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Chapters 11, 12

Chapters 11, 12. Announcements. Homework Assignments. Charge Balance Equations.

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Chapters 11, 12

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  1. Chapters 11, 12

  2. Announcements • Homework Assignments

  3. Charge Balance Equations The charge balance equation is an algebraic statement that the sum of all positive charges in the solution must equal the sum of all of the negative charges. In writing charge balance equations, the coefficient in front of each species is always the magnitude of the charge on that ion; example, for the phosphate ion, 3[PO4].

  4. Charge balance in a solution that contains 0.0250 M KH2PO4 and 0.0300 M KOH. The sum of positive charges = sum of negative charges.

  5. Mass Balance (or material-balance) Equations The mass-balance equation is statement of the conversation of matter. For example in considering a 0.050 M solution of acetic acid CH3CO2H which dissociates CH3CO2H(aq) < == > CH3CO2 (aq)+ H+(aq) 0.050 M = [CH3CO2H] + [CH3CO2] If there are several products as in the ionization of a polyprotic acid, the mass balance must include all possible species.

  6. Fractional Composition Equations In the ionization of polyprotic acids is best viewed in terms of fractional composition equations. For a weak acid HA, the total concentration of HA = the sum of [A] + [HA] The fractional part of the entire concentration is given by the symbol x which is defined as the ratio of [x] / [total] For HA, A- = [A] / [A] + [HA] Likewise we could represent the fraction of HA as HA = [HA] / [A] + [HA]

  7. Fractional Composition Equations Page 249 of the textbook shows the derivation of useful expressions for alpha values. HA = [H+] / [H+] + Ka and A- = Ka / [H+] + Ka

  8. Alpha plot (pH vs ) for a system HA whose pKa = 5.00. Below a pH of 5.00 the predominant form is HA; above a pH of 5.00, the predominant form is A.

  9. Fractional Composition Equations In general for any polyprotic acid HnX Hn A = [H+]n / [H+]n + Ka1 [H+]n-1 + Ka1 Ka2[H+]n-2 … Ka1 Ka2 X….Kan Alpha Values for Diprotic Acids – an Excel sheet

  10. Chapter 12 – EDTA Titrations

  11. EDTA Titrations EDTA (abbreviation for ethylenediaminetetra- acetic acid) is the most widely used reagent of a class of materials known as chelates, or polydentate ligands. Ligands are compounds that may act as good Lewis Bases, i.e., electron pair donors. If the ligand has several pairs of electrons, it may function as a polydentate ligand.

  12. EDTA form strong 1 : 1 complexes with many metal ions; the coordination is through the 4 O atoms and 2 N atoms

  13. EDTA is a hexa-pronated ligand; the 6 of the pKa values of EDTA are given below:

  14. Predominant form of EDTA as a function of pH

  15. Fractional composition of multi-protonated ligands The fraction () of a possible form of a multi-pronated ligand is a function of the pH or [H+] of the solution. Y-4 = [Y -4] / [EDTA] Total We will not derive it at this time, but Y-4 = (K1…Kn) / [H+]n + [H+]n-1K1 + [H+]n-2K1 K2 + …  (K1…Kn) It is a plot of  values, (0 to n) that is plotted in the next slide.

  16. Graph of Predominant form of EDTA as a function of pH

  17. Formation Constants Kf for polydentate metal-complexes The formation reaction for polydentate ligands that form a 1 : 1 complex with metal ions is defined by the reaction M+n + Y-4 < === > MY n-4 and the formation constant Kf = [MY n-4 ] / [M+n] [Y-4] Since the Kf values for most metal-edta complexes are few large > 105, it is customary to report them as log10 Kf ;these are given for several metal-EDTA complexes in the next slide.

  18. Formation Constants Kf for polydentate metal-complexes For the metal-EDTA complexes, it is the tetra-negative form of EDTA (edta4) that reacts (or chelates) with the metal. Because the  value for edta4 is dependent on the pH as shown in the alpha plot, complexation general occurs at pH > 10 where edta-4 is large.

  19. Conditional Formation Constants Kf’ for polydentate metal-complexes The conditional formation constants Kf’ are then dependent on the actual [edta-4], or may be written as Kf’=edta-4 Kf Conditional formation constants will always be less than the Kf values for the same metal-EDTA complex. The practical consequence of this is shown in the next slide.

  20. Effect of pH on the end point in the EDTA titration of Ca+2

  21. Titration curve for the EDTA titration of Ca and Mg

  22. Metal Ion Indicators A metal ion indicator (metallochromic indictor) are compounds that change color when they bind to a metal ion. In order to be useful as an metal ion indicator, the compound must bind less strongly than EDTA. In a typical titration of Mg+2 with EDTA Eriochrom Black T might be used as the indicator. This reaction is shown as MgIn + EDTA  MgEDTA + In The color of MgIn is wine red; the color of the freed indicator In is blue. Thus the color change at the end point.

  23. pH regions and suggested indicators for EDTA titrations of several metal ions.

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