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JournalofThermalAnalysis,Vol.45(1995)1285-1296©1995AkadémiaiKiadó,BudapestJohnWiley&Sons,LimitedChicesterTEMPERATURECALIBRATIONOFDIFFERENTIALSCANNINGCALORIMETERSD.M.PriceCourtauldsResearch&Technology,POBox111,LockhurstLane,COVENTRYCV65RS,UK(ReceivedNovember30,1994)AbstractAprocedureforcalibratingthetemperaturescaleofaDSCisdescribed.Adifferentcalibrationtrendwasobtainedusingthetransitionpointsoforganiccompoundscomparedtothatfoundusingthemeltingpointsofhighlypuremetals.Thecrystal-crystaltransitionsofthreeammoniumsaltswerestudiedbythismethod;ammoniumdihydrogenphosphateandammoniumsulphatewerefoundtobesuitablereferencematerialsintheregion-150to0°C,butammoniumnitratewasfoundtobeunsuitableduetopoorlydefinedtransitionpointsandinconsistentthermalbehaviour.Keywords:ammoniumsalts,crystal-crystaltransitions,DSC,meltingtransitions,reproduciblebehaviour,temperaturecalibrationIntroductionDifferentialscanningcalorimetry(DSC)isprobablythemostpopularthermoanalyticaltechniquewithinmaterialsscience.Itpartlyowesthispositiontoitsabilitytodetermineusefulmaterialparameters(e.g.meltingpoint,Tgetc.)rapidly,andwithhighprecision.Theaccuracyofsuchmeasurementsisdependantoncalibrationwithsubstanceswhichhavewell-definedtransitiontemperatures.Thisisespeciallyimportantwheremeasurementsmaybecarriedoutinvariouslaboratories,ondifferentinstruments,perhapsoveralongperiodoftime.Suitableexperimentalprocedureshavebeencomprehensivelydescribedintheliterature[1-3]althoughthechoiceofreferencematerialsisstillopentodebate[4,5].HighpuritymetalsexhibitsharpmeltingpeaksbyDSCwhichmakesthempopularforcalibratingtheinstrumentalresponseagainstaccuratevaluesdeterminedbyclassicalcalorimetry.Formanypracticalpurposes,thismaynotbeappropriate-whenstudyingorganicmaterials,suchashighpolymers,thereisanargumentforcalibratinglikewithlikesincetherearesignificantdifferencesinthermalconductivity,heatcapacityandtransitionenthalpybetweenmetalsandorganicsubstances[6].Inaddition,somemetalsaredifficulttoworkwithbecauseoftheirtoxicity(mercury)oreaseofhandling(gallium)andevencommonly-usedorganiccompounds,suchasdichloroethane,presentproblemsduetoenvironmentalissues[7]andvolatility.Evenso,itislikelythatindium,tin,lead,etc.willremainasconvenienttemperature(andenthalpy)standardsforDSCforthesometimetocome.TheuseofDSCbytheindustrialchemistpresentsaneedforsecondaryorworkingstandardsforDSCcalibration.Thesewouldhavethepropertiesofhavingone(ormore)transitionpointsintheregionofmostcommoninterest(ca.-100to400°C),beinginexpensive,readilyavailableinhighpurity,easytohandleandpresentinglittledifficultywithtoxicityanddisposal.Someofthesecriteriacanbemetbyexistingmaterialse.g.leadhasameltingpointclosetothatofpoly(tetrafluoroethlyene)andmakesaconvenientsingle-pointPRICE:TEMPERATURECALIBRATIONJ.ThermalAnal.,45,19952temperature(andenthalpy)calibrantforstudiesonthissubstance[8]despiteoxidisingreadily.Indiumandtinareusefulintheintermediatetemperaturerange,howevermeasurementsintheregionbelow150°Careproblematicsincethereislittlepublisheddataonconvenientreferencematerialswhichfittheabovecriteria.Thepurposesofthisstudywerethereforethreefold;i.tocalibrateaDSCinstrumentaccordingtoarecommendedprocedure[9]usinghighpuritymetalstandardswhosemeltingtemperaturesrepresentfixedpointsintheInternationalTemperatureScaleof1990[10,11]oritspredecessors[12],ii.toexploretheuseofsomelowmolecularweightorganicmaterialsastemperaturecalibrants,andiii.toinvestigatethesuitabilityofthephasetransitionsofreadilyavailableinorganicammoniumsaltsforsub-ambientandmediumtemperaturecalibration[13].MaterialsMercury,gallium,indium,tin,bismuthandlead(99.999%+pure)wereobtainedfromJohnson-Matthey.Cyclohexane,octane,decaneanddodecanewere99.9%+puritywereobtainedfromtheAldrichChemicalCo.Benzoicacid(99.99%),ammoniumdihydrogenphosphate,ammoniumsulphateandammoniumnitrate(all98%+pure)wereobtainedfromBDH.Freshlydistilledwaterwasuseddirectfromthestill.ExperimentalAllmeasurementsweremadewithaMettlerDSC-30heat-fluxdifferentialscanningcalorimeterandTC11controller.DatastorageandanalysiswasperformedonaIBMpersonalcomputerrunningMettlerTA72.1GraphWarewhichdisplayedtransitiontemperaturestothenearest0.1°C.Sampleswereweighedintoaluminiumcrucibleswhichhadbeenpreviouslybakedat500°Cfor30minutestoestablishaninertoxidecoatingonthecontainer.Metallicsampleswerefreshlycutandcleanedtoremoveanyexistingoxidelayer.Thecruciblewasthenhermeticallysealedtominimiseinteractionwiththeatmosphereandinhibitevaporationofthemorevolatilematerialsstudied.Thecrucibleswereweighedbeforeandaftereachseriesofexperimentstoconfirmthatnochangeinmasshadtakenplace.Measurementofthesample'stransitiontemperaturewasmadebyheatingtheDSCcellfrom50°Cbelowtheonsetofthetransition(toensurethatthematerialwasinthedesiredphasepriortoheating)to30°Cabovetheendofthetransition.Eachcruciblewassubjectedtosevenheatingprogrammesintheorder10,10,1,2,5,20and10°C/min,quench-coolingthecellwithliquidnitrogenin-betweenscans.Evenatthehighestheatingratesemployed,thestartingtemperaturewassufficientlybelowtheonsetofthetransitionfortheD