Calorimeter: From Wikipedia, The Free Encyclopedia
Calorimeter: From Wikipedia, The Free Encyclopedia
Calorimeter: From Wikipedia, The Free Encyclopedia
CalorimeterWikipedia,thefreeencyclopedia
Calorimeter
FromWikipedia,thefreeencyclopedia
Acalorimeterisanobjectusedforcalorimetry,ortheprocessofmeasuring
theheatofchemicalreactionsorphysicalchangesaswellasheatcapacity.
Differentialscanningcalorimeters,isothermalmicrocalorimeters,titration
calorimetersandacceleratedratecalorimetersareamongthemostcommon
types.Asimplecalorimeterjustconsistsofathermometerattachedtoametal
containerfullofwatersuspendedaboveacombustionchamber.
TofindtheenthalpychangepermoleofasubstanceAinareactionbetween
twosubstancesAandB,thesubstancesareaddedtoacalorimeterandthe
initialandfinaltemperatures(beforethereactionstartedandafterithas
finished)arenoted.Multiplyingthetemperaturechangebythemassand
specificheatcapacitiesofthesubstancesgivesavaluefortheenergygivenoff
orabsorbedduringthereaction.Dividingtheenergychangebyhowmany
molesofAwerepresentgivesitsenthalpychangeofreaction.Thismethodis
usedprimarilyinacademicteachingasitdescribesthetheoryofcalorimetry.It
doesnotaccountfortheheatlossthroughthecontainerortheheatcapacityof
thethermometerandcontaineritself.
Contents
1History
2Adiabaticcalorimeters
3Reactioncalorimeters
3.1Heatflowcalorimeter
3.2Heatbalancecalorimeter
Theworldsfirstice
calorimeter,usedinthe
winterof178283,by
AntoineLavoisierand
PierreSimonLaplace,to
determinetheheat
evolvedinvarious
chemicalchanges
calculationswhichwere
basedonJosephBlacks
priordiscoveryoflatent
heat.Theseexperiments
markthefoundationof
thermochemistry.
3.3Powercompensation
3.4Constantflux
4Bombcalorimeters
5Calvettypecalorimeters
6Constantpressurecalorimeter
7Differentialscanningcalorimeter
8Isothermaltitrationcalorimeter
9Seealso
10References
History
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ThenamecalorimeterwasmadeupbyAntoineLavoisier.[1]In1780,heusedaguineapiginhis
experimentswiththisdevicetomeasureheatproduction.Theheatfromtheguineapig'srespirationmelted
snowsurroundingthecalorimeter,showingthatrespiratorygasexchangeisacombustion,similartoa
candleburning.[2]
Adiabaticcalorimeters
Anadiabaticcalorimeterisacalorimeterusedtoexaminearunawayreaction.Sincethecalorimeterrunsin
anadiabaticenvironment,anyheatgeneratedbythematerialsampleundertestcausesthesampleto
increaseintemperature,thusfuellingthereaction.
Noadiabaticcalorimeterisfullyadiabaticsomeheatwillbelostbythesampletothesampleholder.A
mathematicalcorrectionfactor,knownasthephifactor,canbeusedtoadjustthecalorimetricresultto
accountfortheseheatlosses.Thephifactoristheratioofthethermalmassofthesampleandsample
holdertothethermalmassofthesamplealone.
Reactioncalorimeters
Areactioncalorimeterisacalorimeterinwhichachemicalreactionisinitiatedwithinaclosedinsulated
container.Reactionheatsaremeasuredandthetotalheatisobtainedbyintegratingheatflowversustime.
Thisisthestandardusedinindustrytomeasureheatssinceindustrialprocessesareengineeredtorunat
constanttemperatures.Reactioncalorimetrycanalsobeusedtodeterminemaximumheatreleaseratefor
chemicalprocessengineeringandfortrackingtheglobalkineticsofreactions.Therearefourmainmethods
formeasuringtheheatinreactioncalorimeter:
Heatflowcalorimeter
Thecooling/heatingjacketcontrolseitherthetemperatureoftheprocessorthetemperatureofthejacket.
Heatismeasuredbymonitoringthetemperaturedifferencebetweenheattransferfluidandtheprocess
fluid.Inaddition,fillvolumes(i.e.wettedarea),specificheat,heattransfercoefficienthavetobe
determinedtoarriveatacorrectvalue.Itispossiblewiththistypeofcalorimetertodoreactionsatreflux,
althoughtheaccuracyisnotasgood.
