熔沸点的测定

MeltingPointandBoilingPoint1.Purpose1.1Learntheknowledgeofmeltingpointandboilingpoint.1.2GraspthemethodstotestmeltingpointandboilingpointwithCapillaryTubes.1.3GrasptheusageofDigitalmelting-pointapparatus.2.Principle2.1MeltingPointDefinition:Themeltingpointofasubstanceisdefinedasthetemperatureatwhichtheliquidandsolidphasesexistinequilibriumwithoneanother.Figure1.PhasechangewithtimeandtemperatureSharpincreaseaftermelting,soheatslowlyIfheatisaddedtoamixtureofthesolidandliquidphasesofapuresubstanceatthemeltingpoint,ideallynoriseintemperaturewilloccuruntilallthesolidhasmeltedandbeenconvertedtoliquid.Themeltingpointandfreezingpointofapuresubstanceareidentical.Apureorganiccompoundhaveasettledmeltingpoint,usuallylowerthan350℃,withatemperaturerangeof0.5～1℃.Figure2.TheeffectsofimpuritiesonmeltingpointAccordingtoRaoult'slaw,atacertainpressureandtemperatureconditions,ifweputsoluteintoasolvent,thepartialpressureofsolventvaporwillobviouslydecreased.ThesolidandliquidphasesintersectatpointsM1,whichrepresentstheboilingpointofcompoundscontainingimpuritiesarelowerthanthatofpurecompounds.

当含杂质时（假定两者不形成固熔体），根据拉乌耳定律可知，在一定的压力和温度条件下，在溶剂中增加溶质，导致溶剂蒸气分压降低，固液两相交点M´即代表含有杂质化合物达到熔点时的固液相平衡共存点，T1为含杂质时的熔点，显然，此时的熔点较纯粹者低，熔点距扩大。因此，熔点是判断晶体化合物纯度的重要指标之一。2.2Meltingpointmethodsandapparatus(1)CapillarytubesandsamplepreparationCommerciallyavailablecapillarytubeshaveoneendalreadysealedandtheotheropenedendtopermitintroductionofthesample（2-3mm）。熔点管制备：将毛细管一端用酒精灯烧熔封口，呈45°角，不停旋转。注意事项：样品需研细，可填装结实，有空隙则不易传热，影响结果。样品装实后2～3mm。样品装填需迅速，防治吸潮。SamplesPreparations表面皿桌面玻璃管样品(2~3mm)样品样品管样品只可用一次（2）Thieletubeapparatusb形管石蜡油浴橡皮圈熔点管样品温度计12435ProperuseoftheThieletubeiscriticaltoobtainreliablemeltingpoints。Heatingrate.Firstly,theheatingshouldbequickly.Whenthetemperatureis10～15℃lowerthanthemeltingpoint,heattheThieletubeattherateof1～2℃/mininordertodeterminethemeltingpoint。通过剪短灯芯或移远酒精灯调节加热速度。NotesFiveimportantpositions.（3）MeltingpointdeterminationprocedureMeltinginvariablyoccursoveratemperature,andthemeltingpointrangeisdefinedasthetemperatureatwhichthefirsttinydropofliquidappearsuptoandincludingthetemperatureatwhichthesolidhasmeltedcompletely。Payattentiontothefirsttinydropofliquidappearisthebeginningofthemeltingprocess。2.3Digitalmeltingpointapparatus开机稳定20min设起始温度（低于熔点10℃）放样（电流归零）升温（1℃/min）初熔灯亮出终熔温度降温后关机2.4BoilingPointDefinition:whenthevaporpressureoftheliquidbeingheatedequalstheatmosphericpressureonthesurfaceoftheliquid,itboils.Thetemperatureatthatmoment,iscalledboilingpoint.Figure4.TherelationshipbetweenvaporpressureandboilingpointEverypureorganiccompoundhasafixedboilingpointatacertainpressure.Butsometimesanorganiccompoundtogetherwithothercomponentscanformabinaryorternaryazeotropemixture(二元或三元共沸物）,whichalsohasadefiniteboilingpoint.沸点管分内管与外管，外管回收。外管内液体量理论为1/5，实际装1/2，以免挥发损失。内管长度为外管的2/3，开口端向下，但勿接触管底。开口端位于温度计水银球中部。加热时，内管中会有小汽泡缓缓逸出，在达到沸点时，将有一连串的小气泡快速地逸出。此时停止加热，当最后一个气泡刚欲缩回至内管中时，此时温度即为该液体的沸点。b形管石蜡油浴橡皮圈沸点内管样品液面沸点外管外管回收Figure5MicroDeterminationofboilingpoint3.ExperimentsandR