化学-配位滴定

1、Chapter Five: ComplexometrylObjectives and Demands： 1) Grasp stability of complexes of EDTA with metal ions; 2) Master the variation of metal ion concentration during titration process.Teaching Contents and Arrangements：l1) Complexes and Formation ConstantsHow Stable Are Complexes?l2) Chelates: EDTA

2、The Ultimate Titrating Agent for Metals;l3) MetalEDTA Titration Curves;l4) Detection of the End Point: IndicatorsThey Are Chelating Agents;l5) Other Uses of Complexes;l6) Fraction of Dissociating Species in Polyligand Complexes: ValuesHow Much of Species?l7) Calculation of Conditional Stability Cons

3、tants of Complex Equilibria. Emphases and Difficulties：lEmphases： 1) Understanding Coefficients Caused by Acidity (lgY(H), Coexisting ions (lgY(N), and Complex Reaction (lgM(L); Effect on Complex Equilibria with Side Reaction; 2) Calculation of Conditional Stability Constants of Complex Equilibria.l

4、Difficulties： Selectivity and Accuracy of Metal ions Titration while Coexisting with Other Ions. 5.1. Generality of Complexometric TitrationlComplexometric titration is a type of titration based on complex formation between the analyte and titrant.lMany metal ions form slightly dissociated complexes

5、 with various ligands (complexing agents). lThe analytical chemist makes judicious use of complexes to mask undesired reactions. lThe formation of complexes can also serve as the basis of accurate and convenient titrations for metal ions in which the titrant is a complexing agent. lComplexometric ti

6、trations are useful for determining a large number of metals. Selectivity can be achieved by appropriate use of masking agents (addition of other complexing agents that react with interfering metal ions) and by pH control, since most complexing agents are weak acids or weak bases whose equilibria ar

7、e influenced by the pH. 1) Complex ReactionlComplexometric titration is a type of titration based on complex formation between the analyte and titrant.)(22CNAgCNAg)()()(22WhiteCNAgAgAgCNAgThe stability of complex can be expressed as Kstab, stability constant, or Kf, formation constant. 1 .212210)(CN

8、AgCNAgKstab2) Theory BaselThe metal ion is a Lewis acid (electron pair acceptor), and the complexer is a Lewis base (electron pair donor).lThe number of molecules of the complexing agent, called the ligand, will depended on the coordination number of the metal and on the number of complexing groups

9、on the ligand molecules.lMany cations will form complexes in solution with a variety of substances that have a pair of unshared electrons (e.g., on N, O, S atoms in the molecule) capable of satisfying the coordination number of the metal.Example of Complexing Agent Ammonia (NH3) Ammonia (NH3) is a s

10、imple complexing agent with one pair of unshared electrons that will complex copper ion: Cu2+ + 4:NH3:Cu:.H3NNH3.NH3NH32+Here, the copper ion acts as a Lewis acid, and the ammonia is a Lewis base. The Cu2+ (hydrated) ion is pale blue in solution, while the ammonia (the ammine) complex is deep blue.

11、A similar reaction occurs with the green hydrated nickel ion to form a deep blue ammine complex. Pale BlueDeep Blue3) Complex and Complexing AgentlComplex(1) Simple complex: It is composed of a center ion and mono-ligand, without cycle structure(2) Chelate: multi-ligand, with cycle lComplexing agent

12、(1) Inorganic complexing agent:(2) Organic complexing agent: 24322)()()()(CNCdCNCdCNCdCNCdCNCdN.COO:.:4) Types of Chelantsl“OO” Type Chelant l“NN” Type Chelant l“NO” Type Chelant lSulfur containing Chelant lTrilon Chelant (氨羧配合剂) 4a) “OO” Type Chelant Bonding metal ion with two oxygen atoms, e.g. Ox

