沉淀滴定與重量分析

1、沉淀滴定與重量分析沉淀滴定與重量分析 Precipitation Titration and Gravimetric AnalysisContents1 微溶化合物的溶解度和溶度積微溶化合物的溶解度和溶度積 Solubility and solubility product constant2 沉淀滴定沉淀滴定Precipitation Titration 5.3 重量分析重量分析Gravimetric Analysis 微溶化合物的溶解度和溶度積微溶化合物的溶解度和溶度積 Solubility and solubility product constant1. Solubility (s)

2、and solubility product constant (Ksp)CaSO4(s) = Ca2+(aq) + SO42-(aq)Ksp = Ca2+SO42- = 9.110-6 at 25CnThe equilibrium constant for the equilibrium established between a solid solute and its ions in a saturated solution.1.1 Solubility (s) and solubility product MA(固固) MA(水水) M+A- M+A-MA水水: 固有溶解度固有溶解度(

3、分子溶解度分子溶解度),用用s0表示表示Solubility : s=s0+M+=s0+A-()()aaKa+-MA=MA水水()()Kapa(M) a(A) 活度積常數(shù)活度積常數(shù)(activity product constant), 只與只與t有關(guān)有關(guān)S(HgCl2)=0.25molL-1Hg2+=1.710-5 molL-1s0solubility product constant and conditional solubility product constantM A MA=MAK spM A M MA A=Ksp M Aconditional solubility product

4、 constant, 與條件有關(guān)與條件有關(guān)MmAn？K sp Ksp Kapsolubility product constant ， 與與t、I有關(guān)有關(guān)SKsp=MA:MA2: K spM A 2 =Ksp M A2 3/4SKsp2M=A :apsp(M) (A)MA (M) (A)Ka aK= = = (M) (A)Table Several Solubility Product Constants at 25CQuestion 298K時(shí)鉻酸銀時(shí)鉻酸銀Ag2CrO4的溶度積常數(shù)的溶度積常數(shù)為為2.010-12，求該溫度下的溶解度。，求該溫度下的溶解度。 SolutionspK5342s

5、p10974CrOAg ./ )(K 設(shè)飽和溶液中設(shè)飽和溶液中 離子的濃度離子的濃度x mol/L，則代入溶，則代入溶度積常數(shù)表達(dá)式得：度積常數(shù)表達(dá)式得：Ksp (Ag2CrO4) = c(Ag+)/mol/L2c( )/mol/L = (2x)2x = 4x3 x/moldm-3 = 溶液中溶液中 離子的濃度為離子的濃度為7.910-5 mol/L, 也是也是Ag2CrO4以以mol/L為單位的溶解度。為單位的溶解度。242OCr242OCr242OCrQuestion AgCl 和和 Ag2CrO4的溶度積分別為的溶度積分別為 1.810-10 和和 1.1 10-12 ，則下面敘述中，則

6、下面敘述中正確的是：正確的是： (A) AgCl與與Ag2CrO4的溶度積相等的溶度積相等 (B) AgCl的溶解度大于的溶解度大于Ag2CrO4 (C) 二者類型不同，不能由二者類型不同，不能由 大小直接判斷溶解度大小大小直接判斷溶解度大小Solution分子式分子式溶度積溶度積溶解度溶解度/ AgBr AgIAgClmol dm-31.810 -10 Ag2CrO41.310 -55.010 -137.110 -78.310 -179.110 -101.110 -126.510 -5spK1.2 Factors which affect s(1). Salt effects increas

7、es s沉淀重量法中，用沉淀重量法中，用I=0.1時(shí)的時(shí)的Ksp計(jì)算計(jì)算;計(jì)算難溶鹽在純水中的計(jì)算難溶鹽在純水中的溶解度用溶解度用Kap S/S01.61.41.21.00.001 0.005 0.01c(KNO3)/(molL-1)BaSO4AgClI , S apsp(M) (A)MA (M) (A)Ka aK= = = (M) (A)(2) The Common-Ion Effect in Solubility Equilibrium 同離子效應(yīng)同離子效應(yīng)The Common-Ion Effect沉淀重量法總要加過量沉淀劑沉淀重量法總要加過量沉淀劑.例例: 測(cè)測(cè)SO42- 若加入過量若加

