chapter 4 acid-base-- titrimetry.ppt

1、Chapter 3 Titrimetric 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 determining acidic or basic sub

2、stances 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 1903)Acid:any substance that io

3、nizes 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 Theory Acid=Proton Donor Base=Proton Acceptor Acid BaseAcid Base HCl +H2O H3O+Cl-(Strong)(Weak)H2CO3 +H2O H3O+HC

4、O3-(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 =H3O+A-HAAHKa A-+H2O =HA+OH

5、-AOHHAKb 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-AHHAHK22a2 AHAHHK32a1 HAAHK3a3 A3-+H2O=HA2-+OH-HA2-+H2O=H2A-+OH-H2A-+H2O=H3A+OH-HAOHAHK222b AOHHAK321b AHOHAHK233b Ka1 Kb3=Ka2 Kb2=Ka3 Kb1=KwH3PO4 H2PO4-HPO42

6、-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 equation(PBE)a)H+obtained=H+lostb)Selecting zero levelsc)Judging the species tha

7、t lose protons and 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-+OH-not containing HA,H2OAnother Example Write the PBE for the Na2CO3 solution

8、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.Calculation of pH(2)Solutions of Strong Acids and BasesStrong Acid(HA):HA +H2O =H3O+A-H

9、2O+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 Equation24KOHw2 bbccExample What is the pH of a 0.004 mol/L solution of hydrochloric acid?cHCl=0.004 mol/L H+=0.004 M pH =-log(0.004)=2.4(3)Solutions of Weak Ac

10、id/BasewaKHAKH For Monoprotic Weak Acid HA PBE：H+=A-+OH-Ka=H+A-/HA OH-=Kw/H+)H(KHAKHaaa cIf caKa 20 Kw,Kw is negligible,thus2K4KKHaa2aac -Approximate EquationaaKHc If the concentration of the weak acid is not too low,that is ca/Ka 500bbKc OHSimilarly,the simplified equation of a weak base concentrat

11、ion is ExampleA 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.Solution1050.01221.6 Lmolca2.410050.0H aaKc75.2pH1024.2H10.3 (4)Dissociation of Polyprotic Acid/BaseHAAHKAHHAAHHAHKHAHAH22a221a2 Polyprotic acids ionize in

12、steps:The solution contains H2A,HA-and A2-,calculating its pH is complicated.100KK2a1a 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 molL-1 s

13、olution of malonic acid,CH2(COOH)2.The dissociation constants for malonic acid are Ka1=10-2.85 and Ka2=10-5.66.Solution100101010KK81.266.585.22a1a The second dissociation is ignored and H2A=0.15 H+2.8522a110H0.15HAHHAHK 1.86pH101.38Hergo2 OHAHOHHA:BaseAHHA:Acid222 Amphiprotic Solution -can accept or

14、 donate a protonSuch as HCO3-,HPO42-,H2PO4-,NH4Ac,etc.OHHOHKOHAHOHHAKAHHA22b222a2PBE:H+H2A=A2-+OH-Dissociations in HA-solution:OH HHAK OHHAK H-2a2b w-wa1b2KOH HKKK HK HHAK KHAH Hwa2a1 PBE:H+H2A=A2-+OH-HAKKKHAKK Ha1wa1a2a12 HAK)KHA(KK Ha1wa2a12 a2a1a2a12a1awa2aKKHAHAKK H20Kand20KKIf cc2pKpK pHKK Ha2a

15、1a2a1 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 dilute26.4266.585.22pKpKpH2a1a a1awa2a20Kand20KK ccExample Calculate the pH of a solution having,at eq

16、uilibrium,a hydrogen malonate ion HA-concentration of 0.15 mol/L and a malonate ion A2-concentration of 0.05 mol/L.Solutions:18.5pH106.6H15.005.0H102.2HAAHK6622a (5)Buffers A buffered solution resists changes in pH when acids or bases are added or when dilution occurs.Buffer=Acid+its Conjugate Basee

17、.g.HAc-NaAc H2CO3-HCO3-H2PO4-HPO42-Henderson-Hasselbalch Equation:)HA(n)A(nlgpKHAAlgpKpHaa The Derivative ProcessHAAHKa HAAlglgHlgKa Ka:the HA acid dissociationTake logsRearrangeFor any acidsHAAlgpKpHHAAlglgKlgHaa ExampleFind the pH of a solution prepared by dissolving 12.43 g of tris-(hydroxymethyl

18、)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 the solution aremol0296.0597.15767.4)BH(nmol1026.0136.12143.12)B(n Using the Henderson-Hasselbalch equation:61.80296.01026.0lg075.8)HA(n)A(nlgpKpHa Attention!SolutionpH102538

19、0.05mol/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/L Na2CO310.17910.0129.903Saturated solution Ca(OH)213.0012.4512.04Table 3-1 Stan

20、dard 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.cii Distribution Coefficient-the fraction of each species at equilibrium,expressed as The distribution of the acidic a

21、nd 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 process that the fraction of each species and the pH change continuously.For examp

22、le,a monoprotic acid HA AOHOHHA32HAAHKandAHAatotal c)HKHA(1HHAKHAaatotal cFor example,a monoprotic acid HAHKHHK11HA(HA)aa cHKKA)(Aaa cThen Distribution Graph of HADiscuss：HA+A-=1 When pH=pKa；HA=A-=0.5 When pH pKa；A-is predominateFor example,a Diprotic Acid H2A2a1a1a22AHKKKHHH2 2a1a1a22a1aAKKKHHKK2 2