13、yacid (羟基酸羟基酸), Polyprotic acid, Polyhydric alcohol, Polyphenol, CHCOHOHCOHCOHOOH+13Al3+CHCOOHCOHCOHOOH13Al3+H+Tartaric Acid (酒石酸酒石酸) 4b) “NN” Type Chelant Bonding metal ion with two nitrogen atoms, e.g. Organic amine, Heterocycle (杂环杂环) compound like phenanthroline NN3Fe2+Complex of phenanthroline

14、(邻二氮菲邻二氮菲) and Fe 4c) “NO” Type Chelant Bonding metal ion with oxygen and nitrogen atoms, e.g. Trilon (氨羧配合剂氨羧配合剂) Chelant, hydroxyquinoline, and some azo-dyes, +13Al3+13Al3+H+NOHNO8-Hydroxyquinoline (8-羟基喹啉羟基喹啉) 4d) Sulfur containing Chelant: “SS”, “SO”, “SN” types “SS” Type Tetratomic ring:NCC2H5C

15、2H5SSNa+12Cu2+NCC2H5C2H5S12Cu2+S+Na+Copper Reagent (二乙胺基二硫代甲酸钠) +12Cd2+H+COHH2COSH12Cd2+COH2COSHThioglycolic acid (巯基乙酸) “SO”, “SN” Types Penta-ring:4e) Trilon Chelant (氨羧配合剂氨羧配合剂) Capable of binding with many metal ions. Examples are listed below: NNCH2COOHCH2COOHCH2COOHCH2COOHN.COO:.:CyDTA (环己烷二胺四

16、乙酸环己烷二胺四乙酸) H2CH2COOH2CH2CH2CH2CNNCH2COOHCH2COOHCH2COOHCH2COOHEGTA (乙二醇二乙醚二胺四乙酸乙二醇二乙醚二胺四乙酸) H2CH2CNNCH2CH2COOHCH2CH2COOHCH2CH2COOHCH2CH2COOHEDTP (乙二胺四丙酸乙二胺四丙酸) H2CH2CNNCH2COOHCH2COOHCH2COOHCH2COOHEDTA (乙二胺四乙酸乙二胺四乙酸) Basic functional group:5.2. Complexes of EDTA with Metal Ionsl1) Feature of EDTAl2)

17、Dissociation equilibrium of H4Yl3) Metal Complexes with EDTA1) Feature of EDTAlEDTA or (ethylenediamminetetraacetic acid, 乙二胺四乙酸 or EDTA酸), otherwise known as edetate, versene, or diaminoethanetetraacetic acid disodium salt (Na2H2Y2H2O=372.24). lIt has four carboxyl groups and two nitrogen atoms tha

18、t can act as electron pair donors, therefore it is also a bidentate (有二齿的) ligand or Lewis base. This changes in an alkali solution where the EDTA molecule is more stable. It can be written as H4Y. lIt is also a hydrophilic compound and its recommended storage is in a dry area with a desiccator. NH2

19、C-OOCH2CHOOCH2CH+H2CNCH2COOHCH2COO-H+Uses of EDTAl1) Scavenging (v. 净化净化, 清除清除 ) metal ions. -In biochemistry and molecular biology, this is a common method to inactivate enzymes. l2) Complexometric titrations. l3) Buffer solutions. l4) Determination of water hardness. l5) Use as a water softener. l

20、6) Used in medicine as a treatment (brand name Endrate, marketed by Hospira, Inc.; generic product is also on the market) for acute hypercalcemia (n. 医医血钙过多血钙过多 ) and lead poisoning. Uses of EDTAl7) Used in medical and laboratory equipment as an anticoagulant (a. 抗凝的抗凝的; n. 药药抗凝血剂抗凝血剂). l8) Added to

21、 some processed foods and especially cosmetics as a preservative and a sequestrant (n. 多价螯合多价螯合剂剂). l9) A somewhat controversial therapy (n. 疗法疗法,治疗治疗), called chelation therapy has evolved around metal scavengers such as EDTA. l10) Used in dentistry as a root canal irrigant to remove compounds of o

