大一學(xué)科任務(wù)分析化學(xué)section2acid-base

4B.1Acid-basetitrationcurves4B.2SelectingandEvaluatingtheendpoint4B.3TitrationsinNonaqueoussolvents4B.4Representativemethod4B.5Quantitativeapplications4B.6Qualitativeapplications4B.7Characterizationapplications4B.8Evaluatingofacid-basetitrimetryCapter4TitrationsSection2

Acid-baseTitrations

Acid-basetitration:Atitrationinwhichthereactionbetweentheanalyteandtitrantisanacid-basereaction.Acid–basetitrationsuseastrongacidorstrongbaseasatitrant.4B.1Acid-basetitrationcurves

p2784B.1Acid-basetitrationcurvesp2794B.1.1Titratingstrongacidsandstrongbases4B.1.2Titratingaweakacidwithastrongbase4B.1.3Sketchinganacid-basetitrationcurveCapter4TitrationsThefollowingfigureshowsthetitrationofastrongacidwith0.1000MNaOH.Fortitrationofastrongacidwithastrongbase,theequivalencepointoccursatapHof7.00.StrongAcidandStrongBasetitrationcurve4B.1.1Titratingstrongacidsandstrongbasesp280Wecanidentifyfourdifferentregionsinthistitrationexperiment.

p280Beforethetitration,thepHisdeterminedbytheinitialconcentrationofstrongacid.Beforetheequivalencepoint,thepHisdeterminedbytheconcentrationofunneutralizedstrongacid.Attheequivalencepoint,thepH=7,isdeterminedbythedissociationofwater.Aftertheequivalencepoint,thepHisdeterminedbytheconcentrationofexcessstrongbasethatweareadding.4B.1.1Titratingstrongacidsandstrongbases酸堿滴定曲線的計算1.強堿滴定強酸

例：0.1000mol·L-1NaOH溶液滴定20.00ml0.1000mol·L-1HCl溶液.

(1)滴定前，加入滴定劑(NaOH)體積為0.00mL時：

0.1000mol·L-1鹽酸溶液pH=1.00

(2)滴定中，加入滴定劑體積為18.00ml時：

[H+]=0.1000×(20.00-18.00)/(20.00+18.00)

=5.3×10-3mol·L-1

pH=2.28

加入滴定劑體積為19.98mL時(離化學(xué)計量點差約半滴):[H+]=c×

VHCl/V=0.1000×(20.00-19.98)/(20.00+19.98)=5.0×10-5mol·L-1

pH=4.30強堿滴定強酸滴定曲線的計算

(3)化學(xué)計量點，加入滴定劑體積為：20.00mL反應(yīng)完全,溶液中[H+]=10-7mol·L-1,

溶液pH=7

(4)化學(xué)計量點后，加入滴定劑體積為20.02mL,過量0.02mL(約半滴)22.00mL如何計算？

[OH-]=nNaOH/V

=(0.1000×0.02)/(20.00+20.02)=5.0×10-5mol·L-1pOH=4.3,

pH=14-4.3=9.7滴定突躍:4.3~9.7Table:Datafortitrationof50.00mLof0.1000MHClwith0.2000MNaOH

Volumeoftitrant(mL)

0.005.0010.0015.0020.0022.0024.0025.0026.0028.0030.0035.0040.0045.0050.00pH

1.001.141.301.511.852.082.577.0011.4211.8912.5012.3712.5212.6212.70Fig.

p276Titrationcurvefor50.00mLof0.1000MHClwith0.2000MNaOHp280~281強堿滴定強酸滴定曲線的討論：（3）指示劑變色點（滴定終點）與化學(xué)計量點并不一定相同，但相差不超過±0.02mL,相對誤差不超過±0.1%。符合滴定分析要求。

（1）滴定過程中，溶液總體積不斷增加，計算時應(yīng)注意。（2）滴定的前18.00mL,pH變化：2.28-1=1.28；而化學(xué)計量點前后0.04mL(約1滴),

pH變化：9.70-4.30=5.4Thefollowingplotshowsthetitrationcurvesof0.1Msolutionsofastrongacidandaweakacid(aceticacid).FirstnotetheinitialpHofthesesolutions(theinitialpHofthe0.1Maceticacidis2.87).Forthestrongacid,thedominantpH-determiningspeciesweredescribed.WeakAcidTitrationCurveback4B.1.2Titratingaweakacidwithastrongbasep281wecanalsoidentifyfourdominantequilibriaastitrantisadded.Beforethetitration,thepHisdeterminedbythedisociationofaciticacid.Beforetheequivalencepointthedominantspeciesareaceticacidandacetateion.Thepresenceofthesetwospeciesformabuffer,andthepHcouldbecalculated.Attheequivalencepoint,alloftheaceticacidhasbeenneutralizedandonlyacetateionremainsinsolution.ThepHisthereforedeterminedbythebasehydrolysisreactionoftheacetatewithwater.ThepHisapproximately8.72(not7!).