8、入過量Ba2+, Ba2+=0.01molL-1S=SO42-=Ksp/Ba2+=610-10/0.01=610-8 molL-1 m(BaSO4)=610- 8300233.4=0.004mg可揮發(fā)性沉淀劑過量可揮發(fā)性沉淀劑過量50%100%非揮發(fā)性沉淀劑過量非揮發(fā)性沉淀劑過量20%30%The Common-Ion Effect Decreases the SolubilityAgCl在在KNO3溶液中的溶解度溶液中的溶解度 (25 )0.00 0.00100 0.00500 0.0100 1.278 1.325 1.385 1.427c(KNO3)/mol /LAgCl溶解度溶解度/10

9、- -5(mol/L)0.15 0.24 0.016 0.014 0.013 0.016 0.023c(Na2SO4)/mol/LS(PbSO4)/mmol/LPbSO4在在Na2SO4 溶液中的溶解度溶液中的溶解度 (25 ) 0 0.001 0.01 0.02 0.04 0.1000 0.2000(3). Acidic Effects Increase s例計(jì)算例計(jì)算CaC2O4在不同情況下的溶解度在不同情況下的溶解度CaC2O4 Ca2+C2O42- S SKap(CaC2O4)=10-8.6 (I0) H2C2O4 pKa1=1.3 pKa2=4.3 (I0)在純水中在純水中S=Ca2

10、+=C2O42-= =510-5 molL-1apK在在pH=1.0的酸性溶液中的酸性溶液中CaC2O4 Ca2+C2O42- S S H+HC2O4-H2C2O4用用I0.1的常數(shù)的常數(shù)K sp= Ca2+C2O4 =S2=C2O42- a (H)2-24C O Ksp= Ksp =10-7.8+3.4 =10-4.42-24C O(H)S = = 10-2.2 = 610-3 (molL-1)spK a (H) =1/ =1+H+b1+H+2b22-24C O-1.0+4.0-2.0+5.13.4=1+10+10=102-24C O副反應(yīng)系數(shù)副反應(yīng)系數(shù) 若若pH=4.0，過量，過量H2C2

11、O4 ( c=0.10molL-1) 240.3C O (H)10 sp24-7.5+1.0-6.5-7-1mol L=10=10= 310C O KS Ca2沉淀完全沉淀完全Ksp= Ksp a = 10-7.8+0.3=10-7.52-24C O(H) Ag2S =2Ag+S2 2S S/as(H)Ag2SKsp8 10-51, H2S pKa1=7.1 pKa2=12.9pH = 7.0, as(H) = 2.5 107Ksp = Ksp as(H) = Ag+2Cs2- = (2S)2S = 4S3S = 1.1 10-14 mol/LKsp較大較大, S2-定量成為定量成為HS-,

12、產(chǎn)生同量產(chǎn)生同量OH-假設(shè)假設(shè) MnS+H2O=Mn2+HS+OH S S S2+2-s+-2+-2wp-aa2-12.6-14.0-13.7-12.9H OH =MnHS OH =10 =1010MnS KKK KK -1-13.7/3- 4.63=10=10mol LSKMnSKap10-12.6, H2S pKa1=7.1 pKa2=12.9檢驗(yàn)：檢驗(yàn)：OH-=S=10-4.6 molL-1, H+=10-9.4 molL-1 S(H)110-9.4+12.9+10-18.8+20.01 + 103.5 + 101.2103.5 S2- HS- H2S 主要以主要以HS-存在存在故假設(shè)合

13、理。故假設(shè)合理。7.1pKa112.9pKa2pHS2H2SHS9.4亦可不必計(jì)算，由優(yōu)勢(shì)區(qū)域圖可知：亦可不必計(jì)算，由優(yōu)勢(shì)區(qū)域圖可知：S最小最小同離子效應(yīng)同離子效應(yīng)絡(luò)合作用絡(luò)合作用pCl=2.4pCl5 4 3 2 1 01086420Sx106 mol/LS(AgCl)-pCl曲線曲線(4). Complex effects increase sKsp =Ag Cl-= Ag+Cl- Ag(Cl)=Ksp(1+Cl 1+Cl2 2 +-Ag+ + Cl- AgCl Cl-AgCl, AgCl2-,-1Cl S= Ag = Ksp / Cl-=Ksp( + 1+Cl 2 +H+YHiYHPbC

