chapter 4 acid-base__ titrimetry.ppt

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1、Chapter 3 Chapter 3 TitrimetricTitrimetric Analysis Analysis3.2 Acid-Base Titration3.2.1 Acid-Base Equilibria 1)Acid-Base Theory 2)PBE pH Calculation 3)Distribution Coefficient 3.2.2 Titration Curves3.3.3 Acid-Base Indicators3.3.4 Applications Acid-Base TitrationThis is a quick accurate method for d

2、etermining acidic or basic substances in many samples.Several inorganic acids and basesHundreds of organic speciesThis titrant is typically a strong acid or base.The sample species can be either a strong or weak acid or base.3.2.1 Acid-Base Equilibria1.Acid-Base Theory Arrhenius Theory(Nobel Prize 1

3、903)Acid:any substance that ionizes in water to give hydrogen ions(H+).Hydrogen ions associates with the solvent to increase hydronium ion(H3O+)in solution.Base:ionizes in water to give hydroxyl ions(OH-)Brnsted Lowry TheoryAcid=Proton DonorBase=Proton Acceptor Acid BaseAcid Base HCl +H2O H3O+Cl-(St

4、rong)(Weak)H2CO3 +H2O H3O+HCO3-(Weak)(Strong)A Brnsted acid is a proton donor.A Brnsted base is a proton acceptor.Acid=H+BaseThese are conjugate pairs.A“strong”electrolyte is completely dissociated.A“weak”electrolyte is partially dissociated.Acid Dissociation ConstantBase Dissociation ConstantHA+H2O

5、 =H3O+A-A-+H2O =HA+OH-For any acid-base pair:Ka Kb=H+A-HAHA OH-A-=KwStrong acid-Ka Strong base-Kb Weak acid-Ka Weak base-Kb H3A=H+H2A-H2A-=H+HA2-HA2-=H+A3-For For PolyproticPolyprotic Acids:Acids:A3-+H2O=HA2-+OH-HA2-+H2O=H2A-+OH-H2A-+H2O=H3A+OH-For its Conjugate Bases:For its Conjugate Bases:Ka1 Kb3

6、=Ka2 Kb2=Ka3 Kb1=KwFor For PolyproticPolyprotic Acids:Acids:H3PO4 H2PO4-HPO42-PO43-Kb2Kb1Kb3Ka1Ka2Ka33.2.1 Acid-Base Equilibria2.Calculation of pH pH=-lgH+pH is the acidity expression of an aqueous solution:25 H+=1.010-7 mol/L pH=7.00 OH-=1.010-3 mol/L H+=1.010-11 mol/L pH=11.00(1)Proton balance equ

7、ation(PBE)a)H+obtained=H+lostb)Selecting zero levelsc)Judging the species that lose protons and d)obtain protonsd)Writing the PBE equationExample Write the PBE for the acid HA solutionIn the solution：the species that are present in sufficient amount are HA,H2O HA+H2O=H3O+A-H2O+H2O=H3O+OH-PBE:H3O+=A-

8、+OH-not containing HA,H2OAnother Example Write the PBE for the Na2CO3 solution In the solution：the species that are present in sufficient amount are CO32-,H2O CO32-+H2O=HCO3-+OH-HCO3-+H2O=H2CO3+OH-H2O+H2O=H3O+OH-PBE:HCO3-+H2CO3 +H+=OH-not containingCO32-,H2OWrite the PBE for Na2S solution?2.Calculat

9、ion of pH(2)Solutions of Strong Acids and BasesStrong Acid(HA):HA +H2O =H3O+A-H2O+H2O =H3O+OH-PBE:H3O+=A-+OH-Suppose A-=ca,at 25 C If c 10-6 mol/L,H3O+=A-=ca If c 10-6 mol/L,OH-=HB=cb If c 10-6 mol/L,Kw/OH-+cb=OH-Exact EquationExample What is the pH of a 0.004 mol/L solution of hydrochloric acid?cHC