Aftertheequivalencepoint,thesituationisthesameasforastrongacidandthepHisdeterminedbytheconcentrationofexcessstrongbase.4B.1.2Titratingaweakacidwithastrongbasep281滴定進程溶液組成滴定前：HAc等量點前：HAc-NaAc等量點時：NaAc等量點后：NaOH+NaAc選擇指示劑:

酚酞(8.0~10.0，無色→粉紅色)百里酚酞（9.4~10.6，無色→藍色）

強堿滴定弱酸強堿滴定弱酸

2）化學(xué)計量點前，加入滴定劑體積19.98mL

開始滴定后,溶液即變?yōu)镠Ac(ca)---NaAc(cb)緩沖溶液

ca=0.020.1000/(20.00+19.98)=5.0010-5mol·L-1

cb=19.980.1000/(20.00+19.98)=5.0010-2mol·L-1[H+]=Kaca/cb

例:0.1000mol·L-1NaOH溶液滴定20.00mL0.1000mol·L-1HAc溶液

繪制滴定曲線時,通常用最簡式來計算溶液的pH值.

1）滴定開始前,一元弱酸(ca/Ka=5.56×10-3≥400,最簡式計算)pH突躍范圍：7.74~9.70Comparingfigure4）化學(xué)計量點后

加入滴定劑體積20.02mL[OH-]=(0.10000.02)/(20.00+20.02)=5.010-5mol·L-1pOH=4.30；

pH=14-4.30=9.703）化學(xué)計量點

生成HAc的共軛堿NaAc（弱堿），其濃度為：

cb=20.000.1000/(20.00+20.00)=5.0010-2mol·L-1pKb=14-pKa=14-4.74=9.26[OH-]=(cb

Kb)1/2=(5.0010-210-9.26)1/2

=5.2410-6mol·L-1

pOH=5.28;pH=14-5.28=8.72pH突躍范圍：7.74~9.70Table:Datafortitrationof50.00mLof0.1000MAceticAcidwith0.1000MNaOHVolumeoftitrant(mL)

0.005.0025.0045.0049.9550.0050.0555.00100.0pH

2.883.814.765.717.748.729.7011.6812.52Fig.6

p283Titrationcurvefor50.00mLof0.1000Maceticacid(pKa=4.76)with0.1000MNaOHP281~283滴定曲線九個點:0%,10%,50%,90%,99.9%,100%,100.1%,110%,200%Table:Datafortitrationof50.00mLof0.1000MAceticAcidwith0.1000MNaOHVolumeoftitrant(mL)

0.005.0010.0015.0020.0025.0030.0035.0040.0045.0048.0050.0052.0055.0060.0065.0070.0075.0080.0085.0090.0095.00100.00pH

2.883.814.164.394.584.764.945.135.365.716.148.7211.2911.6811.9612.1212.2212.3012.3612.4112.4612.4912.52Fig.6

p283Titrationcurvefor50.00mLof0.1000Maceticacid(pKa=4.76)with0.1000MNaOHP281~283滴定曲線九個點:0%,10%,50%,90%,99.9%,100%,100.1%,110%,200%1）滴定前，弱酸在溶液中部分電離，與強酸相比，曲線開始點提高；2）滴定開始時，溶液pH升高較快，這是由于中和生成的Ac-產(chǎn)生同離子效應(yīng)，使HAc更難離解，[H+]降低較快；弱酸滴定曲線的討論：3）繼續(xù)滴加NaOH，溶液形成緩沖體系，曲線變化平緩；4）接近化學(xué)計量點時，溶液中剩余的HAc已很少，pH變化加快;）滴定條件：c·Ka≥10-8

6）化學(xué)計量點前后，產(chǎn)生pH突躍，與強酸相比，突躍變??；Fig.7

Howtosketchanacid-basetitrationcurve;seetextforexplanation.p284~2854B.1.3Sketchinganacid-basetitrationcurve4B.2SelectingandEvaluatingtheendpoint

p287Whereistheequivalencepoint?