14、2O4=Pb2+C2O42-PbYHC2O4,H2C2O4 酸效應(yīng)酸效應(yīng) 絡(luò)合效應(yīng)共存絡(luò)合效應(yīng)共存Ksp10-9.7 pH=4.0，C2O4 =0.2molL-1 , Y =0.01molL-1 C2O4(H) 100.3 Y(H)108.6Y=Y/ Y(H)=10-10.6 mol L-1 Pb(Y)1+10-10.6+18.0107.4Ksp =Ksp Pb(Y) C2O4(H) =10-9.7+7.4+0.3=10-2.0-124sp-2.0=Pb =10/0.2 = 0.05mol LC O KS在此條件下，在此條件下，PbC2O4不沉淀不沉淀 Ca(Y)=1+10-10.6+10.7

15、=100.4Ksp =Ksp Ca(Y) C2O4(H) =10-7.8+0.4+0.310-7.124sp-7.1-6.1-1mol L=Ca =10/0.2 =10C O KS此時(shí)，此時(shí)，CaC2O4沉淀否？沉淀否？Ca2沉淀完全，則沉淀完全，則Pb2，Ca2可分離。可分離。(5). 影響影響s 的其他因素的其他因素溫度溫度: t， s 溶解熱不同溶解熱不同, 影響不同影響不同. 室溫過濾可減少損失室溫過濾可減少損失.溶劑溶劑: 相似者相溶相似者相溶, 加入有機(jī)溶劑，加入有機(jī)溶劑，s顆粒大小顆粒大小: 小顆粒溶解度大小顆粒溶解度大, 需陳化需陳化.形成膠束：形成膠束： s 沉淀析出時(shí)形態(tài)：

16、沉淀析出時(shí)形態(tài)：2. 沉淀滴定沉淀滴定 Precipitation Titration沉淀反應(yīng)為基礎(chǔ)的分析方法沉淀反應(yīng)為基礎(chǔ)的分析方法沉淀反應(yīng)很多，但能用于沉淀滴定的不多，沉淀反應(yīng)很多，但能用于沉淀滴定的不多，應(yīng)用較多的是生成難溶應(yīng)用較多的是生成難溶Ag鹽的反應(yīng)鹽的反應(yīng)Ag+ + Cl- = AgCl Ag+ + Br- = AgBr Ag+ + SCN- = AgSCN 銀量法，銀量法， Cl-, Br-, I-, CN-, SCN-, Ag+argentometric titrationsPrecipitation Titration CurveEXAMPLE: Sketch a curv

17、e for the titration of 20.00 mL of 0.1000 M NaCl with 0.1000 M AgNO3.titration curve = pAg vs. vol. AgNO3 addedAg+ + X- = AgX Ksp= Ag+X+ pAg+pX=pKspSPpCl = pAg = 1/2 pKsp = 4.91+ 0.1%Ag+ = 5.0 10-5 mol/L, pAg = 4.30, pCl = pKsp - pAg = 9.81 - 4.30 = 5.51-0.1%Cl- = 5.0 10-5 mol/L, pCl = 4.30, pAg = p

18、Ksp - pCl = 9.81 - 4.30 = 5.51Initial point Cl- = 0.1000 mol/L, pCl = 1.00 Taking account of the dissolution of the precipitation-0.1%Cl- =Cl-sur+Cl-solCl-sol=Ag+=aKsp=Cl-Ag+=(Cl-sur+a)aa= 2.9 10-6 mol/L, pCl = 4.28, pAg = pKsp - pCl = 9.81 - 4.28 = 5.53AgNO3NaCl (I-)0 50 100 150 200 T %pAg0.1mol/L0