10、l=0.004 mol/L H+=0.004 M pH =-log(0.004)=2.4(3)Solutions of Weak Acid/BaseFor Monoprotic Weak Acid HA PBE：H+=A-+OH-Ka=H+A-/HA OH-=Kw/H+If caKa 20 Kw,Kw is negligible,thus-Approximate EquationIf the concentration of the weak acid is not too low,that is ca/Ka 500-Simplified Equation-Simplified Equatio

11、nSimilarly,the simplified equation of a weak base concentration is-Simplified Equation-Simplified EquationExampleA solution contains 6.1gL-1 of benzonic acid,C6H5COOH(苯甲酸、安息香苯甲酸、安息香),the Ka for benzonic acid is 6.310-5=10-4.2.calculate the pH.Solution(4)Dissociation of Polyprotic Acid/BasePolyprotic

12、 acids ionize in steps:The solution contains H2A,HA-and A2-,calculating its pH is complicated.For a solution containing H2A or H2A+HA-The second dissociation will have little effect and can be ignored.The pH of the solution is calculated from the Ka1 expression.If ExampleCalculate the pH of a 0.15 m

13、olL-1 solution of malonic acid,CH2(COOH)2.The dissociation constants for malonic acid are Ka1=10-2.85 and Ka2=10-5.66.SolutionThe second dissociation is ignored and H2A=0.15 H+Amphiprotic Solution -can accept or donate a protonSuch as HCO3-,HPO42-,H2PO4-,NH4Ac,etc.PBE:H+H2A=A2-+OH-Dissociations in H

14、A-solution:PBE:H+H2A=A2-+OH-Example Calculate the pH of a solution of sodium hydrogen malonate,HOOC-CH2-COONa.The dissociation constants for malonic acid are pKa1=2.85 and pKa2=5.66.Solutions:unless the solution is very diluteExample Calculate the pH of a solution having,at equilibrium,a hydrogen ma

15、lonate ion HA-concentration of 0.15 mol/L and a malonate ion A2-concentration of 0.05 mol/L.Solutions:(5)Buffers A buffered solution resists changes in pH when acids or bases are added or when dilution occurs.Buffer=Acid+its Conjugate Basee.g.HAc-NaAc H2CO3-HCO3-H2PO4-HPO42-Henderson-Hasselbalch Equ

16、ation:The Derivative ProcessKa:the HA acid dissociationTake logsRearrangeFor any acidsExampleFind the pH of a solution prepared by dissolving 12.43 g of tris-(hydroxymethyl)aminomethane(FW 121.136)plus 4.67g tris-hydrochloride(FW 157.597)in 1.00L water.(pKa=8.075)Solution:the moles of B and BH+in th

17、e solution areUsing the Henderson-Hasselbalch equation:Attention!SolutionpH1025380.05mol/L K tetroxalate1.671.681.690.05mol/L KH2 citrate3.8203.7763.7750.05mol/L KH Phthalate3.9984.0084.0300.025mol/L KH2PO4+0.0025mol/L Na2HPO46.9236.8656.8400.01mol/L Na2B4O79.3329.1809.0810.025mol/L NaHCO3+0.0025mol

18、/L Na2CO310.17910.0129.903Saturated solution Ca(OH)213.0012.4512.04Table 3-1 Standard Buffers and their pH Values3.Calculation of Species Present As a strong base reacts with a diprotic acid,the pH increases and the proportion of H2A,HA-and A2-in solution change.Distribution Coefficient-the fraction

19、 of each species at equilibrium,expressed as The distribution of the acidic and basic forms as a function of pH -Distribution GraphThe Role of Distribution Graph：1.Understand the acid-base titration deeply 2.Judge whether the polyprotic acid(base)can be titrated step by step The titration is a proce

20、ss that the fraction of each species and the pH change continuously.For example,a monoprotic acid HAFor example,a monoprotic acid HAThen Distribution Graph of HADiscuss：HA+A-=1 When pH=pKa；HA=A-=0.5 When pH pKa；A-is predominateFor example,a Diprotic Acid H2ASimilarlya.pH pKa1，H2A predominateb.pKa1 p