22、rganic and inorganic debris (smearlayer) l11) Used in photography as a component of bleach-fix used to dissolve elemental silver produced during development. l12) Used as a soil conditioner to allow calcifuge (n. 植植避钙避钙植物植物, 嫌钙植物嫌钙植物) plants to grow in base rich soils. Use as Anticoagulant (n. 药药抗凝血

23、剂抗凝血剂)lEDTA works as an anticoagulant by chelating all the calcium contained in blood. Calcium is needed for coagulation to occur; without calcium blood will not clot. The calcium levels below which clotting ceases are low enough to be lethal (a. 致命的), so EDTA is only used as an anticoagulant outsid

24、e the body; for instance in tubes of blood, and medical machinery. lEDTA is contained in purple, lavender and pink Vacutainer (tubes that blood is taken in), and can be in the form of a powder, or a small amount of liquid, already in the tube. lThe sodium or potassium salts of EDTA (K2EDTA, K3EDTA,

25、Na2EDTA) are used in Vacutainer tubes. 2) Dissociation equilibrium of H4Y NH2C-OOCH2CHOOCH2CH+H2CNCH2COOHCH2COO-H+pH=1pH12 However in high pH EDTA forms a Y4- as the alkali pH accepts the H+ ions from the carboxyl groups. This is now a hexadentate ligand and can form a octahedral complex with most m

26、etal 2+ ions (M2+), hence the name complexometeric titration. 463522433425126YHYYHYHYHYHYHKKKKKK2) Dissociation equilibrium of H4Y NH2C-OOCH2CHOOCH2CH+H2CNCH2COOHCH2COO-H+pH=1pH12 However in high pH EDTA forms a Y4- as the alkali pH accepts theH+ ions from the carboxyl groups. This is now a hexadent

27、ate ligandand can form a octahedral complex with most metal 2+ ions (M2+), hence the name complexometeric titration. The complex formed with the ion MY2- will be in a 1:1 ratio and isa reason why EDTA is so widely used to determine amounts of metalions such as manganese and calcium in water or to he

28、lp treat leadpoisoning in humans as the EDTA binds with the heavy, toxic metal. 463522433425126YHYYHYHYHYHYHKKKKKKEDTA Equilibria463522433425126YHYYHYHYHYHYHKKKKKKYHHYH5269 . 0265110YHYHHKaYHHYH456 . 154210YHYHHKaYHHYH340 . 243310YHYHHKa223YHHYH67. 2322410YHYHHKa322HYHYH16. 6223510YHHYHKa43YHHY26.10

29、34610HYYHKa pH=1pH123) Complexes of EDTA with Metal IonslEDTA or its disodium salt is a chelating agent, forming coordination compounds with most divalent (or trivalent) metal ions, such as calcium (Ca2+) and magnesium (Mg2+) or copper (Cu2+). MYYMYMMYKMYFormation Constant,or Stability Constant:3) C

30、omplexes of EDTA with Metal IonsStructure Diagramof Metal-EDTA Chelate Concluded Features for EDTA lEDTA stands for ethylenediamintetraacetic acid .lEDTA forms strong 1:1 hexa dental complexes with most metal ions (= six-coordinate geometry). The metal is coordinated to oxygen and nitrogen of EDTA.l

31、EDTA has six deprotonation constants (K1-K6), with K1-K4 are referring to the deprotonation of the carboxylic acids and K5&K6 refer to the amins. lThe neutral compound is H4Y, the most acidic one is H6Y2+ and the most basic ones is Y4-. lThe commonly used compound is the disodium salt Na2H2Y2H2O. lA