23、a1a1a21aHAKKKHHKH Similarlya.pH pKa1，H2A predominateb.pKa1 pH 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 H3Aa3a2a1a2a12a13a3a2a1Aa3a2a1a2a12a13a2a1HAa3a2a1a2a12a132a1AHa3a2a1a2a12a133AHKKK HKK HK HKKKKKK HK

24、K HK HHKKKKK HKK HK HHKKKK HKK HK HH3223 Distribution Graph of H3PO4For Acid HnAana2a12-na2a1-1na1nana2a1Aana2a12-na2a1-1na1n2-na2a1AHana2a12-na2a1-1na1n-1na1AHana2a12-na2a1-1na1nnAHKKK HKK HK HKKKKKK HKK HK HHKKKKK HKK HK H HKKKK HKK HK HHn22n1nn 阿司匹林阿司匹林pH=1.0pH=8.4 假设假设 H+,A-等离子不能穿等离子不能穿透隔膜透隔膜,HA

25、分子可自由通过分子可自由通过隔膜隔膜.达平衡时达平衡时,隔膜两边的隔膜两边的HA浓度相等，浓度相等，即即HA血血=HA胃胃 阿司匹林是一种弱酸阿司匹林是一种弱酸(即即乙酰水杨酸乙酰水杨酸),pKa=3.5.计算计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 Ti

26、tration Curves For each type of titration,our goal is to construct 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 sm

27、art enough to use either a strong acid or base as your titrant.This 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 th

28、e equivalence point(2)At the equivalence point(3)After the equivalence point Titration Curves A plot of ml titrant(or%titration)vs pH will result in a typical titration curve.Titration Curves 1.09pH0.082mol/L10ml100ml0.10mol/L10ml0.10mol/L100mlOH3 Example:Construct a titration curve for the titratio

29、n of 100ml of 0.10M HCl with 0.10M NaOH.0%Titration pH=-log0.10=1.00After 10ml NaOH100%Titration Equivalence point pH=7Note 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 acco

30、unt for the dilution of our titrant.10ml Overtitration68.112.3214pH2.32pOHL0.0048mol/210ml10ml0.10mol/LOH Titration Curves Figure.Dependence of the magnitude of end-point break on concentrationThe concentrations of sample and titrant are the same.Titration Curves of Weak Acid/Base Titration of a wea

31、k 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.Example(acid)HA+H2O H3O+A-(base)We will have the same four general regions for our titration curve.Calculation will require that

32、 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 titration curve.For Benzoic AcidKa=6.31 10-5pKa=4.20 Titration Curves of Weak Acid/Base0%Titration 2.60pHso0.025mol/L)(0.10)10(6.31

33、KH500/K20KK5aaaawaa ccc10%Titration Here we can use the Henderson-Hasselbalch equation.We can calculate other points by repeating this process.3.259010lg4.20pHnnlgpKpHHAAa Titration Curves of Weak Acid/Base Titration Curves of Weak Acid/Base100%Titration At this point,all of our acid has been conver

34、ted to the conjugate base benzoate.We need to use the Kb relationship to readily solve for this point.1010581.K/KKAHAOHKaWbb 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 200ml.Finally,we can

35、 assume that benzoic acid is negligible compared to benzoate100%Titration 8.45pOH14pH5.55pOHsomol/L102.81)(0.050)10(1.58KOH500/K20KK610bbbbwbb ccc 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)11.68

36、2.3214pH2.32pOHL0.0048mol/210ml10ml0.10mol/LOH-Titration Curves of Weak Acid/Base Titration Curves of Weak 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

37、 a buffer mixture of NH4+and NH3.At the equivalence point,we have NH4+,a weak acid.Beyond the e.p.,we have 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

38、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H3

39、A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:A3-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 3.1H3

40、COH-H+黄色红色H3CNN NSO3-H3CH3CNN NSO3-HAssume that)blue()red(InHHInHInInHKHIn HInInlgpKpHHIn Transition Range of an IndicatorWhen only the color of the unionized form is seen,In-/HIn 10/1.So Transition range of an indicatorUnder these conditions,the pH transition range for our indicator is pKHIn 1.Most

41、 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M

42、ethyl 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

43、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)and the true equivalence point is called the indicator error.Selection of IndicatorsBecause indicators are acids

44、 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 drops of dilute indicator solution.Selection of IndicatorsIn general,we seek an indicator whose transition range ov

45、erlaps 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标准溶液酸标准溶液: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: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),用除去用除去CO2

46、 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)HCl consumed is V1 mL The reactions are:HCl+Na2CO3 =NaHCO3+NaCl HCl+NaOH =NaCl+H2ONaOH,Na2CO3,NaHCO31.Titration of Mixtures Containing Sodium Carbonate1.Ti

47、tration 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:NaOH(V1-V2),Na2CO3(V2)（2）V1=V2:Na2CO3（3）V1V2:NaOH(V1-V2),Na2CO3(V2)%100m10)NaOH(M)VV)(HCl(c)NaOH(%100m10)CONa(MV)HCl

48、(c)CONa(321332232 1.Titration of Mixtures Containing Sodium Carbonate(2)V1V2:NaHCO3(V1-V2),Na2CO3(V2)%10010)()()(%10010)()()(33213332132 mNaHCOMVVHClcNaHCOmCONaMVHClcCONa 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

49、基橙加过量的加过量的BaCl2溶液溶液溶解，定容溶解，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

50、point is about 9.3.Kjeldahl 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 422CuSO,SOHnmNHOHCONHC442Step 3:DistillationOHNHNHOH234