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Ch 13 - EDTA Titrations

Ch 13 - EDTA Titrations. E thylene D iamine T etraacetic A cid. Formation Constants, K f. EDTA Formation Constants. EDTA is a hexaprotic weak acid that complexes 1:1 with metal cations -.

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Ch 13 - EDTA Titrations

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  1. Ch 13 - EDTA Titrations EthyleneDiamineTetraacetic Acid

  2. Formation Constants, Kf

  3. EDTA Formation Constants EDTA is a hexaprotic weak acid that complexes 1:1 with metal cations -

  4. Notice that the first 4 protons are much more acidic than the last two, so the dominant form of EDTA in solution will be H2Y2- .... Mn+ + H2Y2- = MYn-4 + 2H+ By Le Chatelier's Principle, the complex will dissociate at low pH's, and it will be more stable at high pH's. EDTA titrations are therefore pH dependent and analyte solutions must be buffered to the optimum pH. low pH high pH

  5. Minimum pH for Titration from "Fundamentals of Analytical Chemistry", Skoog and West, 4th ed.

  6. The pH-Dependent Metal-EDTA Equilibrium (Sec. 12-5 and Sec 13-5) Fractional Composition Equations - the fraction (percentage) of each species of an acid or base existing at a given pH HA = H+ + A- CT = [HA] + [A-] A- = fraction of HA dissociated to A- HA = fraction of HA still existing as HA

  7. the mass balance equation is - rearranging and substituting into Ka -

  8. solve for [HA] - to solve for A- substitute [HA] = CT - [A-] into Ka

  9. Fractional Composition Diagram for Monoprotic Acids cross where pH = pKa

  10. Fractional Composition Diagrams for Polyprotic Acids: General Forms diprotic triprotic

  11. Ka1 = 4.46 x 10-7 Ka2 = 4.69 x 10-11 Ka1 = 7.11 x 10-3 Ka2 = 6.34 x 10-8 Ka3 = 4.50 x 10-13

  12. Fractional Composition Diagrams for EDTA

  13. Conditional (Effective) Formation Constant, K'f For EDTA titration curves, it's convenient to base calcuations on the Y4- form of EDTA and derive a new, pH-dependent formation constant K'f CT = all forms of EDTA (Y4-, HY3- etc)

  14. All EDTA equilibrium calculations will use K'f at the pH of the titration. The value of Y4- at this pH is taken from Table 13-3.

  15. Example, p. 300 The formation constant for FeY- is 1.3 x 1025 (Fe3+). Calculate the concentration of free Fe3+ in a solution of 0.10 M FeY- at pH = 4.00 and pH = 1.00.

  16. EDTA Titration Curves, Sec 13-6 A complex formation titration curve plots pM (analogous to pH) vs. volume of titrant (see next slide). To save time, we will only calculate the pM = -log[Mn+] at the equivalence point in order to select the correct indicator. p.302 - Calculate the titration curve for the reaction of 50.0 mL of 0.0500 M Mg2+, buffered to a pH of 10.0, with 0.0500 M EDTA. The equivalence pt. volume is 50.0 mL.

  17. At the equivalence pt. virtually all of the Mg is in the form MgY2-. [MgY2-] at the eq. pt. = The ICE table for the reaction is: Initial: Change: Equilibrium:

  18. Kf CaY2- > Kf MgY2- so the endpoint is sharper for Ca2+

  19. The equiv. pt. becomes sharper as the pH of the titration approaches the optimal value for the analyte, e.g. for Ca2+….. The equiv. pt. becomes sharper as the Kf of the EDTA-metal complexes becomes larger…..

  20. H2In- + H2O = HIn2- + H3O+ pKa2 = 8.1 redblue HIn2- + H2O = In3- + H3O+ pKa3 = 12.4 blueorange end point reaction with metal cation….. MIn- + Y4- = MY2- + In3-Kf < Kf analyte redblue

  21. In the titration of Ca, Mg is added to the titrant in order to sharpen the end point.

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