32、 metal-EDTA complex becomes less stable at lower pH. NH2C-OOCH2CHOOCH2CH+H2CNCH2COOHCH2COO-H+432234526YHYYHYHYHYHYHK1K2K3K4K5K65.3. Effect on Stability of EDTA Complexes with Metal ions M+YMYOHLHNHOHMLM(OH)HYNYMHYMOHYM(OH)nH6YMLnHydroxideEffectAuxiliaryEffectAcidityEffectInterferingEffectMixedEffect

33、MainSide Reaction1) Acidity Effect and Its CoefficientMain reaction: 44nnMYYMAcidity effect: The ability of Y4- in main reaction is lowered by theinteraction of Y4- and H+.Acidity effect coefficient: 4)(YYHYwhere, Y represents the analytical concentration of EDTA,Y4- represents the concentration of

34、component Y4-, which canform a complex with metal ion. Then, 265432234YHYHYHYHYHHYYY265432234YHYHYHYHYHHYHYSide ReactionAcidity effect coefficient66554433221123456623456534564456356264265432234)(11HHHHHHKKKKKKHKKKKKHKKKKHKKKHKKHKHYYHYHYHYHYHHYYaaaaaaaaaaaaaaaaaaaaaHYAs 1, 2, , 6 are cumulative stabi

35、lity constants, then Y(H) is correlated with acidity of solution. If there is no secondary reaction between H+ and Y4-, then Y(H) = 1.Example 5.1 Example 5.2 Example 5.1：Please calculate the acidity effect coefficient of cyanide while at pH=5.00? Solution: HCNHCN10102 . 6aK4105)(106 . 1102 . 61000.

36、111 aHCNKHCNHCNCNCNCNlgCN(H) = 4.21 Example 5.2: Please calculate the acidity effect coefficient of EDTA while at pH=2.00? Solution: pKa1=-0.9, pKa2=-1.6, pKa3=-2.0, pKa4=-2.67, pKa5=-6.16, pKa6=-10.26 66554433221123456623456534564456356264265432234)(11HHHHHHKKKKKKHKKKKKHKKKKHKKKHKKHKHYYHYHYHYHYHHYY

37、aaaaaaaaaaaaaaaaaaaaaHYExample 5.2: Please calculate the acidity effect coefficient of EDTA while at pH=2.00? Solution: pKa1=-0.9, pKa2=-1.6, pKa3=-2.0, pKa4=-2.67, pKa5=-6.16, pKa6=-10.26 1359.1169.1209.1309.1342.1226. 80.9-6 . 10 . 267. 216. 626.10626 . 10 . 267. 216. 626.10520 . 267. 216. 626.104

38、267. 216. 626.103216. 626.102226.102)(1025. 310101010101011010101010101000. 1 10101010101000. 1 101010101000. 1 1010101000. 1 10101000. 1 10101.001HYlgY(H) = 13.51 2) Effect of Metal Ion Hydroxide effect or electrolysis effect: nnnOHMOHOHOHMOHMOHMMOHM.1)(.)(2212)(Auxiliary effect: nnnLMLLLMLMLMMLM.1

39、)(.)(2212)(Effect of Metal IonThe total coefficient of secondary reaction for metal ion can be expressed as: MMMWhere, M=M+MOH+M(OH)2+M(OH)n+ML+ML2+MLnThus, M =M(L) + M(OH) 1 If existing multi-complexing agent, then 123.(1)nMM LM LM LM LaaaaanFor example,2 )()()(CMBMAMMMMExample 5.3 Example 5.4 Exam

40、ple 5.3: In 0.10mol/L of AlF63- solution, the concentration of free F- is 0.010mol/L, Please calculate the concentration of Al3+? Solution: The cumulative stability constants of AlF63- are 1=1.4106, 2=1.41011, 3=1.01015, 4=5.61017, 5=2.31019, 6=6.91019; F-=0.010mol/L, cAlF63- =0.10mol/L 979997461951