p287Findingtheendpointwithavisualindicator

p288FindingtheendpointbymonitoringpHp290Findingtheendpointbymonitoringtemperaturep293當(dāng)弱酸或弱堿的酸堿性太小時：用某些分析儀器來判斷終點,準確度會高一些?？刹捎梅撬味ǚ?。

滴定突躍：等量點前后差半滴，Er=±0.1%范圍內(nèi)，溶液pH值的變化范圍。突躍范圍：強強互滴，突躍范圍跨越兩區(qū)；強弱滴定，強堿滴定弱酸，突躍范圍一般落在堿區(qū)，強酸滴定弱堿，突躍范圍落在酸區(qū)。等量點：等于突躍范圍上下限的平均值。Whereistheequivalencepoint?

p287Fig.9p287Titrationcurvesfor50.00mLof0.1000Mweakacidwith0.1000Mstrongbase.ThepKaoftheweakacidsare(a)1,(b)3,(c)5,(d)7,(e)9,(f)11.

影響突躍范圍的因素①弱酸的Ka

Ka越大，突躍范圍越大.隨著弱酸的Ka變小，突躍變小，Ka在10-9左右,突躍消失;影響突躍范圍的因素(兩個)Fig.Titrationcurvesfor(a)10-1MHCl;(b)10-2MHCl;(c)10-3MHCl;(d)10-4MHCl.IneachcasethetitrantisanequimolarsolutionofNaOH.影響突躍范圍的因素②弱酸的濃度，濃度越大，突躍范圍越大。p311濃度/mol·L-1pH突躍范圍ΔpH0.14.3-9.75.4

0.015.3-8.73.4

0.0016.3-7.71.4

0.00016.9-7.10.2結(jié)論:弱酸直接準確滴定的條件：ca.Ka≥10-8弱堿直接準確滴定的條件：cb.Kb≥10-8常用標(biāo)準溶液的濃度：0.1mol?L-1當(dāng)弱酸或弱堿的酸堿性太小時：用某些分析儀器來判斷終點,準確度會高一些?？刹捎梅撬味ǚā?/p>

例：能否用酸堿滴定法準確滴定下列濃度為0.1mol?L-1的溶液？若能，計算等量點時的pH，并選擇合適的指示劑。(1)H3BO3;(2)NH4Cl;(3)NaCN解：(1)H3BO3為弱酸,pKa

=9.14;

ca·Ka

﹤10-8，不能準確滴定。

(2)NH4+為弱酸,pKa=14-4.75=9.25;

ca·Ka

﹤10-8不能準確滴定。

(3)CN-為弱堿,pKb

=14-9.31=4.69;

cb·Kb

>10-8，能準確滴定。

等量點:HCN,pH=5.30

指示劑:甲基紅(黃→橙紅)Indicators

areweakacidsorbasesthatchangecolorwhentheygainorlosetheiracidicproton(s).Fig.11

LadderdiagramshowingtherangeofpHlevelswhichatypicalacid-baseindicatorchangescolor.Acid-BaseIndicatorsP288~2894B.2SelectingandEvaluatingtheendpoint

p287Findingtheendpointwithavisualindicator

p288(a)(b)(c)Fig.24

p314Titrationcurvesfora50.00mLmixtureof0.1000MHAand0.1000MofHBwith0.1000MofNaOH,wherepKa,HA=3andpKa,HB=8;pKa,HA=5andpKa,HB=6.Thedashedlinesindicatethelocationofthetwoequivalencepoints(a)(b)例：用0.1000mol·L-1HCl溶液滴定20.00ml0.1000mol·L-1Na2CO3溶液。pH1=8.31酚酞(紅-無)甲酚紅-百里酚藍(8.2-8.4,玫瑰紅-清紫)。pH2=3.9甲基橙(3.1~4.4,黃-橙)，劇烈搖動。第一等量點誤差約為0.5%混合酸的滴定強酸和弱酸混合：例:用NaOH滴定20mL0.1mol?L-1HCl與0.1mol?L-1HAc混合液。0.1mol?L-1HClpH=1.00.1mol?L-1HAcpH=2.87二者不能分步滴定HA+HB：用c·Ka≥10-8，判斷能否準確滴定;用cHA·Ka1