Heatbalancecalorimeter
Thecooling/heatingjacketcontrolsthetemperatureoftheprocess.Heatismeasuredbymonitoringtheheat
gainedorlostbytheheattransferfluid.
Powercompensation
Powercompensationusesaheaterplacedwithinthevesseltomaintainaconstanttemperature.Theenergy
suppliedtothisheatercanbevariedasreactionsrequireandthecalorimetrysignalispurelyderivedfrom
thiselectricalpower.
Constantflux
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Constantfluxcalorimetry(orCOFLUXasitisoftentermed)isderivedfromheatbalancecalorimetryand
usesspecializedcontrolmechanismstomaintainaconstantheatflow(orflux)acrossthevesselwall.
Bombcalorimeters
Abombcalorimeterisatypeofconstantvolumecalorimeterused
inmeasuringtheheatofcombustionofaparticularreaction.Bomb
calorimetershavetowithstandthelargepressurewithinthe
calorimeterasthereactionisbeingmeasured.Electricalenergyis
usedtoignitethefuelasthefuelisburning,itwillheatupthe
surroundingair,whichexpandsandescapesthroughatubethat
leadstheairoutofthecalorimeter.Whentheairisescapingthrough
thecoppertubeitwillalsoheatupthewateroutsidethetube.The
changeintemperatureofthewaterallowsforcalculatingcalorie
contentofthefuel.
Inmorerecentcalorimeterdesigns,thewholebomb,pressurized
withexcesspureoxygen(typicallyat30atm)andcontaininga
Bombcalorimeter
weighedmassofasample(typically11.5g)andasmallfixed
amountofwater(tosaturatetheinternalatmosphere,thusensuring
thatallwaterproducedisliquid,andremovingtheneedtoinclude
enthalpyofvaporizationincalculations),issubmergedundera
knownvolumeofwater(ca.2000ml)beforethechargeis
electricallyignited.Thebomb,withtheknownmassofthesample
andoxygen,formaclosedsystemnogasesescapeduringthe
reaction.Theweightedreactantputinsidethesteelcontaineristhen
ignited.Energyisreleasedbythecombustionandheatflowfrom
thiscrossesthestainlesssteelwall,thusraisingthetemperatureof
thesteelbomb,itscontents,andthesurroundingwaterjacket.The
Bombcalorimeter
temperaturechangeinthewateristhenaccuratelymeasuredwitha
thermometer.Thisreading,alongwithabombfactor(whichis
dependentontheheatcapacityofthemetalbombparts),isusedtocalculatetheenergygivenoutbythe
sampleburn.Asmallcorrectionismadetoaccountfortheelectricalenergyinput,theburningfuse,and
acidproduction(bytitrationoftheresidualliquid).Afterthetemperaturerisehasbeenmeasured,the
excesspressureinthebombisreleased.
Basically,abombcalorimeterconsistsofasmallcuptocontainthesample,oxygen,astainlesssteelbomb,
water,astirrer,athermometer,thedewarorinsulatingcontainer(topreventheatflowfromthecalorimeter
tothesurroundings)andignitioncircuitconnectedtothebomb.Byusingstainlesssteelforthebomb,the
reactionwilloccurwithnovolumechangeobserved.
SincethereisnoheatexchangebetweenthecalorimeterandsurroundingsQ=0(adiabatic)nowork
performedW=0Thus,thetotalinternalenergychangeU(total)=Q+W=0
Also,totalinternalenergychangeU(total)=U(system)+U(surroundings)=0U(system)=
U(surroundings)=CvT(constantvolumedV=0)
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whereCv=heatcapacityofthebomb
Beforethebombcanbeusedtodetermineheatofcombustionofanycompound,itmustbecalibrated.The
valueofCvcanbeestimatedbyCv(calorimeter)=m(water).Cv(water)+m(steel).Cv(steel)
m(water)andm(steel)canbemeasured
Cv(water)=1cal/g.K
Cv(steel)=0.1cal/g.K
Inlaboratory,Cvisdeterminedbyrunningacompoundwithknownheatofcombustionvalue:Cv=Hc/T
Commoncompoundsarebenzoicacid(Hc=6318cal/g)orpmethylbenzoicacid(Hc=6957cal/g).
Temperature(T)isrecordedeveryminuteandT=T(final)T(initial)
Asmallfactorcontributestothecorrectionofthetotalheatofcombustionisthefusewire.Nickelfusewire
isoftenusedandhasheatofcombustion=981.3cal/g
Inordertocalibratethebomb,asmallamount(~1g)ofbenzoicacid,orpmethylbenzoicacidisweighed.
