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

7

8 Amino Acids H 2 L + = HL = L 2- Glycine.HCl CAS Glycine CAS Sodium Glycinate CAS

9 Ionic Structures Glycine HCl

10 Sodium Glycinate

11 Intermediate Form Zwitterion form

12 Constants H 2 L + = HL = L - K a1 K a2 K b2 K b1 K a1 * K b2 = K w or K a2 * K b1 = K w

13 How do we then calculate the pH’s Acid Form H 2 L + Treat was a weak monoprotic acid. Base Form L - Treat as a weak monoprotic base

14 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 …

15 Intermediate Form

16 Approximations In many cases the K 1 K w term will be much smaller than the other term in the numerator so the equation becomes: Then often K 1 will be much smaller than [HL] so we end up with:

17 Intermediate Form Which when taken to the log form gives us. pH = (pK a1 + pK a2 )/2

18 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.

19 Buffers Phosphoric acid system H 3 PO 4 and H 2 PO 4 - use pK a1 H 2 PO 4 - and HPO 4 2- use pK a2 HPO 4 2- and PO 4 3- use pK a3

20 Principle Species Monoprotic

21 Principle Species Diprotic Fumaric acid

22 Principle Species