21、H pKa2，A2-predominated.pH=2.75,HA-is biggest HA-=0.938;H2A=0.028;A2-=0.034(H2C2O4)Discussion of H2A Distribution GraphFor a Triprotic Acid H3ADistribution Graph of H3PO4For Acid HnA阿司匹林阿司匹林pH=1.0pH=8.4 假假设设 H+,A-等等离离子子不不能能穿穿透透隔隔膜膜,HA分分子子可可自自由由通通过过隔隔膜膜.达达平平衡衡时时,隔隔膜膜两两边边的的HA浓度相等，浓度相等，即即HA血血=HA胃胃 阿阿司司匹

22、匹林林是是一一种种弱弱酸酸(即即乙乙酰酰水水杨杨酸酸),pKa=3.5.计计算算阿司匹林在胃中的吸收比率。阿司匹林在胃中的吸收比率。pH=7.4 pH=1.0 血浆血浆 隔膜隔膜 胃胃药物的吸收方式可表示为药物的吸收方式可表示为H+A-HAHA H+A-HA=血血HA c血血=胃胃HAc胃胃=c血血c总药总药c血血c血血+c胃胃c血血c胃胃=1+血血HA胃胃HA=1+H胃胃(H血血+Ka)(H胃胃+Ka)H血血=1+=c血血c总药总药99.99%3.2.2 Titration Curves For each type of titration,our goal is to construct

23、a graph showing how the pH changes as the titrant is added.By doing this,we understand what is happening during the titration and are able to interpret an experimental titration curve.Titration curves We assume that you are always smart enough to use either a strong acid or base as your titrant.This

24、 will results in your types of titrations for simple acids or bases.Strong Acid vs.Strong Base Strong Acid vs.Weak Base Strong Base vs.Strong Acid Strong Base vs.Weak AcidIn the titration,there are three regions:(1)Before reaching the equivalence point(2)At the equivalence point(3)After the equivale

25、nce point Titration Curves A plot of ml titrant(or%titration)vs pH will result in a typical titration curve.Titration Curves Example:Construct a titration curve for the titration of 100ml of 0.10M HCl with 0.10M NaOH.0%Titration pH=-log0.10=1.00After 10ml NaOH100%Titration Equivalence point pH=7Note

26、 that for the first 90%of our titration,we only saw a change of 1.28 pH units.Now we have a jump of 4.72 pH units.Titration Curves OvertitrationAll we need to do here is to account for the dilution of our titrant.10ml OvertitrationTitration Curves Figure.Dependence of the magnitude of end-point brea

27、k on concentrationThe concentrations of sample and titrant are the same.Titration Curves of Weak Acid/Base Titration of a weak acid or base with a strong titrant is a bit more complex than the strong acid/strong base sample.We must concern with the conjugate acid/base pairs and their equilibrium.Exa

28、mple(acid)HA+H2O H3O+A-(base)We will have the same four general regions for our titration curve.Calculation will require that you use the appropriate Ka or Kb relationship.Titration Curves of Weak Acid/BaseExample:A 100 ml solution of 0.10M benzoic acid is titrated with 0.10M NaOH.Construct a titrat

29、ion curve.For Benzoic AcidKa=6.31 10-5pKa=4.20 Titration Curves of Weak Acid/Base0%Titration 10%Titration Here we can use the Henderson-Hasselbalch equation.We can calculate other points by repeating this process.Titration Curves of Weak Acid/Base Titration Curves of Weak Acid/Base100%Titration At t

30、his point,all of our acid has been converted to the conjugate base benzoate.We need to use the Kb relationship to readily solve for this point.Titration Curves of Weak Acid/Base100%Titration At the equivalence point:HA=OH-HA+A-=0.05MWe have diluted the sample and the total volume at this point is 20

31、0ml.Finally,we can assume that benzoic acid is negligible compared to benzoate100%Titration Titration Curves of Weak Acid/BaseOver-titration All we need to do here is to account for the dilution of our titrant.10%over-titration(10ml excess)Titration Curves of Weak Acid/Base Titration Curves of Weak

32、Acid/BaseThe weaker the acid,the smaller the break and the more alkaline the equivalence point.Fig.Titration curve for 100 mL 0.1 M NH3 versus 0.1 M HCl.We start with NH3 weak base.Then we have a buffer mixture of NH4+and NH3.At the equivalence point,we have NH4+,a weak acid.Beyond the e.p.,we have

