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

HO

OH

O

O

4

Cadaverine

H2N NH2

5

Generic Amino Acid

H2N

OH

O

R

6

7

8

Amino Acids

H2L+ = HL = L2-

Glycine.HCl CAS 6000-43-7

Glycine CAS 56-40-6

Sodium Glycinate CAS 6000-44-8

9

Ionic Structures

Glycine HCl

+H3N

OH

O

Cl-

10

Sodium Glycinate

H2N

O-

O

Na+

11

Intermediate Form

+H3N

O-

O

H2N

OH

O

Zwitterion form

12

Constants

H2L+ = HL = L-

Ka1 Ka2

Kb2 Kb1

Ka1 * Kb2 = Kw

or Ka2 * Kb1 = Kw

13

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

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

][

][][

1

121

HLK

KKHLKKH w

16

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:

21][ KKH

][

][][

1

121

HLK

KKHLKKH w

][

][][

1

21

HLK

HLKKH

17

Intermediate Form

Which when taken to the log form gives us.

pH = (pKa1 + pKa2)/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 H3PO4 and H2PO4

- use pKa1

H2PO4- and HPO4

2- use pKa2

HPO42- and PO4

3- use pKa3

20

Principle Species

Monoprotic

21

Principle Species

Diprotic Fumaric acid

22

Principle Species