Acid/base chemistry

1
Acid/base chemistry

• Buffers
• Henderson-Hasselbalch equation

2

• Weak acid (eg. acetic acid)
• Acetic acid ? acetate proton
• CH3COOH ? CH3COO- H

Weak acid
Conjugate base
3
CH3COOH ? CH3COO- H
Ratio holds true at equilibrium if you
neutralize H (decrease numerator), the
concentrations will adjust to get back to
Ka equilibrium shifts to the right
4
Ka/pKa

• Relative strength of an acid
• Higher Ka/lower pKa stronger
• pH of the pure acid
• pH range at which the weak acid is an effective
  buffer
• pH range at which acid and conjugate base are
  both present in appreciable quantities

5
Initial pH
Dissolve a weak acid in water (acetic, Ka 1.74
x 10-5) HAc ? H Ac- Reaction proceeds
until it reaches equilibrium H depends on Ka,
HAc equilibrium H 0.0013M pH 2.9
6
Buffer functional definition

• Solution that resists change in pH upon addition
  of small amounts of H or OH- (strong acid or
  base)
• Resists change in pH upon dilution

7
Buffering region(pKa 1 pH unit)
HAc ? Ac- H
Neutralizing H
8
Buffering region(pKa 1 pH unit)
9
What happens when pH pKa?

• First add strong acid to pure water
• 0.1 M HCl ? pH?
• pH 7 ? 1
• Second add strong acid to 0.5 M solution of
  acetic acid at pH 4.76
• pH 4.76 ? 4.38

10
(No Transcript)
11
Henderson-Hasselbalch
A-H HA
Ka
HA A-
H Ka
HA A-
-logH -log(Ka) - log
HA A-
A- HA
pH pKa - log
pH pKa log
12
Henderson-Hasselbalch

• Calculate pH, given pKa and ratio of conjugate
  base/weak acid
• Calculate pKa, given pH and ratio
• Calculate ratio, given pKa and pH

13

• Whats the ratio of base/acid of aspartic acids
  side chain at neutral pH? (pKa 3.65)
• Whats the ratio of base/acid of histidines side
  chain at neutral pH? (pKa 6.00)