19、.1mol/L1mol/L1mol/L4.96.53.35.54.30246810AgIpAg Jump increase by 2 when the concentration increase 10 fold.pAg Jump increase by n when the Ksp dcrease 10n End-Point Detectionargentometric titrations （銀量法）（銀量法）a. Mohr (potassuim chromate as indicator) titration of Cl- with AgNO3Titration reaction : A

20、g+Cl- AgCl(s) whiteIndication reaction: Ag+CrO42- Ag2CrO4(s) brick redSomething Notable:Indicator dosage Acidity of solutionsInterferencesExperimental dosage： 5.2-2.6 10-3 mol/L , Indicator dosage:sp, Ag+=Cl-=Ksp(AgCl)1/2 = 1.2510-5 mol/L CrO42-=Ksp(Ag2CrO4)/ Ag+2 = 0.7010-2 mol/L用用0.1000 mol/L Ag+

21、滴定滴定0.1000 mol/L KCl, 指示劑指示劑CrO42-= 5.0010-3 mol/L。計(jì)算由指示劑引入的終點(diǎn)。計(jì)算由指示劑引入的終點(diǎn)誤差。誤差。(生成可判斷終點(diǎn)的生成可判斷終點(diǎn)的AgCrO4需消耗需消耗2.0010-5 mol/L的的Ag+)sp時(shí)時(shí), Ag+sp=Cl-sp=Ksp(AgCl)1/2 =1.2510-5 mol/L CrO42- Ag+2 =Ksp(Ag2CrO4)出現(xiàn)沉淀時(shí)出現(xiàn)沉淀時(shí): Ag+= Ksp(Ag2CrO4)/ CrO42- 1/2 =1.4810-5 mol/L明顯明顯Ag+過量了過量了, 此時(shí)此時(shí)Cl-=Ksp/Ag+=1.0510-5 mo

22、l/LAg+過量過量= Ag+-Cl-=1.4810-5-1.0510-5=0.4310-5總多消耗的總多消耗的Ag+:Ag+總總=0.4310-5+2.0010-5= 2.4310-5mol/L終點(diǎn)誤差：終點(diǎn)誤差：滴定劑過量的量滴定劑過量的量被測(cè)物的量被測(cè)物的量2.4310-5 2V0.1000V100% = 100% =0.049%Acidity of solutionsAcidity：pH 6.5 10.5；pH 6.5 7.2 when NH3 exist Strong acid： H+CrO4 Cr2O72- (K=4.31014) Strong base： Ag2O precipi

23、tationAdvantage：direct, simple, accurate for measurement of Cl- and Br-Disadvantage: interferences (precipitation of AgmAn 、Mm(CrO4 )n、 M (OH)n）; I-、SCN- can not be detected since I- and SCN are strongly adsorbed on the surface of the precipitates； b. Volhard (NH4Fe(SO4)2 as indicator)i). Direct tit

24、ration (determining Ag+) Titration reaction: Ag+SCN- AgSCN(s) white Indication reaction: Fe3+ SCN- FeSCN2+(s) redSomething Notable: Acidity of solutions (0.3 mol/L HNO3 )Concentration of indicator (1-2 ml 40%)Interference of adsorptionb. Volhardii). Back titration (for determination of haloid ions)

25、Error occurs due to the reaction:SCN- - + Fe3+AgSCN AgCl Ag+ + Cl- -+FeSCN2+Ksp(AgSCN) Ksp(AgCl)(2.0 10-12) (3. 210-10)Ways to avoid the error: c. Fajans - titration of Cl- with AgNO3 - absorption indicator, dye absorbs on to the surface of the particles of AgCl, attracted by excess Cl- absorbed on

