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Chapter 11

Chapter 11. Polyprotic Acid and Bases. Diprotic Acids. Compounds with two acid/base groups Can be two acids groups Oxalic Acid Can be two basic groups Cadaverine Or and acid and base group Amino acids. Oxalic Acid. Cadaverine. Generic Amino Acid. Amino Acids. H 2 L + = HL = L 2-

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Chapter 11

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  1. Chapter 11 Polyprotic Acid and Bases

  2. Diprotic Acids • Compounds with two acid/base groups • Can be two acids groups • Oxalic Acid • Can be two basic groups • Cadaverine • Or and acid and base group • Amino acids

  3. Oxalic Acid

  4. Cadaverine

  5. Generic Amino Acid

  6. Amino Acids • H2L+ = HL = L2- • Glycine.HCl CAS 6000-43-7 • Glycine CAS 56-40-6 • Sodium Glycinate CAS 6000-44-8

  7. Ionic Structures • Glycine HCl

  8. Sodium Glycinate

  9. Zwitterion form Intermediate Form

  10. Constants • H2L+ = HL = L- • Ka1 Ka2 • Kb2 Kb1 • Ka1 * Kb2 = Kw • or • Ka2 * Kb1 = Kw

  11. How do we then calculate the pH’s • Acid Form H2L+ • Treat was a weak monoprotic acid. • Base Form L- • Treat as a weak monoprotic base

  12. Intermediate Form pH • These compounds can either gain a proton (work as a base) or they can donate a proton (work as an acid). • The magnitude of the K values will determine the pH. • The following equation can be derived …

  13. Intermediate Form

  14. Approximations • In many cases the K1Kw term will be much smaller than the other term in the numerator so the equation becomes: • Then often K1 will be much smaller than [HL] so we end up with:

  15. Intermediate Form • Which when taken to the log form gives us. • pH = (pKa1 + pKa2)/2

  16. Diprotic Buffer Systems • Same treatment as with monoprotic systems but we use the pKa that is between the two major forms that are in the solution.

  17. Buffers • Phosphoric acid system • H3PO4 and H2PO4- use pKa1 • H2PO4- and HPO42- use pKa2 • HPO42- and PO43- use pKa3

  18. Principle Species • Monoprotic

  19. Principle Species • Diprotic • Fumaric acid

  20. Principle Species

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