33、excess HCl,which suppresses the hydrolysis of NH4+,and the curve follows that for a strong base titration.Figure.Titration curves for 100 mL 0.1 M weakbases of different Kb values versus 0.1 M HCl.The weaker the base,the smaller the break and more acid the equivalence point.多元酸多元酸(碱碱)的滴定的滴定 多元酸能分步滴定

34、的条件多元酸能分步滴定的条件 滴定计量点的滴定计量点的pH值的计算值的计算 指示剂的选择指示剂的选择多元酸能分步滴定的条件多元酸能分步滴定的条件被滴定的酸足够强被滴定的酸足够强,cKan10-8相邻两步解离相互不影响，相邻两步解离相互不影响，lgKa足够大足够大,若若pH=0.2,允许允许TE%=0.3%,则需则需lgKa4滴定化学计量点的滴定化学计量点的pH值的计算值的计算以以H3A为例：为例：前提：可以分步滴定前提：可以分步滴定 sp1:H2A-按按酸式盐酸式盐计算计算 pH=1/2(pKa1+pKa2)sp2:HA2-按按酸式盐酸式盐计算计算 pH=1/2(pKa2+pKa3)sp3:A

35、3-按按多元碱多元碱计算计算OH-=Kb1c Kw/Ka3 c NaOH滴定滴定0.1molL-1H3PO4HCl滴定滴定0.1molL-1Na2CO3混合酸分步滴定混合酸分步滴定两弱酸混合（两弱酸混合（HAHB）被滴定的酸足够强被滴定的酸足够强,cKa10-8 c1Ka/c2Ka105强酸弱酸（强酸弱酸（H+HA）Ka 10-7,测总量测总量 Ka 4.4醌式醌式pH 3.1Assume that Transition Range of an IndicatorWhen only the color of the unionized form is seen,In-/HIn 10/1.So

36、Transition range of an indicatorUnder these conditions,the pH transition range for our indicator is pKHIn 1.Most indicators require a transition range of about two pH units.Some commonly used indicators are listed in table 3-3.Methyl violet甲基紫甲基紫P-nitrophenol硝基酚硝基酚Cresol red甲酚红甲酚红Bromothymol blue溴百里

37、酚蓝溴百里酚蓝Thymol blue百里酚蓝百里酚蓝Neutral red中性红中性红Methyl orange甲基橙甲基橙Phenol red酚红酚红Congo red 刚果红刚果红Phenolphthalein酚酞酚酞Methyl red甲基红甲基红Thymolphthalein百里酚酞百里酚酞Litmus石蕊石蕊Alizarin yellow R茜黄茜黄Methyl purple甲基紫甲基紫1,3,5-trinitrobenzene三硝基苯三硝基苯Table 3-3 Common Indicators 影响指示剂变色范围的因素影响指示剂变色范围的因素指示剂用量指示剂用量:宜少不宜多宜少不

38、宜多，对单色指示剂影响较大，对单色指示剂影响较大 例：例：50100mL溶液中溶液中23滴滴PP，pH9变色，变色，而而1015滴滴PP，pH8变色变色温度温度离子强度离子强度：影响：影响pKHInSelection of IndicatorsThe closer the point of the indicator color change is to the equivalence point,the more accurate the end point will be.The difference between the observed end point(color change)

39、and the true equivalence point is called the indicator error.Selection of IndicatorsBecause indicators are acids or bases,they react with analyte or titrant,we use indicator under the assumption that the moles of indicator are negligible compared to the moles of analyte.Never use more than a few dro

40、ps of dilute indicator solution.Selection of IndicatorsIn general,we seek an indicator whose transition range overlaps the steepest part of titration curve as closely as possible.常用酸碱标准溶液的配制与标定常用酸碱标准溶液的配制与标定酸酸标准溶液标准溶液:HCl(HNO3,H2SO4)配制配制:用用市售市售HCl(12 molL-1),HNO3(16 molL-1),H2SO4(18 molL-1)稀释稀释.标定标定