26、the surface of the precipitate Fajans法常用吸附指示劑法常用吸附指示劑指示劑指示劑pKa測(cè)定對(duì)象測(cè)定對(duì)象滴定劑滴定劑顏色變化顏色變化滴定條滴定條件件(pH)熒光黃熒光黃7Cl-, Br-, I- Ag+黃綠黃綠 粉紅粉紅710二氯二氯熒光黃熒光黃 4Cl-, Br-, I-Ag+黃綠黃綠 粉紅粉紅410曙紅曙紅2Br-, I-, SCN-Ag+粉紅粉紅 紅紫紅紫210甲基紫甲基紫Ag+Cl-紅紅 紫紫酸性酸性沉淀對(duì)鹵離子及指示劑的吸附能力：沉淀對(duì)鹵離子及指示劑的吸附能力：ISCNBrCl 曙曙紅紅熒熒光光黃黃Step titration (分布滴定）分布滴定

27、）標(biāo)準(zhǔn)溶液的配制與標(biāo)定標(biāo)準(zhǔn)溶液的配制與標(biāo)定nAgNO3: （棕色瓶中保存）（棕色瓶中保存）n純品直接配制純品直接配制(貴貴）n粗配后用粗配后用NaCl標(biāo)液標(biāo)定其濃度標(biāo)液標(biāo)定其濃度nNH4SCN: 以以AgNO3標(biāo)液標(biāo)液, Volhard法標(biāo)定法標(biāo)定nNaCl工作基準(zhǔn)或優(yōu)級(jí)純，直接配制工作基準(zhǔn)或優(yōu)級(jí)純，直接配制n高溫電爐中于高溫電爐中于550C干燥干燥2h.n置于瓷坩堝中，在石棉網(wǎng)上用天然氣燈加熱置于瓷坩堝中，在石棉網(wǎng)上用天然氣燈加熱.indicatorK2CrO4FeNH4(SO4)2absorption indicatortitrantAg SCN-Cl- or Ag+Titration r

28、eaction Ag+Cl-=AgClSCN-+Ag+=AgSCN Ag+Cl-=AgCl Indication reaction2Ag+CrO4-=Ag2CrO4 (brick red)Fe3+SCN-=FeSCN2+(red)Structure change resulted from the indicator adsorptionpHpH6.510.50.3mol/L HNO3Depend on pKa of indicatoranalytes Cl-, Br-, CN-,Ag+Ag+, Cl-, Br-, I-, SCN- et.alCl-, Br-, SCN- ,Ag+ et.al

29、End point detection of precipitation titration by scanometry method without using indicatorAnal. Methods, 2012, 4, 923-932 3. 重量分析法重量分析法 Gravimetric Analysis1 Overview of Gravimetry1) Characters Measuring Mass or change in Mass Most fundamental of all analytical measurements The Oldest analytical te

30、chnique2) Types of Gravimetric Methodsb. Electrogravimetry （電重量法）（電重量法）c. Volatilization gravimetry （揮發(fā)重量法）（揮發(fā)重量法）Why Gravimetry is Important? Overview cont.特點(diǎn)優(yōu)點(diǎn)：Er: 0.10.2，準(zhǔn)，不需標(biāo)準(zhǔn)溶液。缺點(diǎn)：慢，耗時(shí)，繁瑣。 (S，Si, Ni的仲裁分析仍用重量法）1)Basis2.Precipitation GravimetryprecipitantPrecipitateAnalyte (in solution)Filtration

31、, Quantifiable formMassrinsing, dryingmetage2) Attributes of Precipitatea. Low Solubilityb. Easy to separatec. High Purityd. Easy to Form QF2.Precipitation Gravimetry cont.3) Attributes of QFa. Known composition b. High stabilityc. High molecular weight2.Precipitation Gravimetry cont.Representative

32、Gravimetric AnalysisRepresentative Gravimetric Analysis cont.4) Styles and formation of precipitate晶形沉淀無定形沉淀顆粒直徑0.11m凝乳狀沉淀膠體沉淀顆粒直徑0.020.1m顆粒直徑0.02mFe(OH)3AgClCaC2O4，BaSO4Controlling particle size (of crystalloid): Nucleation: homogeneous (均相) & heterogeneous （異相） inoculating crystal（晶種晶種 ） criti

33、cal cQ/s Growth: Relative Supersaturation: （相對(duì)過飽和度）（相對(duì)過飽和度） RSS=(cQ-s)/s2.Precipitation Gravimetry cont.formation of precipitate2.Precipitation Gravimetry cont.ionnucleationcrystal nucleusgrowthparticlescoacervationAmorphous precipitatedirectionalismCrystalloid precipitateWays to form precipitatesVo

