chapter 11
DESCRIPTION
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- - PowerPoint PPT PresentationTRANSCRIPT
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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
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Amino Acids
H2L+ = HL = L2-
Glycine.HCl CAS 6000-43-7
Glycine CAS 56-40-6
Sodium Glycinate CAS 6000-44-8
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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
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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 …
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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
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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.
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Buffers
Phosphoric acid system H3PO4 and H2PO4
- use pKa1
H2PO4- and HPO4
2- use pKa2
HPO42- and PO4
3- use pKa3