AlengthofNickelfusewire(~10cm)isweighedbothbeforeandafterthecombustionprocess.Massof
fusewireburnedm=m(before)m(after)
Thecombustionofsample(benzoicacid)insidethebombHc=Hc(benzoicacid)xm(benzoicacid)+
Hc(Nifusewire)xm(Nifusewire)
Hc=Cv.TCv=Hc/T
OnceCvvalueofthebombisdetermined,thebombisreadytousetocalculateheatofcombustionofany
compoundsbyHc=Cv.T
[3][4]
Calvettypecalorimeters
Thedetectionisbasedonathreedimensionalfluxmetersensor.Thefluxmeterelementconsistsofaringof
severalthermocouplesinseries.Thecorrespondingthermopileofhighthermalconductivitysurroundsthe
experimentalspacewithinthecalorimetricblock.Theradialarrangementofthethermopilesguaranteesan
almostcompleteintegrationoftheheat.Thisisverifiedbythecalculationoftheefficiencyratiothat
indicatesthatanaveragevalueof94%+/1%ofheatistransmittedthroughthesensoronthefullrangeof
temperatureoftheCalvettypecalorimeter.Inthissetup,thesensitivityofthecalorimeterisnotaffectedby
thecrucible,thetypeofpurgegas,ortheflowrate.Themainadvantageofthesetupistheincreaseofthe
experimentalvessel'ssizeandconsequentlythesizeofthesample,withoutaffectingtheaccuracyofthe
calorimetricmeasurement.
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Thecalibrationofthecalorimetricdetectorsisakeyparameterandhastobeperformedverycarefully.For
Calvettypecalorimeters,aspecificcalibration,socalledJouleeffectorelectricalcalibration,hasbeen
developedtoovercomealltheproblemsencounteredbyacalibrationdonewithstandardmaterials.The
mainadvantagesofthistypeofcalibrationareasfollows:
Itisanabsolutecalibration.
Theuseofstandardmaterialsforcalibrationisnotnecessary.Thecalibrationcanbeperformedata
constanttemperature,intheheatingmodeandinthecoolingmode.
Itcanbeappliedtoanyexperimentalvesselvolume.
Itisaveryaccuratecalibration.
AnexampleofCalvettypecalorimeteristheC80Calorimeter(reaction,isothermalandscanning
calorimeter).[5]
Constantpressurecalorimeter
Aconstantpressurecalorimetermeasuresthechangeinenthalpyofareactionoccurringinsolution
duringwhichtheatmosphericpressureremainsconstant.
Anexampleisacoffeecupcalorimeter,whichisconstructedfromtwonestedStyrofoamcupsandalid
withtwoholes,allowinginsertionofathermometerandastirringrod.Theinnercupholdsaknownamount
ofasolute,usuallywater,thatabsorbstheheatfromthereaction.Whenthereactionoccurs,theoutercup
providesinsulation.Then
where
=Specificheatatconstantpressure
=Enthalpyofsolution
=Changeintemperature
=massofsolute
=molecularmassofsolute
Themeasurementofheatusingasimplecalorimeter,likethecoffeecupcalorimeter,isanexampleof
constantpressurecalorimetry,sincethepressure(atmosphericpressure)remainsconstantduringthe
process.Constantpressurecalorimetryisusedindeterminingthechangesinenthalpyoccurringinsolution.
Undertheseconditionsthechangeinenthalpyequalstheheat.
Differentialscanningcalorimeter
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Inadifferentialscanningcalorimeter(DSC),heatflowintoasampleusuallycontainedinasmall
aluminiumcapsuleor'pan'ismeasureddifferentially,i.e.,bycomparingittotheflowintoanempty
referencepan.
InaheatfluxDSC,bothpanssitonasmallslabofmaterialwithaknown(calibrated)heatresistanceK.
Thetemperatureofthecalorimeterisraisedlinearlywithtime(scanned),i.e.,theheatingratedT/dt=is
keptconstant.Thistimelinearityrequiresgooddesignandgood(computerized)temperaturecontrol.Of
course,controlledcoolingandisothermalexperimentsarealsopossible.
