基础化学-梁逸增-第四章课件.ppt

中南大学化学化工学院 主讲人 刘绍乾 BufferSolutions 第4章 缓冲溶液 本章教学内容 4 1缓冲溶液与缓冲原理4 2缓冲溶液的pH值计算4 3缓冲容量和缓冲范围4 4缓冲溶液的配置4 5缓冲溶液在医学上的意义 内容提要 1 缓冲溶液的组成及缓冲机制 5 血液中的缓冲系 2 缓冲溶液pH的计算 3 缓冲容量和缓冲范围及其影响因素 4 缓冲溶液的配制 重点 难点 1 缓冲溶液pH值的计算2 缓冲容量的概念3 缓冲溶液的选择与配制方法 缓冲容量的概念 solutionsthatresistchangeinhydroniumion H andthehydroxideion OH concentration andconsequentlypH uponadditionofsmallamountsofacidorbase orupondilution 4 1 1TheEssentialFeatureofaBuffer BufferSolution 4 1缓冲溶液与缓冲原理 Buffersaremixturescontainingacommonion eitherweakacidsandtheirconjugatebasesorweakbasesandtheirconjugateacid Twocommonbuffers ammonium ammonia carbonate bicarbonate NH4 aq H aq NH3 aq CO32 aq H2O l HCO3 aq OH aq 共轭酸 共轭碱 HAcNH4ClH2PO4 NaAcNH3 H2OHPO42 抗酸成分 缓冲系 抗碱成分 缓冲溶液组成示意图 BuffersystemsthatareusefulatvariouspHvalues Tris Tris Hydroxymethy methanamin 三羟甲基氨基甲烷 下列情况均需pH一定的缓冲溶液 大多数为酶所控制的生化反应 微生物的培养 组织切片 细胞染色 药物调剂 研制等 Bufferwithequalconcentrationsofconjugatebaseandacid OH H3O BufferafteradditionofH3O H2O CH3COOHH3O CH3COO BufferafteradditionofOH 4 1 2HowaBufferWorks HAc H2O H3O Ac H Shiftleft OH H2O Shiftright Anti acid Anti base anti acidmechanism anti basemechanism TheamountsofweakacidandweakbaseinthebuffermustbesignificantlylargerthantheamountsofH3O orOH thatwillbeadded otherwisethepHcannotremainapproximatelyconstant Thusadditionoflimitedamountsofastrongacidorbaseiscounteractedbythespeciespresentinthebuffersolution andthepHchangesverylittle NosolutioncankeepthepHapproximatelyconstantifyouaddlargeramountsofeitheracidorbasethatarepresentintheoriginalbuffer ForaHB NaBbuffersystem HB H2O H3O B NaB Na B Ka H B HB H Ka HB B Apply logonbothsidesofaboveequation TheHenderson HasselbalchEquation 4 2缓冲溶液pH值的计算 pH pKa lg B HB pKa lg conjugatebase conjugateacid pKa the logofKaoftheconjugateacid B HB equilibriumconcentration B HB bufferratio B HB totalconcentration HB cHB cHB dissociated B cNaB cHB dissociated cHB cNaB TheHenderson HasselbalchEquation pKa lg nB V nHB V Iftheconcentrationsofconjugateacidandbaseusedareequal i e cB cHB pH pKa lg cB VB cHB VHB pKa lg VB VHB threedifferenttypesofofHenderson HasselbalchEquation Fightdilution 如果用活度代替浓度 校正因数 CalculatingthepHofaBufferSolution 1 PROBLEM SampleProblem4 1 Abufferispreparedbymixingequalvolumesof0 2mol L 1NaAcand0 4mol L 1HAc WhatisthepHofthefinalsolution ThepKaofHAcis4 74 WhatwillthepHbeaftertheadditionof0 005molofNaOH s to500mLofthebuffersolutiondescribedabove SOLUTION Beforetheadditionof0 005molofNaOH s to500mLofthebuffersolution CalculatingthepHofaBufferSolution 1 SampleProblem4 1 Aftertheadditionof0 005molofNaOH s to500mLofthebuffersolution CalculatingthepHofaBufferSolution 1 SampleProblem4 1 SOLUTION 例4 2 将0 10mol L 1的H3PO4溶液20mL与0 10mol L 1的NaOH溶液30mL混合 混合溶液是缓冲溶液吗 求混合溶液的pH 已知 pKa1 2 16 pKa2 7 21 pKa3 12 32 4 3 1TheConceptsofBufferCapacity BuffercapacityisdefinedastheamountofstrongacidorbaseneededtochangethepHofoneliterofbufferby1unit BuffercapacityistheabilitytoresistpHchange Or morespecifically 4 3缓冲容量和缓冲范围 where isthebuffercapacityandhasunitsofmolesperliterperpH mol L 1 pH 1 dna orb standsformolesofstrongacidorstrongbasewhichareaddedtoabuffersolutiontocausethechangeinpH dpH Thefollowingcanbederivedfromaboveone 2 303 HB B ctotal unit mol L 1 pH 1 Themagnitudeof indicatestherelativestrengthofbuffercapacity Thelargerthevalueof