/cHB·Ka2

≥105,

判斷能否分步滴定。多元酸的滴定l能否直接準確滴定：

ca·Ka1

≥10-8

ca·Ka2

≥10-8能

ca·Ka3

≥10-8l能否分步滴定的條件：

Ka1

/

Ka2

≥105能

Ka2

/

Ka3

≥105

能選擇指示劑：不作滴定曲線，只通過計算等量點，以指示劑的理論變色點與等量點接近為原則選擇指示劑。l能否直接準確滴定：

cb·Kb1

≥10-8

cb·Kb2

≥10-8能

cb·Kb3

≥10-8l能否分步滴定的條件：

Kb1

/

Kb2

≥105能

Kb2

/

Kb3

≥105

能多元堿的滴定4B.3TitrationsinNonaqueoussolvents

p295

Amphotericsolvent(兩性溶劑）SH(質(zhì)子溶劑)Autoprotolysis（質(zhì)子自遞）neutralsolvent(中性溶劑）2SHSH2++S-KsaffectspHchangeintitrations.pKs↑ΔpH↑突躍↑Leveling:HCl&HAcinNH3;Distinguish:HCl&HAcinH2OAllotherthingsbeingequal,thestrengthofaweakacidincreasesifitisplacedinasolventthatismorebasicthanwater,whereas…….p296Fig.16

p296Titrationcurvesfor50.00mLof10-4MHClwith10-4MNaOHin(a)water,Kw=1×10-14,and(b)nonaqueoussolvent,Ks=1×10-20.NonaqueoustitrationsLeveling:Acidsthatarebetterprotondonorsthanthesolventareleveledtotheacidstrengthoftheprotonatedsolvent;Leveling:basesthatarebetterprotonacceptorsthanthesolventareleveledtothebasestrengthofthedeprotonatedsolvent.Fig.17

p296Titrationcurvesfor50.00mLof0.1000Mweakacid(pKa=11)with0.1000MNaOHin(a)water,Kw=1×10-14,and(b)nonaqueoussolvent,Ks=1×10-20.Thetitrationcurvein(b)assumesthatthechangeinsolventhasnoeffectontheaciddissociationconstantoftheweakacid.4B.4Representativemethod

p296Kjeldahlmethod:克氏定氮法

氨基酸、蛋白質(zhì)、生物堿中的氮常用克氏法測定。將適量濃硫酸加入試樣中加熱，使C、H轉(zhuǎn)化為CO2和H2O，N元素在銅鹽或汞鹽催化下生成NH+4，消化分解后的溶液中加入過量NaOH溶液，再用蒸餾法測定NH3。AmmoniawillbedistilledintoHClsolutionwhosevolumeandconcentrationiswellknown.Hg2+willberemovedbyS2-.Anylossofammoniaislossofanalyteandanegativedeterminateerror.4B.5Quantitativeapplications

p298Selectingandstandardizingatitrantp298

primarystandardp299Inorganicanalysisp300

multiproticacids&mixtureofacidsandbasesOrganicanalysisp302

Kjeldahlanalysis&OrganicfunctionalgroupsQuantitantivecalculationsp304Example9.2(304);9.3(305);9.4(306);9.5(307)Alkalinity:Ameasureofawater’sabilitytoneutralizeacid.Acidity:Ameasureofawater’sabilitytoneutralizebase.Fig.18

p301table9.8p302Titrationcurvesfor(a)50.00mLof0.1000MNaOHwith0.1000MHCl;(b)50.00mLof0.1000MNa2CO3with0.1000MHCl;and(c)50.00mLof0.1000MNaHCO3with0.1000MHCl.ThedashedlinesindicatethepH8.3andpH4.5endpoints.(a)(b)(c)4B.5Quantitativeapplications

p298Organicanalysisp303table9.9,9.10Fig.9.19p304(a)Titrationcurvefor50.00mLofa0.00489Msolutionofcriticacid,using0.04166MNaOHasthetitrant;(b)ladderdiagramforcitricacid.(a)(b)4B.6Qualitativeapplications

p308Table9.114B.7Characterizationapplicationsp309Equivalentweight（當(dāng)量）

p309Equilibriumconstants（平衡常數(shù)）

p310Fig.9.20EstimatingthepKaforaweakacidfromitstitrationcurvewithastrongbase.Theweakacidisneithertoostrongnortooweak.Despitethis,wecanuseGranplot(p292)togainthepKa.Alinearizedformofatitrationcurve.p3104B.8Evaluatingofacid-basetitrimetryp311ScaleofoperationsAccuracy312Precision