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Buffer Solutions
Section 8.
A buffer solution is one that resists changes in
pH when small quantities of acid or base are
added to it.
- Can be acidic or basic
- Contains entities that can remove any H+^ or OH-^ that are
added to it
Titration curve of a weak acid with a strong base
Buffering zone – very little change in pH
Acidic buffers
- Made of a weak acid , and a salt of its conjugate base
Eg. HC 2 H 3 O 2 and NaC 2 H 3 O (^2)
Basic buffers
- Made of a weak base , and a salt of its conjugate acid
Eg. NH 3 and NH 4 Cl
How does a buffer work?
Consider the acidic buffer of acetic acid/acetate ion.
The two entities exist in equilibrium:
HC 2 H 3 O 2 H +^ + C 2 H 3 O 2 -
The buffer solution contains these entities:
- Unionized acetic acid
- Ethanoate ions
- Protons (^) Determines pH
Also present, but
not important:
Buffer System:
HC 2 H 3 O 2 H +^ + C 2 H 3 O 2 -
Add an acid
Acetate ion will react
with the H +
C 2 H 3 O 2 -^ + H +^ HC 2 H 3 O
Add a base
Acetic acid will react
with the OH -
HC 2 H 3 O 2 + OH-^ C 2 H 3 O 2 -^ + H 2 O
…but why doesn’t the pH change?
The pH of the solution is determined by the RATIO of acid to
conjugate base.
K a =
H +^ [C 2 H 3 O 2 − ]
[HC 2 H 3 O 2 ]
[H+] = Ka ×
[HC 2 H 3 O 2 ]
[C 2 H 3 O 2 − ]
HC 2 H 3 O 2 ⇌ H +^ + C 2 H 3 O 2 -
Re-arrange for [H +^ ]...
As long as the ratio of
[HC 2 H 3 O 2 ]
[C 2 H 3 O 2 − ]
is fairly constant, the pH won’t change.
The concentrations of [HA] and [A -^ ] change so minimally that the
ratio of [HA]/[A - ] remains fairly constant ∴ pH does not change
Example 1.
What ratio of
[C 2 H 3 O 2 − ]
[HC 2 H 3 O 2 ]
would produce a buffer with a pH of 4.46?
[H+] = 10-4.
[H +] = 3.5 × 10 -5^ mol/L
Ka =
H+^ [C 2 H 3 O 2 − ]
[HC 2 H 3 O 2 ]
1.74 × 10 -5^ = 3.5 × 10 -5^ mol/L
[C 2 H 3 O 2 − ]
[HC 2 H 3 O 2 ]
[C 2 H 3 O 2 − ]
[HC 2 H 3 O 2 ] (rounded to 2 SD)
Find the [H+] of a solution with pH 4.46.
Sub known values into K (^) a expression. Solve for ratio.
= [H+] [C^2 H^3 O^2
− ]
[HC 2 H 3 O 2 ]
The buffering capacity of a solution refers to HOW MUCH acid or base it can handle, without changing its pH.
- A solution with a large buffering capacity can handle a lot of added
hydrogen or hydroxide ions.
- Buffering capacity is determined by the ABSOLUTE MAGNITUDES
of [HA] and [A-^ ].
- Larger concentrations of acid HA and A -^ are able to neutralize a larger amount of added base or acid.
Learning Checkpoint
- A buffer solution is one that maintains a fairly constant pH, even when
extra acid or base is added to it.
- Made of a weak acid and its conjugate base (acidic buffer), or a weak
base and its conjugate acid (basic buffer).
- The entities in solution are able to neutralize any added H +^ or OH-.
- The pH of a buffer solution is determined by two things:
- The Ka of the acid (or K (^) b of the base)
- The RATIO of HA to A-^ (or B or BH+, for a basic buffer)
- The buffering capacity of the solution will be determined by the absolute
magnitudes of these concentrations (as opposed to the ratio).
pH Calculations for a Buffer System
- Buffer systems are equilibrium systems
- Have K values
- Analyze using ICE table
- However ….
K (^) a =
H +^ [C 2 H 3 O 2 − ] [HC 2 H 3 O 2 ]
H+^ and C 2 H 3 O 2 -^ are not equal in these cases
Same as the concentration of the salt
Same as the original concentration of the acid