Heatflowsintothetwopansbyconduction.Theflowofheatintothesampleislargerbecauseofitsheat
capacityCp.Thedifferenceinflowdq/dtinducesasmalltemperaturedifferenceTacrosstheslab.This
temperaturedifferenceismeasuredusingathermocouple.Theheatcapacitycaninprinciplebedetermined
fromthissignal:
Notethatthisformula(equivalenttoNewton'slawofheatflow)isanalogousto,andmucholderthan,
Ohm'slawofelectricflow:V=RdQ/dt=RI.
Whensuddenlyheatisabsorbedbythesample(e.g.,whenthesamplemelts),thesignalwillrespondand
exhibitapeak.
Fromtheintegralofthispeaktheenthalpyofmeltingcanbedetermined,andfromitsonsetthemelting
temperature.
Differentialscanningcalorimetryisaworkhorsetechniqueinmanyfields,particularlyinpolymer
characterization.
Amodulatedtemperaturedifferentialscanningcalorimeter(MTDSC)isatypeofDSCinwhicha
smalloscillationisimposedupontheotherwiselinearheatingrate.
Thishasanumberofadvantages.Itfacilitatesthedirectmeasurementoftheheatcapacityinone
measurement,evenin(quasi)isothermalconditions.Itpermitsthesimultaneousmeasurementofheat
effectsthatrespondtoachangingheatingrate(reversing)andthatdon'trespondtothechangingheating
rate(nonreversing).Itallowsfortheoptimizationofbothsensitivityandresolutioninasingletestby
allowingforaslowaverageheatingrate(optimizingresolution)andafastchangingheatingrate
(optimizingsensitivity).[6]
SafetyScreening:DSCmayalsobeusedasaninitialsafetyscreeningtool.Inthismodethesamplewill
behousedinanonreactivecrucible(oftenGold,orGoldplatedsteel),andwhichwillbeabletowithstand
pressure(typicallyupto100bar).Thepresenceofanexothermiceventcanthenbeusedtoassessthe
stabilityofasubstancetoheat.However,duetoacombinationofrelativelypoorsensitivity,slowerthan
normalscanrates(typically23/minduetomuchheaviercrucible)andunknownactivationenergy,itis
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necessarytodeductabout75100Cfromtheinitialstartoftheobservedexothermtosuggestamaximum
temperatureforthematerial.Amuchmoreaccuratedatasetcanbeobtainedfromanadiabaticcalorimeter,
butsuchatestmaytake23daysfromambientatarateof3Cincrementperhalfhour.
Isothermaltitrationcalorimeter
Inanisothermaltitrationcalorimeter,theheatofreactionisusedtofollowatitrationexperiment.This
permitsdeterminationofthemidpoint(stoichiometry)(N)ofareactionaswellasitsenthalpy(deltaH),
entropy(deltaS)andofprimaryconcernthebindingaffinity(Ka)
Thetechniqueisgaininginimportanceparticularlyinthefieldofbiochemistry,becauseitfacilitates
determinationofsubstratebindingtoenzymes.Thetechniqueiscommonlyusedinthepharmaceutical
industrytocharacterizepotentialdrugcandidates.
Seealso
Enthalpy
Heat
Calorie
WikimediaCommonshas
mediarelatedto
Calorimeters.
Heatofcombustion
Calorimeterconstant
References
1. AntoineLaurentLavoisier,ElementsofChemistry:InaNewSystematicOrderContainingAlltheModern
Discoveries,1789:"IacknowledgethenameofCalorimeter,whichIhavegivenit,asderivedpartlyfromGreek
andpartlyfromLatin,isinsomedegreeopentocriticismbutinmattersofscience,aslightdeviationfromstrict
etymology,forthesakeofgivingdistinctnessofidea,isexcusableandIcouldnotderivethenameentirelyfrom
Greekwithoutapproachingtooneartothenamesofknowninstrumentsemployedforotherpurposes."
2. Buchholz,AndreaCSchoeller,DaleA.(2004)."IsaCalorieaCalorie?".AmericanJournalofClinicalNutrition
79(5):899S906S.PMID15113737.Retrieved20070312(http://www.ajcn.org/cgi/content/full/79/5/899S)
3. Polik,W.(1997).BombCalorimetery.Retrievedfromhttp://www.chem.hope.edu/~polik/Chem345
2000/bombcalorimetry.htm
4. Bozzelli,J.(2010).HeatofCombustionviaCalorimetry:DetailedProcedures.Chem339PhysicalChemistry
LabforChemicalEngineersLabManual.
5. C80CalorimeterfromSetaramInstrumentation(http://www.setaram.com/C80.htm)
6. http://csacs.mcgill.ca/francais/docs/CHEM634/DSC_Hunt.pdf
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