thegreaterthecapacityofthebuffertoresistchangesinpH 4 3 2FactorsAffectingBufferCapacity Buffercapacitydependsontwofactors Relativeone bufferratio B HB Absoluteone totalconc ofbuffer B HB WhenCtotaliffixed cB cHB 1 1 max cB cHB 10 1 min cB cHB 1 10 decrease min Foragivenbufferpair themorethebufferratioapproach1 thestrongerthecapacity whenthebufferratioequals1 thecapacityreachesitsmaximum decrease max 0 576ctotal 因为 2 303 HB B c总 即 max 0 576ctotal 缓冲比等于1时 HB B 1 2c总 所以 Whenthebufferratio c B c HB isfixed themoreconcentratedthecomponentsofabuffer thegreaterthebuffercapacity ThepHofabufferisdistinctfromitsbuffercapacity Whenthetotalconcentrationofbufferisfixed themoretheratioofc B c HB approaches1 themorethebuffercapacity Whenc B c HB 1 thebufferhasthehighestcapacity Conclusion 1 HCl 2 0 1mol L 1HAc NaOH 3 0 2mol L 1HAc NaOH 4 0 05mol L 1KH2PO4 NaOH 5 0 05mol L 1H2BO3 NaOH 6 NaOH 缓冲容量与pH的关系 Bufferrange thepHrangeoverwhichthebufferactseffectively Buffershaveausablerangewithin 1pHunitofthepKaofitsacidcomponent 4 3 3BufferRange max min decrease min decrease pH pKa 1 buffereffectiverange 1 Choosetheconjugateacid basepair Calculatetheratioofbuffercomponentconcentrations Determinethebufferconcentration 0 05mol L 1 0 2mol L 1 渗透压 4 MixthesolutionandadjustthepH Generalstepsforabufferpreparation 4 4缓冲溶液的配置 例4 3 如何配制pH 5 00的缓冲溶液100mL 解 1 选择缓冲系 表4 1 由于HAc的pKa 4 76 接近所配缓冲溶液pH 5 00 所以可选用HAc Ac 缓冲系 2 确定总浓度 计算所需缓冲系的量一般具备中等缓冲能力 0 05 0 2mol L 1 即可 并考虑计算方便 选用0 10mol L 1的HAc和0 10mol L 1NaAc溶液 应用式 4 2 可得 设V NaAc xmL则 V NaAc xmL 64mLV HAc 100mL 64mL 36mL AnenvironmentalchemistneedsacarbonatebufferofpH10 00tostudytheeffectsoftheacidrainonlimestone richsoils HowmanygramsofNa2CO3mustbeaddedto1 5Loffreshlyprepared0 20MNaHCO3tomakethebuffer KaofHCO3 is4 7x10 11 PLAN WeknowtheKaandtheconjugateacid basepair ConvertpHto H3O findthenumberofmolesofcarbonateandconverttomass Preparingabuffer 1 PROBLEM SampleProblem4 4 WeknowthepHofthebufferis10 0 Theconc ofNaHCO3is0 20M UsingHenderson HasselbalchEquationwecanfindouttheconc ofCO32 inthebuffer KaofHCO3 is4 7x10 11 CO32 0 094M molesofNa2CO3 1 5L 0 094mols L 0 14 15gNa2CO3 SOLUTION PROBLEM Preparingabuffer 2 SampleProblem4 5 Thereis2literof0 50mol L 1NH3 H2Oand2literof0 50mol L 1HCl hydrochloricacid inalaboratory AtechnicianwantstousethemtoprepareabufferwithpH 9 00withouttheadditionofwater Howmanylitersofbuffercanthetechnicianprepareatmost WhataretheconcentrationsofNH3 H2OandNH4 inthebuffer Thepkb NH3 H2O 4 74 SOLUTION Topreparethebufferwiththevolumeasmoreaspossible 2literof0 500mol L 1NH3 H2Omustbeutilizedcompletely whileonlyapartoftheHClcanbeused LetthevolumeofHClusedbexL so thetotalvolumeofthebufferpreparedis 2 00 x L Aftertheneutralization Preparingabuffer 2 SampleProblem4 5 NH3 aq HCl aq NH4 aq Cl l Initial mol 1 00 5V0 change mol 0 5V 0 5V 0 5V Equil mol 1 0 0 5V00 5V V 1 3L sothebiggestvolumeofbufferis3 3L Preparingabuffer 2 SampleProblem4 5 用0 030mol L 1H3PO4溶液和同浓度的NaOH溶液配制pH 7 40的生理缓冲溶液200mL 需要H3PO4和NaOH各多少毫升 ThepHofthebloodinahealthyindividualremainsremarkablyconstantat7 35to7 45 Thisisbecausethe bloodcontainsanumberofbuffersthatprotectagainstpHchangeduetothepresenceofacidicorbasicmetabolites Fromaphysiologicalviewpoint achangeof 0 3pHunitisextreme Acidmetabolitesareordinarilyproducedingreaterquanti