41、94173152116662213)(109 . 8109 . 6103 . 2106 . 5100 . 1104 . 1104 . 11)010. 0(109 . 6)010. 0(103 . 2)010. 0(106 . 5)010. 0(100 . 1)010. 0(104 . 1010. 0104 . 11.1FFFAlcFAl AlF3 AlF4- AlF52- main complexesLmolcAlFAl/101 . 1109 . 810. 0119)(3Example 5.4: In 0.010mol/L of zinc-ammonia solution, the conce

42、ntration of free NH3 is 0.10mol/L while pH=10, Please calculate the total coefficient of secondary reaction for zinc ion? Solution: The cumulative stability constants of Zn(NH3)42+ are log1log4: 2.27, 4.61, 7.01, 9.06; While pH=10, Zn(OH) = 102.4; 1 . 5406. 9301. 7261. 427. 2434333232312)(10)10. 0(1

43、0)10. 0(10)10. 0(1010. 010113NHNHNHNHZncNHZn1 . 54 . 21 . 5)()(101101013OHZnNHZnZn3) Conditional Stability/Formation ConstantM+YMYOHLHNHOHMLM(OH)HYNYMHYMOHYM(OH)nH6YMLnHydroxideEffectAuxiliaryEffectAcidityEffectInterferingEffectMixedEffectMainSide ReactionAccording to main reaction, the formation co

44、nstant or stability constant is expressed as: YMMYKMY3.1) Considering acidity effect of EDTA: The conditional stability constant value holds for only a specified pH. 4)(YYYYHY)( HYMYYMMYYMMYK)( HYMYMYKYMMYKWhere, Y represents the analytical concentration of EDTA including all the existing forms. Y4-

45、 or Y represents the concentration of only component Y4-, which can form a complex with metal ion. Relationship between pH and KMY: pH logY(H) KMY Complex reaction completely Benefit for complexometric titration 3.2) Considering effect of acidity and metal ion: )( HYMYYMMYYMMYK 4)(YYYYHY MMM)( HYMYM

46、YKYMMYK)( HYMMYYMMYKYMMYKK3.3) Effect on complexometric titration: a) For metal ion: hydroxide effect (MOH) and auxiliary effect (ML) from other complexing agents;b) For EDTA: acidity effect and interfering effect from other coexisting metal ions;c) For MY: mixed effect (MHY and MOHY)It is very impo

47、rtant to control pH value in solution for complexometric titration. 4) Appropriate pH Condition The control of appropriate pH condition is depended on the acidityeffect of EDTA and the hydroxide effect of metal ion.End-point error permitted: pM (0.20.5), RE 0.1% Then, This is the condition for deter

48、mination of single component metalion with complexometric titration. 6lgMYcK6lglglglglglg)(HYMYMYMYKcKccK6lglglg)(MYHYKcExample 5.5Example 5.5lWhat is the permitted lowest pH value when titrating 0.01mol/L Ca2+ solution with EDTA? (logKCaY=10.69) lSolution:69. 2669.1001. 016lglglg)(gKcCaYHYCheck in

49、Table 5-2, pHmin 7.6 Ringbom Curve: Curve of acidity effect Ringbom Curve: Curve of acidity effect5.4. Titration Curve l1) Similar to acid-base titrationl2) Transition range at stoichiometric point, pM=-lgMn+ l3) Considering side reactionsl4) Metal indicator is decided by the experimentExample 5.6:

50、Titration with 0.01000mol/L EDTA into 20.00mL 0.01000mol/L Ca2+ solutionl(1) Before titration: Only Ca2+ existing in solution, the pCa is depended on the original concentration. 1210000 . 0LmolCaThen, pCa=2.00 Example 5.6: Titration with 0.01000mol/L EDTA into 20.00mL 0.01000mol/L Ca2+ solutionl(2)

51、Starting titration till stoichiometric point: lDropping EDTA and form CaY complex till completed, the pCa is depended on the residual Ca2+.lIf adding 18.00 mL EDTA, the residual Ca2+ of 2.00 mL has not formed complex with EDTA. lThen, 510)00.1800.20(10)00.1800.20(01000. 0LmolLLLmolCapCa

