Chapter 11 EDTA Titrations

1. Chapter 11 EDTA Titrations

2. Polar groups have positive and negative regions that attract neighboring molecules by electrostatic forces. Nonpolar groups have little charge separation and are soluble inside the nonpolar cell membrane. Metal cations dissolve in water and are said to be hydrophilic (“water loving”). Cell membranes exclude water and are described as hydrophobic (“water hating”).

3. EDTA is a merciful abbreviation for ethylenediaminetetraacetic acid, a compound that forms strong 1:1 complexes with most metal ions (Figure 12-1) and finds wide use in quantitative analysis.

4. 11-1 Metal-Chelate Complexs

5. Metal ions are Lewis acids, accepting electron pairs from electron-donating ligands that are Lewis bases. Cyanide (CN-) is called a monodentate ligand because it binds to a metal ion through only one atom (the carbon atom). A ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate (“many toothed”) , or a chelating ligand ( pronounced KEE-late-ing). We say that ethylenediamine is bidentate because it binds to the metal through two ligand atoms. The chelate effect is the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands.4

8. An important tetradentate ligand is adenosine triphosphate (ATP), which binds to divalent metal ions (such as Mg2+, Mn2+, Co2+, and Ni2+) through four of their six coordination positions (Figure 12-2).

9. The octadentate ligand in Figure 12-3 is being evaluated as an anticancer agent.5 The chelate is covalently attached to a monoclonal antibody, which is a protein produced by one specific type of cell in response to one specific foreign substance called an antigen.

12. A titration based on complex formation is called a complexometric titration. The stoichiometry is 1:1 regardless of the charge on the ion.

15. BOX 11-1 Chelation Therapy and Thalassemia

19. 11-2 EDTA Acid-Base Properties pK applies at 25oC and µ = 0.1, except pK1 applies at µ = 1 M

20. Fraction of EDTA in the form Y4-: where [EDTA] is the total concentration of all free EDTA species in the solution.

22. EDTA Complexes The equilibrium constant for the reaction of a metal with a ligand is called the formation constant,Kf, or the stability constant: Formation constant:

27. Conditional Formation Constant [Y4-] = αY4-[EDTA] Conditional formation constant: The number K’f = αY4-Kf is called the conditional formation constant, or the effective formation constant. Mn+ + EDTA = MYn-4K’f = αY4-Kf

29. 11-3 EDTA Titration Curves Mn+ + EDTA = MYn-4K’f = αY4-Kf (12-7) Region 1: Before the Equivalence Point Region 2: At the Equivalence Point MYn-4 = Mn+ + EDTA Region 3: After the Equivalence Point

30. Titration Calculations Ca2+ + EDTA  CaY2- K’f = αY4-Kf = (0.30)(1010.65) = 1.34X 1010 Region 1: Before the Equivalence Point Initial volume of Ca2+ Total volume of solution Fraction remaining(=4/5) Original concentration of Ca2+ Dilution factor =0.029 1 M  pCa2+ = -log[Ca2+] = 1.54

31. Region 2: At the Equivalence Point Initial volume of Ca2+ Total volume of solution Original concentration of Ca2+ Dilution factor Initial concentration (M) Final concentration (M)

32. Region 3: After the Equivalence Point Volume of excess EDTA Total volume of solution Original concentration of EDTA Dilution factor Initial volume of Ca2+ Total volume of solution Original concentration of Ca2+ Dilution factor

33. The Titration Curve

34. 11-4 Do It with a Spreadsheet Mass balance for M: Mass balance for L:

35. Spreadsheet equation for titration of M with L: Spreadsheet equation for titration of M with L:

37. 11-5 Auxiliary Complexing Agents To permit many metals to be titrated in alkaline solutions with EDTA, we use an auxiliary complexing agent.

38. Metal-Ligand Equilibria17 M + L = ML β1 = [ML]/([M][L]) (12-13) M + 2L = ML2 β2 = [ML2]/([M][L]2) (12-14) The equilibrium constants, βi, are called overall or cumulative formation constants. αM = [M]/CM (12-15) CM = [M] + [ML] + [ML2] CM = [M] + β1[M][L] + β2[M][L]2 = [M]{1 + β1[L] + β2[L]2} Fraction of free metal ion:

39. EDTA Titration with an Auxiliary complexing Agents K’’f = αZn2+αY4-Kf (12-18)

43. Box 11-2 Metal Ion Hydrolysis Decreases the Effective Formation Constant for EDTA Complexes K’’’f = {(αFe3+αY4-)/αFeY-}Kf Take-home message: In this book, we restrict ourselves to cases in which there is no hydrolysis and αMm+ is controlled by a deliberately added auxiliary ligand. In reality, hydrolysis of Mm+ and MY influences most EDTA titrations and makes the theoretical analysis more complicated than we pretend in this chapter.

44. 11-6 Metal Ion Indicators Metal ion indicators (Table 12-3) are compounds whose color changes when they bind to a metal ion. Useful indicators must bind metal less strongly than EDTA does. MgIn + EDTA  MgEDTA + In (12-19) Red Colorless Colorless Blue

45. Demonstration 11-1 Metal Ion Indicator Color Changes

47. If a metal does not freely dissociate from an indicator, the metal is said to block the indicator.

48. 11-7 EDTA Titration Techniques Direct Titration In a direct titration,analyte is titrated with standard EDTA. Auxiliary complexing agents such as NH3, tartrate, citrate, or triethanolamine may be employed to prevent metal ion from precipitating in the absence of EDTA.

50. Back Titration In a back titration, a known excess of EDTA is added to the analyte.