34、n Weimarn 經(jīng)驗(yàn)公式經(jīng)驗(yàn)公式聚集速度(分散度)相對(duì)過飽和度s:晶核的溶解度Q：加入沉淀劑瞬間溶質(zhì)的總濃度Q - s：過飽和度K：常數(shù)，與沉淀的性質(zhì)、溫度、 介質(zhì)等有關(guān)n =K Q - s s BaSO4 1.110-10 big 1000 CrystalloidAgCl 1.810-10 big 5.5 CurdyCaC2O4H2O 2.310-9 31PbSO4 1.710-8 28 QSS criticalbothPrecipitate K sp S crystal nucleus5) Factors affecting purity of precipitationCo-prec

35、ipitation (共沉淀共沉淀) adsorption (吸附) mixed crystal （混晶） inclusion & occlusion （包夾與吸留）Post-precipitation （后沉淀）（后沉淀）Difference between the above twoWays to minimize the impurity 2.Precipitation Gravimetry cont.Ba2+Cl-Ba2+Ba2+Ba2+Ba2+Ba2+Ba2+Ca2+Ca2+Cl-Cl-K+Na+Adsorption layerDiffusion layer24SO24SO2

36、4SO24SO24SO24SO24SO24SO24SO24SO3NOAdsorption后沉淀后沉淀 主沉淀形成后，主沉淀形成后，“誘導(dǎo)誘導(dǎo)”雜質(zhì)隨后沉淀下來雜質(zhì)隨后沉淀下來 縮短沉淀與母液共置的時(shí)間縮短沉淀與母液共置的時(shí)間吸附共沉淀（服從吸附規(guī)則）吸附共沉淀（服從吸附規(guī)則）是膠體沉淀不純的主要原因，是膠體沉淀不純的主要原因，洗滌洗滌包夾共沉淀包夾共沉淀是晶形沉淀不純的主要原因，是晶形沉淀不純的主要原因，陳化或重結(jié)晶陳化或重結(jié)晶混晶共沉淀混晶共沉淀 預(yù)先將雜質(zhì)分離除去預(yù)先將雜質(zhì)分離除去沉淀沾污對(duì)分析結(jié)果的影響（例沉淀沾污對(duì)分析結(jié)果的影響（例BaSO4法法）吸附物質(zhì)吸附物質(zhì) 測(cè)測(cè)SO42- 測(cè)測(cè)

37、 Ba2+BaCl2 （+） （-）Na2SO4 （-） （+）Ways to minimize the impurity1）選擇適當(dāng)分析步驟）選擇適當(dāng)分析步驟 測(cè)少量組分含量時(shí)，首先沉淀含量少的組分2）改變易被吸附雜質(zhì)的存在形式，降低其濃度）改變易被吸附雜質(zhì)的存在形式，降低其濃度 分離除去，或掩蔽。如將Fe3+還原成Fe2+ 3）選擇合適的沉淀劑）選擇合適的沉淀劑 選用有機(jī)沉淀劑可有效減少共沉淀4）改善沉淀?xiàng)l件）改善沉淀?xiàng)l件 溫度，濃度，試劑加入次序或速度，是否陳化5）再沉淀）再沉淀 有效減小吸留或包埋的共沉淀及后沉淀現(xiàn)象沉淀?xiàng)l件對(duì)沉淀純度的影響沉淀?xiàng)l件沉淀?xiàng)l件表面吸附表面吸附混晶混晶吸留或包夾吸留或包夾后沉淀后沉淀稀釋溶液稀釋溶液00慢沉淀慢沉淀不定不定攪拌攪拌00陳化陳化不定不定加熱加熱不定不定0洗滌洗滌000重結(jié)晶重結(jié)晶不定不定6) Optimization of precipitating conditions For Crystalloid precipitate: diluted, stir, temperature, aging (digestion) For Amorphous precipitate: Concentration, temperature, addition of electrolyte, no aginghomogenous p