1、Designation: E 928 08Standard 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 () 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 SI values are the standard.1.
4、5 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 limitations prior to use.1.6 There is no I
5、SO method equivalent to this method.2. Referenced Documents2.1 ASTM Standards:2E 473 Terminology Relating to Thermal Analysis and Rhe-ologyE 793 Test Method for Enthalpies of Fusion and Crystalli-zation by Differential Scanning CalorimetryE 794 Test Method for Melting And Crystallization Tem-peratur
6、es By Thermal AnalysisE 967 Test Method for Temperature Calibration of Differ-ential Scanning Calorimeters and Differential ThermalAnalyzersE 968 Practice for Heat Flow Calibration of DifferentialScanning CalorimetersE 1970 Practice for Statistical Treatment of Thermoanalyti-cal Data3. Terminology3.
7、1 DefinitionsThe definitions relating to thermal analysisappearing in Terminology E 473 shall be considered applicableto this method.4. Summary of 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 % pur
8、e 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 consists of melting the test specimen that issubjected to a temperature-controlled program while recordingthe heat flow into the specimen as a function of
9、 temperature.The resulting melting endotherm area is measured to yield theenthalpy of fusion, H. The melting endotherm area is thenpartitioned into 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
10、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 = To. 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
11、a straight line. To linearize the plot, an incrementalamount of area is added to the total area and to each fractionalarea to produce a revised value for F. The process ofincremental addition of area is continued until a straight line isobtained.F 5 Apart1 c! / Atotal1 c! (2)1This test method is und
12、er the jurisdiction ofASTM Committee E37 on ThermalMeasurements and is the direct responsibility of Subcommittee E37.01 on ThermalTest Methods and Practices.Current edition approved Sept. 1, 2008. Published November 2008. Originallyapproved in 1983. Last previous edition approved in 2003 as E 928 03
13、.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.3Brennan, W. P., DiVito, M. P., Fynas, R. L., Gray, A. P., “
14、An Overview of theCalorimetric Purity Measurement”, in Purity Determinations by Thermal Methods,R. L. Blaine and C. K. Schoff (Eds.), Special Technical Publication 838, AmericanSociety for Testing and Materials, West Conshohocken, PA 1984, pp. 5 - 15.1Copyright ASTM International, 100 Barr Harbor Dr
15、ive, PO Box C700, West Conshohocken, PA 19428-2959, United States.where:Apart= area of fraction melted, mJAtotal= total area, mJ andc = incremental area, mJ.NOTE 1The best fit straight line may be determined by the leastsquares method. See Practice E 1970.)4.5 The values of mole fraction impurity x
16、and meltingtemperature of the 100 % pure material Toare 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 =
17、molecular weight, g mol1.4.7 Eq 3 has the form of Y = a W+b X+g Z where Y =Apart, a =c,W=1,b =TocRTo2x m/M,X=1/Ts,g = To, andZ=Apart/ 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
18、melting temperature range of a compound broadensas the 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 proces
19、s is a practical way for the generation of datasuitable 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
20、component.5.3 Generally, the repeatability of this test 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
21、 theoriginal detected enthalpy of fusion.5.4 This method 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
22、impurities but only thetotal mol percent of impurity 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 ma
23、jorconstituent is negligible; and6.2.3 The major constituent 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
24、solvent are close in molecular size.6.3 In some cases 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 ga
25、s blanket. Since some materialsdegrade near the melting 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 differenth
26、eat consumption and, perhaps, a change in composition 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 include
27、s:7.1.1 Differential Scanning Calorimeter (DSC), consistingof: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 temperature
28、sensor to provide an indication of the specimen temperature 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 Contr
29、oller, capable of executing aspecific temperature program 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 Data Collection Device, to provide a means ofacquiring, storing, and displaying measured or calc
