Densimetry(密度法).ppt

Physicalmethodsforfoodanalyses MehodsDensimetry 密度法 Refractometry 折光法 Polarimetry 旋光法 Colourimetricmethod 色度 Viscosimetry 粘度 Densimetry Thedensityofmanysubstancesisacharacteristicphysicalpropertyandservesforidentificationpurpose 1 Density isdefinedas mass volume m v2 Absolutedensity 绝对密度 t att isdefinedas t m g Vcm3 cm3 V totalvolumenotincludingairgap3 Relativedensity 相对密度 dt1t2 att isdefinedasdt1t2 m g Vml ml Bydefinition 1ml 0 001partofthevolumeof1kgofpurewaterat3 98 itisdimensionless t2 thetemperatureofsample t1 thetemperatureofwater4 Apparentdensity 表观密度 t1 att isdefinedas t1 m g Vcm3 cm3 V totalvolumeincludingairgapinastateofnature Definitionofdensity Influencingfactorfordensityanalyses1 TemperatureThedensitydecreasesbyabout0 03 per riseforthewateratroomtemperature Measurementofthedensityofliquids PycnometricMethod1 Typesofpycnometer Principle Measuringtheweightofaknownvolumeofliquidinavessel thevolumeofwhichwascalibratedintermsoftheweightofpurewaterthatthevesselholds MattersneedattentionBeforeuse thepycnometermustbethoroughlycleanedwithamixtureofpotassiumdichromateandconcentratedsulfuricacid rinsedthoroughlywithwater anddried Toavoidchangesinvolume thepycnometermustnotbesubjectedtoexcessivetemperatureorpressurechanges Bubblesarethemostcommonsourcesofrelativelylargeerrors Hydrometry Archimedesprinciple Basedontheprinciplethatthesamebodydisplacesequalweightsofallliquidsinwhichitfloats IfV1andV2arethevolumesoftwoliquidsdisplacedbythesamefloatingbody andD1andD2theirrespectivedensities thenV1D1 V2D2D1 D2 V2 V1Thus thevolumesofdifferentliquidsdisplacedbythesamefloatingbodyareinverselyproportionaltothedensitiesoftheliquids Ifthefloatingbodyisanuprightcylinderofuniformdiameter thevolumesdisplacedareproportionaltothedepths H1 H2 towhichthebodysinks D1 D2 H2 H1 Typesofhydrometers OrdinaryDensitometer 普通密度计 Alcoholometers 酒精计 Salometers 盐度计 Oleometers 油度计 Baum hydrometers 波美计 Saccharometer 糖度计 Lactometers 如稠计 Alcoholometers Alcoholometersareusedfordeterminingthepercentageofalcohol SalometersSalometersareusedtodeterminethepercentageofsaturationofsaltbrines OlemetersOlemetersareusedforvegetableandspermoils theyhaveascaleof50 0othatcorrespondingtodensitiesof0 870 0 897 Baum hydrometers Baum hydrometersareoftwokinds heavyB forliquidsheavierthanwater andlightB forliquidslighterthanwater Inthefirst 0ocorrespondstoadensityof1 000and66otoadensityof1 842 Inthelighterthanwater B correspondstoadensityof0 745 ForB degreeonascaleofdensitiesabove1 000 Desnisty 145 145 B reading Saccharometers Theuseofsaccharometersistodeterminethecompositionofvarioussugarblends sucrose cornsyrups invertsugar high fructosesyrups insolutionLactometersLactometersareusedtodeterminedrymilksolids andadulterationbywateraddition ofmilkfromdensitydeterminations TheSoxhletlactometerhasascaleof25 35 density1 025 1 035 subdividedintosuitabledivisions TheQuevennescalehasarangeof14 42oQuevennein1odivisions andcorrespondingscalestoindicatepercentageofwaterinwholeorskimmilk Hand heldDensitometer Measurementofthedensityofsolids Instrumentationfordeterminingapparentdensityofparticles Hand heldDensitometer Refractometry 折光法 Theamountofrefractionisacharacteristicofeverysubstance Itiscommonlyexpressedbytherefractiveindex RI n 折射率 RImeasurementshavelongbeenusedforthequalitative 定性的 identificationofunknowncompoundsbycomparingtheRIoftheunknownwithliteraturevaluesofvariousknownsubstances DefinitionReflectionphenomenaandlaw 反射现象和定律 2 Refractionphenomenaandlaw 折射现象和定律 Whenarayofelectromagneticradiationstrikesaflatsurfaceatanangle theraymaybentupward reflected orbenddownward refracted asillustratedinFig Noticethattheraydoesnotgostraightthroughthematerial TheRefractiveindex RI n definedasn sin 1 sin 2 1 2Because n C velocityinemptyspace So sin 1 sin 2 n2 n1 Totalreflection 全反射 andcriticalangle1 Opticallythinnermedium 光疏介质 2 Opticallythickermedium 光密介质 3 Criticalangle 临界角 sin 1 sin 2 n2 n1n1 n2sin 2n2 refractiveindexofprism 2 criticalangle n1 refractiveindexofliquid Threeraysofmonochromaticradiationstrikeamediumofdifferentdensity Threeoftheseraysarerefractedandwouldproducelightontheothersideofthemedium However thefourthrayandallotherrayshavinganangleofincidenceequaltoorgreaterthanitarenotrefracted Thusnolightgetsthroughthemediumatthispointandadarkfieldisproduced When 2isthecriticalangle 1 90o Refractometers 折射计 AbbeRefractometer 阿贝折光仪 TheAbberefractionisprobablythemostcommontypeemployedtoday Aschematicdiagramofitisshown therefractometercanbeusedfordirectreadingandrequiresonlyoneortwodropsofsample Polarimetry 旋光法 Therearemillionsofrayscomingfromyhourlampandthedirectionoftheelectricandmagneticcomponentsarepuelyrandomandmaylooklike a thisradiationissaidtobeunpolarized If However bysomemeanswecangetalloftheraystohavetheirelectricandmagneticcomponentsallinthesamedirection thentheradiationisplanepolarized asshownin b Opticalrotation opticalactivity Compoundsthatcanrotatepolarizedlight 偏振光 aresaidtobeopticalactive 旋光性 Intheoldsystemofnomenclature ifyouwerelookingtowardthelightsourceandtherotationwasclockwise thenthecompoundwassaidtobedextro 右 ifrotationwascounterclockwise thenthecompoundwaslevo 左 Thenewsystemofnomenclature theCahn Ingold Prelogsystem isnotbasedonrotationatallbutontheconfigurationofatomsaroundaparticularcenter Asanexampleofhowanopticallyactivecompoundisdesignatedinthissystem considerthemoleculeof1 chloro 1 ethanol Opticalrotation 旋光度 Angulardimension 角度大小 ofpolarizationplane 偏振面 ofpolarizedlightthatisrotatedbyopticalsample iscalledopticalrotation 2 Specificrotatorypower 比旋光度 t Lc OpticalrotationL Lengthc Concentrationofsample Polarimeter 旋光计 PolarimeterTheinstrumentisusedformeasuringopticalrotation