Complexation Reactions and Titrations

1. CHAPTER 17 Complexation Reactions and Titrations

2. Complex: [adj.]錯綜複雜的，合成的 [n] 複合物、錯合物 Complexes: 複數 Complexation reaction(複合反應、錯合反應) • One of the first uses of these reactions was for the titrating cations ~ ~ the major topic • Complexes are colored or absorb ultraviolet radiation ~ ~ the basis for spectrophotometric determine

3. [Cr(NH3)6]3+ § The Formation of Complexes • Complex: metal + ligand • Ligand: have at least one pair of unshared electrons available for bond formation. (electron donor)Ex: H2O, NH3, Cl-, Br-, I-…… ~ ~ An ion or a molecule that forms a covalent bond with a cation or neutral metal atom by donating a pair of electrons, which ate then shared by two. (p450) 449

4. § The Formation of Complexes Coordination number (配位數): the number of covalent bonds that a cation tends to form with electron donors Ex: [Cu(NH3)4]2+, [CuCl4]2-, [Cr(NH3)6]3+ Complexometric method: titrimetric methods based on complex formation (錯離子滴定法) Chelate (螯合物) : when a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six-member heterocyclic ring. 450

5. Chelate: when a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six-member heterocyclic ring. (螯合物) • Unidentate: (單牙基) a ligand that has a single donor group Ex: NH3 • Bidentate: (雙牙基) a ligand that has two groups available for covalent bonding Ex: glycine • Tridentate, tetradentate, ……

6. § The Formation of Complexes • Macrocycles : metal ions and cyclic organic compounds ～～the organic compounds contain nine or more atoms in the cycle and include at least three heteroatoms, usually O, N, S.

7. The ions of alkali metals cam form complexes with crown ether and cryptand D. J. Cram, C. J. Pedersen and J.-M. Lehn Nobel prize in Chemistry in 1987

8. § Complexation Equilibria 451

10. β: the overall formation constant 451

11. α: the fraction of the total metal or metal complex concentration existing

12. Calculation of α for metal complexes CT = CM = [M] + [ML] + [ML2] +…… + [MLn] 452

13. [ML] = β1 [M] [L] [ML2] = β2 [M] [L]2 [ML3] = β3 [M] [L]3 [MLn] = βn [M] [L]n CT = CM = [M] + [ML] + [ML2] +…… + [MLn] = [M] +β1[M] [L]+β2[M] [L]2 + ...+βn[M] [L]n = [M] { 1 + β1[L]+β2[L]2 + …...+βn[L]n }

15. § The Formation of Insoluble Species § Ligands That Can Protonate • Side reaction~ involving the metal or the ligand • For ligand ~ if the ligand is weak acid, then ligand can • be protonated.

16. Complexation with protonating ligands Le Châtelier’s principle M + L L : the conjugate base of polyprotic acid Adding acid ~ reduces the concentration of free L available to complex with M, ~ decrease the effectiveness of L as a complexing agent.

17. 草酸

18. Conditional Formation Constant • Conditional Formation Constant (Effective Formation Constant) ~ ~ the effect of pH on the free ligand concentration in a complexation reaction.

19. At a particular pH value , 2 is constant

20. § Titrations with Inorganic Complexing Agents

21. Figure 17-1 Titration curves for complexometric titrations. Titration of 60.0 mL of a solution that is 0.020 M in metal M with (A) a 0.020 M solution of the tetradentate ligand D to give MD as the product; (B) a 0.040 M solution of the bidentate ligand B to give MB2; and (C) a 0.080 M solution of the unidentate ligand A to give MA4. The overall formation constant for each product is 1020. 455

22. 456

23. 458

24. § Aminocarboxylic Acid Titrations • Ethylenediaminetetraacetic acid [EDTA] The EDTA molecule has six potential sites for bonding a metal ion. 458

25. Four carboxyl group ~~~ H4Y 代表EDTA Acidic Properties of EDTA

26. Figure 17-2 Composition of EDTA solutions as a function of pH. 459

27. Figure 17F-1 Structure of H4Y and its dissociation products. Note that the fully protonated species H4Y exists as the double zwitterion with the amine nitrogens and two of the carboxylic acid groups protonated. The first two protons dissociate from the carboxyl groups, while the last two come from the amine groups. 460

28. Figure 17-3 Structure of a metal/EDTA complex. Note that EDTA behaves here as a hexadentate ligand in that six donor atoms are involved in bonding the divalent metal cation. 461

29. § Complexes of EDTA and Metal Ions • The reagent combines with metal ions in a 1:1 ratio regardless of the charge on the cation.

