds 1 %.11. Calculation Method ANOTE 5All calculations shall use all available decimal places beforerounding the final result.11.1 Construct a linear baseline under the melting endot-herm by connecting a straight line between th

41、e baseline beforeand after the the transitions shown in Fig. 1.11.2 Integrate as a function of time the total area under thefusion curve (ABCA) as shown in Fig. 1. Report this value asABCA in mJ.11.3 Partition the total area by drawing at least ten perpen-dicular lines from the baseline to the fusio

42、n curve as illustratedby the typical line (DE) in Fig. 1. Determine the integrated areaof each partial fraction as ADEA in mJ.11.4 Determine the fraction F for each partial area using Eq4.F 5ADEAABCA(4)where:F = fraction of total area,ADEA = area of fraction, mJ, andABCA = total area under fusion cu

43、rve, mJ.11.5 Select at least ten (10) partial area fractions between 10and 50 % of the total area.11.6 From the heat flow value (for example DE) calculatethe temperature, TF, at which each fraction, F, has melted.TF5 TD1 DE / S (5)where:TF= corrected absolute temperature for area fraction, K,TD= mea

44、sured absolute temperature at point D,K,S = slope, mW K1, from section 9.4DE = heat flow corresponding the length DE (mW).11.7 Plot the temperature at which it has melted (TF) versusthe reciprocal of the fraction melted (1/F) as shown in (Fig. 2).The plot may concave upward.NOTE 6The reasons for thi

45、s nonlinear behavior may arise from avariety of causes such as instrumental effects or pre-melting behavior ornondetection of the eutectic melting, or both, that contribute to error in thepartial area data.11.8 By a process of successive approximations, an area c isadded to both the fractional area

46、ADEA and to the total areaABCA until a straight line for the plot of TFversus 1/F isobtained.FIG. 1 Fusion Curve for Method AFIG. 2 Schematic of 1/F Plot for Purity DeterminationsE9280331/F 5 ABCA 1 c! / ADEA 1 c! (6)11.9 Calculate the slope and the intercept, To,oftheTFversus corrected 1/F line whe

47、re the equation for the line isgiven by the following:5TF5 slope! 31F1 To(7)NOTE 7A least squares best fit may be useful for this purpose. SeeStandard E 1970.11.10 Employ Eq 8 to calculate the mole fraction impurityas follows:x52slope! 3 H / RTo2(8)where:x = mole fraction impurity,R = universal gas

48、constant = 8.314510 J mol1K1;,5H = enthalpy of fusion, J mol1(see Note 8) of majorcomponent of the solution,andTo= melting point of pure component, K.NOTE 8If the enthalpy of fusion of the major component is not knownfrom other sources, Test Method E 793 may be used on the sample toobtain a good est

49、imate of the enthalpy of fusion.11.11 Employ Eq 9 to calculate % mole fraction purity X1asfollows:X15 1 2x! 100 % (9)where:X1= percent mole fraction purity12. Calculations Method BNOTE 9All calculations shall use all available decimal places round-ing the final result.12.1 Construct a baseline under the melting endotherm byextrapolating the baseline before the transition into the regionof the melt as shown in Fig. 3.12.2 Create a series of at least ten (10) partial areas bydrawing a series of perpendicular lines, between 10 and 90 %of