# ASTM E928-08 (Reapproved 2014)

Designation: E928 − 08 (Reapproved 2014)Standard Test Method forPurity by Differential Scanning Calorimetry1This standard is issued under the fixed designation E928; 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 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 describes the determination of purity ofmaterials greater than 98.5 mole percent purity using differen-tial scanning calorimetry and the van’t 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 There is no ISO method equivalent to this test method.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 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.2. Referenced Documents2.1 ASTM Standards:2E473 Terminology Relating to Thermal Analysis and Rhe-ologyE793 Test Method for Enthalpies of Fusion and Crystalliza-tion by Differential Scanning CalorimetryE794 Test Method for MeltingAnd Crystallization Tempera-tures By Thermal AnalysisE967 Test Method for Temperature Calibration of Differen-tial Scanning Calorimeters and Differential Thermal Ana-lyzersE968 Practice for Heat Flow Calibration of DifferentialScanning CalorimetersE1970 Practice for StatisticalTreatment ofThermoanalyticalData3. Terminology3.1 Definitions—The definitions relating to thermal analysisappearing in Terminology E473 shall be considered applicableto this test method.4. Summary of Test Method4.1 This test method is based upon the van’t Hoff equation:3Ts5 To2 ~RTo2χ!/~HF! (1)where:Ts= specimen temperature, KTo= melting temperature of 100 % pure material, KR = gas constant (= 8.314 J mol−1K−1),χ = mole fraction of impurity,H = heat of fusion, J mol−1, andF = fraction melted.4.2 This test method consists of melting the test specimenthat is subjected to a temperature-controlled program whilerecording the heat flow into the specimen as a function oftemperature. The resulting melting endotherm area is measuredto yield the enthalpy of fusion, H. The melting endotherm areais then partitioned into a series of fractional areas (about ten,comprising the first 10 to 50 % of the total area). The fractionalarea, divided by the total area, yields the fraction melted, F.Each fractional area is assigned a temperature, Ts.4.3 Eq 1 has the form of Y = mX +b where Y = Ts,X=1/F,m=−(RTo2χ)/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 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 ~Apart1c!/~Atotal1c! (2)1This test method is under the jurisdiction ofASTM Committee E37 on ThermalMeasurements and is the direct responsibility of Subcommittee E37.01 on Calo-rimetry and Mass Loss.Current edition approved Aug. 15, 2014. Published September 2014. Originallyapproved in 1983. Last previous edition approved in 2008 as E928 – 08. DOI:10.1520/E0928-08R14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3Brennan, W. P., DiVito, M. P., Fynas, R. L., Gray, A. P., “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.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1where:Apart= area of fraction melted, mJAtotal= total area, mJ andc = incremental area, mJ.NOTE 1—The best fit straight line may be determined by the leastsquares method. See Practice E1970.)4.5 The values of mole fraction impurity χ 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 van’t Hoff equation is givenby:4Apart52c1@Toc 2 RTo2χ m/M#/Ts1ToApart/Ts(3)where:m = mass of the sample, mg, andM = molecular weight, g mol−1.4.7 Eq 3 has the form of Y = α W+β X+γ Z where Y =Apart, α =−c,W=1,β =[Toc−RTo2χ m/M],X=1/Ts, γ= To, andZ=Apart/ Ts. Eq 3 may be evaluated by multiplelinear regression and χ and Todetermined form the resultantvalues of α, β and γ. This is Method B.5. Significance and Use5.1 The melting temperature range of a compound broadensas the impurity level rises. This phenomenon is describedapproximately by the van’t 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 datasuitable for analysis by the van’t 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 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 theoriginal detected enthalpy of fusion.5.4 This test method is used for quality control, specifica-tion acceptance, and research.6. Interferences6.1 This test method is nonspecific. Many impurities maycause the melting temperature broadening.Thus, it is not usefulin identifying the nature of the impurity or impurities but onlythe total mol percent of impurity present.6.2 The van’t 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 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 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 gas 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 differentheat 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 includes: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 temperaturesensor 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 µW; 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 a spe-cific temperature program by operating the furnace(s) betweenselected temperature limits at a rate of temperature change of0.3 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 calculatedsignals, 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 aluminum.7.2 Planimeter, computer- or electronic-based data treat-ment or other instrumentation to determine area to within61 % 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 sampling and sampled by removing portions from various4Widman, G., Scherrer, O., “ANew Program for DSC PurityAnalysis”, Journalof Thermal Analysis, 371987, pp. 1957–1964.E928 − 08 (2014)2parts 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.8.2 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 calibrations 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 test method (see 10.8) using Test MethodE967. High purity (99.99 %) indium metal is a convenientmaterial to use for this purpose.9.3 Calibrate the apparatus heat flow signal at the heatingrate to be used in this test method (see 10.8) using PracticeE968. High purity (99.99 %) indium metal is a convenientmaterial to use for this purpose.9.4 Determine the leading edge slope (S) in mW/K from theheat flow calibration curve obtained in 9.3. See Fig. 1.NOTE 2—The value of S is negative.10. Procedure10.1 Warning—Toxic 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 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 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, 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 sublimation onto the lid.NOTE 3—If 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 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 of 0.3 to 0.7 K min−1.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 4—All calculations shall use all available decimal places beforerounding the final result.11.1 Construct a linear baseline under the melting endo-therm 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. 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 illustratedFIG. 1 Fusion Curve for Method BE928 − 08 (2014)3by 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 partial area using Eq4.F 5ADEAABCA(4)where:F = fraction of total area,ADEA = area of fraction, mJ, andABCA = total area under fusion curve, mJ.11.5 Select at least ten partial area fractions between 10 and50 % 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 TD1DE/S (5)where:TF= corrected absolute temperature for area fraction, K,TD= measured absolute temperature at point D,K,S = slope, mW K−1, from 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. 1.The plot may concave upward.NOTE 5—The reasons for this 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 ADEA and to the total areaABCA until a straight line for the plot of TFversus 1/F isobtained.1/F 5 ~ABCA1c!/~ADEA1c! (6)11.9 Calculate the slope and the intercept, To,oftheTFversus corrected 1/F line where the equation for the line isgiven by the following:5TF5 ~ slope! 31F1To(7)NOTE 6—A least squares best fit may be useful for this purpose. SeePractice E1970.11.10 Employ Eq 8 to calculate the mole fraction impurityas follows:χ 52~slope! 3H/RTo2(8)where:χ = mole fraction impurity,R = universal gas constant = 8.314510 J mol−1K−1,5H = enthalpy of fusion, J mol−1(see Note 7) of majorcomponent of the solution, andTo= melting point of pure component, K.NOTE 7—If the enthalpy of fusion of the major component is not knownfrom other sources, Test Method E793 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 2 χ! 100% (9)where:X1= percent mole fraction purity12. Calculations – Method BNOTE 8—All calculations shall use all available decimal places round-ing the final result.12.1 Construct a baseline under the melting endotherm byextrapolating the baseline befo