52、=3.28 Example 5.6: Titration with 0.01000mol/L EDTA into 20.00mL 0.01000mol/L Ca2+ solutionl(3) At stoichiometric point:lWhen adding 20.00 mL EDTA solution, Ca2+ is completely formed complex with EDTA. lIn the solution, Ca2+=Y, CaY=0.5CCa, then2225 . 0CaCYCaCaYKCaCaYSo, 150. 669.102101001000. 05 . 0

53、5 . 0LmolKCCaCaYCapCa=6.50 Example 5.6: Titration with 0.01000mol/L EDTA into 20.00mL 0.01000mol/L Ca2+ solutionl(4) After stoichiometric point: lWhen adding excess EDTA, the solution contains CaY and residual Y.lIf adding 20.02 mL EDTA, the residual EDTA is 0.02 mL.lThen, 1633110998. 410)02.2000.20

54、(10)00.2002.20(10000 . 0YLmolLLLmol169. 7669.1021010998. 41002.2000.2000.2001000. 0LmolYKCaYCaCaYpCa = 7.69Table 5.1 Titration with 0.01000mol/L EDTA into 20.00mL 0.01000mol/L Ca2+ solutionAdding EDTApCaWhile pH=12.50pCaWhile pH=8.00pHvariationmL% 0.0000.002.0012.518.0090.03.283.2819.8099.04.304.301

55、9.9899.95.305.1220.00100.06.505.3611.820.02100.17.695.60lgY(H)0.01 20.20101.08.696.42lgCa(OH): small 22.00110.09.697.4240.00200.010.698.42lgKCaY=10.69Titration with 0.01000mol/L EDTA into 0.01000mol/L metal ion solutionCa2+Ni2+NH3+NH4+=0.1mol/LEDTA added / %5.5. Metal Indicators and End-Point l1) Me

56、tal indicatorsCBTEDTAMEDTACBTMRed(Ca2+, Mg2+, Zn2+, Cd2+)BluepH=8-11a. Condition for titration: KM(CBT) KM(EDTA)b. Chose appropriate pH range: 322InHInInHRedpH12 5.5. Metal Indicators and End-Point l2) Rules as metal indicators:a. Both indicator and complex colors are much different;b. Coloring reac

57、tion is sensitive, fast and reversible;c. Coloring complex is stable;d. Complex is water soluble;e. Metal indicator is stable.l3) Common metal indicators Chrome Black T, Cu-PANl4) Other end-point indicating methods5.6. Titration of Metal Ions in Mixtrue1) Titration separately by pH controll(1) For s

58、ingle metal ion, the condition of titration is: lEnd-point error permitted: pM(0.20.5), RE0.1%6lgMYMKc5.6. Titration of Metal Ions in Mixtrue1) Titration separately by pH controll(2) Two components: M and interfering ion NWhen KMY KNY, and Y(N) Y(H), )()()(1NYHYNYYYMMYYMMYKYMMYKK NNYNYYNYcKNKYNYYNYY

59、YY11 )(5.6. Titration of Metal Ions in Mixtrue1) Titration separately by pH controll(2) Two components: M and interfering ion N5lglglglglglgNMNMNYMYMYMCCKCCKKKcpM 0.3, RE 0.5% When KMY KNY, and Y(N) Y(H), Y(N) = KNYCNNYNMMYMNYHYMMYMYMMYMMYMKcKcKcKcKclglglg) 1lg(lglglglglglg)()(Neglecting lgM, Or if

60、CM = CN, lgK 55.6. Titration of Metal Ions in Mixtrue1) Titration separately by pH controll(1) For single metal ion, the condition of titration is: lEnd-point error permitted: pM(0.20.5), RE0.1% l(2) Two components: M and interfering ion Nl(3) Metal components 36lgMYMKcNKKKNYMYMY5lglglglglglgNMNMNYM