30、ulatedsignals, or both. The minimum output signals required for DSCare heat flow, temperature, and time.7.1.2 Containers, that are inert 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 alum
31、inum.7.2 Planimeter, computer- or electronic-based data treat-ment or other instrumentation to determine area to within 61 % precision.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
32、 sampling and sampled by removing portions from variousparts of the container. Combine the portions and mix well toprovide a representative sample for the purity determinations.Only 1 to 3 mg is required for each analysis.4Widman, G., Scherrer, O., “ANew Program for DSC PurityAnalysis”, Journalof Th
33、ermal Analysis, 37 1987, pp. 19571964.E9280828.2 Avoid any physical or mechanical treatment of thematerial that will cause chemical changes. For example,grinding the sample for size reduction often introduces suchchanges as a result of heat generated by friction.9. Calibration9.1 Perform any calibra
34、tions procedures called for by theinstrument manufacturer as described in the operations manual.9.2 Calibrate the apparatus temperature 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 f
35、or this purpose.9.3 Calibrate the apparatus heat flow signal at the heatingrate to be used in this method (see section 10.8) using 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) in mW/K from theheat flow
36、 calibration curve obtained in Section 9.3. See Fig. 1NOTE 2The value of S is negative.10. Procedure10.1 CautionToxic and corrosive effluents 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 specim
37、en 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 desiccatoruntil 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, hermeticall
38、y 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 sublimation onto the lid.NOTE 3If oxidation is suspected, hermetically seal in an inertatmosphere.10.6 Purge the cell with dry nitrogen at a flow rate of 15 t
39、o50 mL/min throughout the experiment.10.7 Place the encapsulated specimen in the specimencontainer and heat rapidly up to 25 K below the meltingtemperature. 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
40、 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-ine contents (see 6.5) and discard. Do not accept data if massloss exceeds 1 %.11. Calculation Method ANOTE 4All calculations shall use all available decimal
41、places beforerounding the final result.11.1 Construct a linear baseline under the melting endot-herm by connecting a straight line between the baseline beforeand after the transitions shown in Fig. 2.11.2 Integrate as a function of time the total area under thefusion curve (ABCA) as shown in Fig. 2.
42、 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 fusion curve as illustratedby the typical line (DE)inFig. 2. Determine the integrated areaof each partial fraction as ADEA in mJ.11.4 Determine the fraction F for each
43、partial area using Eq4.F 5ADEAABCA(4)FIG. 1 Fusion Curve for Method BE928083where:F = fraction of total area,ADEA = area of fraction, mJ, andABCA = total area under fusion curve, mJ.11.5 Select at least ten (10) partial area fractions between 10and 50 % of the total area.11.6 From the heat flow valu
44、e (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= measured absolute temperature at point D,K,S = slope, mW K1, from section 9.4DE = heat flow corresponding the length DE (mW).11.7
45、 Plot the temperature at which it has melted (TF) versusthe reciprocal of the fraction melted (1/F) as shown in (Fig. 1).The plot may concave upward.NOTE 5The reasons for this nonlinear behavior may arise from avariety of causes such as instrumental effects or pre-melting behavior ornondetection of
46、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 ADEA and to the total areaABCA until a straight line for the plot of TFversus 1/F isobtained.1/F 5 ABCA 1 c! / ADEA 1 c! (6)11
47、.9 Calculate the slope and the intercept, To,oftheTFversus corrected 1/F line where the equation for the line isgiven by the following:5TF5 slope! 31F1 To(7)NOTE 6A least squares best fit may be useful for this purpose. SeePractice E 1970.11.10 Employ Eq 8 to calculate the mole fraction impurityas f
48、ollows:x52slope! 3 H / RTo2(8)where:x = mole fraction impurity,R = universal gas constant = 8.314510 J mol1K1;,5H = enthalpy of fusion, J mol1(see Note 7) of majorcomponent of the solution,andTo= melting point of pure component, K.NOTE 7If the enthalpy of fusion of the major component is not knownfr
49、om other sources, Test Method E 793 may be used on the sample toobtain a good estimate 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 8All 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. 1.12.2 Create a series of at least ten (10)