30. 462

31. § Equilibrium Calculations Involving EDTA • A titration curve for the reaction of Mn+ and EDTA ~ ~ ~ a polt of pM versus reagent volume

32. From 15H • Tetraacetic acid ~~~ H4Y H3Y- H2Y2- HY3- Y4- CT = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y]

33. Conditional Formation Constants

35. 464

36. Example 17-1 Calculate the molar Y4- concentration in a 0.0200M EDTA solution buffered to a pH of 10.00 試求下列溶液的[Y4-]， 0.0200 M EDTA 溶液，pH值為10.0

37. Conditional formation constant Calculation of the cation concentration in EDTA solutions

38. Example 17-2 Calculate the equilibrium concentration of Ni2+ in a solution with an analytical NiY2- concentration of 0.0150M at pH (a) 3.0 and (b) 8.0 計算在不同pH值下，0.0150M的[NiY2-]溶液中有多少[Ni2+] 利用conditional formation constant解題

39. Example 17-3 Calculate the concentration of Ni2+ in a solution that was prepared by mixing 50.0mL of 0.0300M Ni2+ with 50.00mL of 0.05M EDTA. The mixture was buffered to a pH of 3.0 將50.0mL，0.0300M Ni2+與50.00mL，0.05M EDTA溶液混 合，並將混合液的pH值調整至3.0。試計算Ni2+的濃度

40. § EDTA Titration Curves Example 17-4 Use a spreadsheet to construct the titration curve of pCa versus volume of EDTA for 50.00mL of 0.00500M Ca2+ being titrated with 0.0100M EDTA in a solution buffered to a constant pH of 10.0 在pH值為10.0時，利用0.0100M EDTA溶液滴定50.00mL 0.00500M Ca2+溶液，並建構其滴定曲線 pCa vs EDTA(體積) 467

41. Figure 17-5 Spreadsheet for the titration of 50.00 mL of 0.00500 M Ca2+ with 0.0100 M EDTA in a solution buffered at pH 10.0. 467

42. Figure 17-6 EDTA titration curves for 50.0 mL of 0.00500 M Ca2+ (K’CaY=1.75×1010) and Mg2+ (K’MgY=1.72×108) at pH 10.0. Note that because of the larger formation constant, the reaction of calcium ion with EDTA is more complete, and a larger change occurs in the equivalence-point region. The shaded areas show the transition range for the indicator Eriochrome Black T. 470

43. Figure 17-7 Influence of pH on the titration of 0.0100 M Ca2+ with 0.0100 M EDTA. Note that the end point becomes less sharp as the pH decreases because the complex formation reaction is less complete under these circumstances. 470

44. Figure 17-8 Titration curves for 50.0 mL of 0.0100 M solutions of various cations at pH 6.0. 471

45. Figure 17-9 Minimum pH needed for satisfactory titration of various cations with EDTA. (From C.N.Reilley and R.W.Schmid, Anal. Chem., 1958,30,947.copyrigh 1958 American Chemical Society. Reprinted with permission of the American Chemical Society.) 471

46. § The Effect of Other Complexing Agents on EDTA Titration Curves • pH increase ~ ~ [OH-] 會產生M(OH)x的化合物 An auxiliary complexing agent is needed to keep the cation in solution, cause the end points to be less sharp. Auxiliary: 輔助的

47. Figure 17-10 Influence of ammonia concentration on the end point for the titration of 50.0 mL of 0.00500 M Zn2+. Solutions are buffered to pH 9.00. The shaded region shows the transition range for Eriochrome Black T. Note that ammonia decreases the change in pZn in the equivalence-point region. 472

48. Feature 17-5EDTA titration curves when a complexing agent is present 鋅一般為4配位 M 只與NH3的濃度，Zn(NH3)x的形成常數有關

49. αM: the fraction of the total metal or metal complex concentration existing K”ZnY :在特定pH與特定NH3濃度下的條件形成常數

50. Example: calculate the pZn of solutions prepared by adding 20.0, 25.0, and 30.0 mL of 0.0100M EDTA to 50.0 mL of 0.00500M Zn2+. Assume that both the Zn2+ and EDTA solutions are 0.100M in NH3 and 0.175M NH4Cl to provide a constant pH of 9.0 在pH值為9.0的緩衝溶液中(NH3 +NH4Cl)，利用0.0100M EDTA溶液滴定50.00mL 0.00500M Zn2+溶液，求pZn值。 EDTA體積(a) 20.0 mL (b)25.0 mL (c) 30.0 mL K”ZnY :在特定pH與特定NH3濃度下的條件形成常數