41、:Na2CO3 或或 硼砂硼砂(Na2B4O710H2O)常用酸碱标准溶液的配制与标定常用酸碱标准溶液的配制与标定碱标准溶液碱标准溶液:NaOH配制配制:以饱和的以饱和的NaOH(约约19 molL-1),用用除去除去CO2 的去离子水稀释的去离子水稀释.标定标定:邻苯二甲酸氢钾邻苯二甲酸氢钾(KHC8H4O4)或草酸或草酸(H2C2O42H2O)3.2.4 Applications of Acid-base Titration1)At the first end point(phenolphthalein)2)HCl consumed is V1 mL The reactions are:H

42、Cl+Na2CO3 =NaHCO3+NaCl HCl+NaOH =NaCl+H2ONaOH,Na2CO3,NaHCO31.Titration of Mixtures Containing Sodium Carbonate1.Titration of Mixtures Containing Sodium CarbonateNaOH,Na2CO3,NaHCO32)At the second end point(methyl orange)HCl consumed V2 mL The reaction is:HCl+NaHCO3 =NaCl+CO2 +H2ODiscussion：（1）V1V2:Na

43、OH(V1-V2),Na2CO3(V2)（2）V1=V2:Na2CO3（3）V1V2:NaOH(V1-V2),Na2CO3(V2)1.Titration of Mixtures Containing Sodium Carbonate(2)V1V2:NaHCO3(V1-V2),Na2CO3(V2)1.Titration of Mixtures Containing Sodium Carbonate药用药用NaOH的测定的测定准确称取一定量样品准确称取一定量样品吸取相同体积的两份吸取相同体积的两份锥形瓶（二）锥形瓶（二）锥形瓶（一）锥形瓶（一）甲基橙甲基橙加过量的加过量的BaCl2溶液溶液溶解，定

44、容溶解，定容酚酞酚酞滴定至终点，滴定至终点，消耗消耗HCl标液标液V1ml滴定至终点，滴定至终点，消耗消耗HCl标液标液V2ml2.Titration of Boric AcidBoric acid(pKa=9.24)is too weak to be directly titrated,however,it can react with polybasic alcohols(glycol,glycerin and mannitol)forming complex acid(pKa is about 6),pH of stoichiometric point is about 9.3.Kjel

45、dahl Nitrogen DeterminationFor analyzing organic samples containing nitrogen,such as pure organic compounds,foods,fertilizers,etc.Step 1:Prereduction Inorganic nitratesOrganic nitroAzo compoundsAmineAmide Step 2:Digestion Step 3:DistillationStep 4:Titration Direct titrationBack titration4.SiO23.2.5

46、Titration Endpoint ErrorTitration Error is the difference between the end point and the equivalence point,expressed by TE%:Titration Errors Determinate Error-excess titrant must be added to the sample to convert the indicator.The effect can be minimized by titration of one or more blank samples.Same

47、 solution volumeIdentical amount of indicatorThe blank amount can then be substracted from the total amount of titrant needed.Titration ErrorsIndeterminate ErrorEach person will differ in how well they can observe a special color change.Running a blank samples will help minimize problems.You can als

48、o prepare a reference solution-a blank adjust to the exact color.1.Titration Endpoint Error of Strong Acid/BaseFor example,a strong acid(HCl)titrated with a strong base(NaOH)The Concentration and Volume of HCl at spThe PBE at any time of titration:For a strong base(NaOH)titrated with a strong acid(H

49、Cl)1.Titration Endpoint Error of Strong Acid/BaseExampleCalculate the TE%of a 0.1000M HCl titrated with 0.1000M NaOH to the methyl orange endpoint(pH=4.0)or to the phenolphthalein endpoint(pH=9.0)Solution:It is a strong acid titrated with a strong base,so we should use the following equationFor meth

50、yl orange endpoint(pH=4.0)H+=10-4 mol/L OH-=10-10 mol/L csp=0.0500mol/LFor phenolphthalein endpoint(pH=9.0)H+=10-9 mol/L OH-=10-5 mol/L csp=0.0500mol/L2.Titration Endpoint Error of Weak Acid/BaseFor a strong base(NaOH)titrated with a weak acid(HA)The PBE at end point of titration:The endpoint is alk