May 06, 2013
Buffer:
-a solution that can resist drastic changes in pH when
small amounts of a strong acid or base are added
-a mixture of a weak acid and its conjugate base
(in approximately equal amounts)
Example: hydrofluoric acid and fluoride ion
HF(aq) and F-(aq)
-the conjugate base is usually in the form of an ionic
compound (ie. NaF ----> Na+ + F-)
Biological Buffers
Blood
Carbonic acid/hydrogen carbonate
Cells
dihydrogen phosphate/hydrogen phosphate
May 06, 2013
What would you mix with sodium oxalate to produce a buffer
solution?
There are two ways to make a buffer solution
1) mix equal amounts of a weak acid and its conjugate base
2)adding a strong base (OH-) to a weak acid solution.
Example: A solution of sodium hydroxide is added to acetic
acid
Net Ionic Equation
Initial
Change
Final
May 06, 2013
Titration Curve
-a buffer can resist changes in pH because it has an acidic
species to neutralize bases and a basic species to neutralize
acids
May 06, 2013
Buffer region on a titration curve:
reaction taking place: HA + OH- ---> A- + HOH
(during the beginning of the titration you are
actually making a buffer)
A buffer is a system at equilibrium.
EQ Reaction: the same as a weak acid in water
May 06, 2013
Example: A Sodium Acetate/Acetic Acid Buffer
Equilibrium taking place:
CH3COOH(aq) + H2O(aq) ⇌ H3O+(aq) + CH3COO-(aq)
There are two ways to examine a buffer system
1) Writing an equilibrium reaction: the same as a weak acid in
water
2) Predicting how a buffer system responds to addition of
strong acdi or strong base
May 06, 2013
Neutralization
Add strong acid
X- + H3O+
HX + H2O
Buffer containing
HX and X-
Neutralization
HX + OH-
X- + H2O
Add strong base
Consider a buffer that contains a weak acid HX and its conjugate base X— .
When a strong acid is added to this buffer, the H3O+ of the strong acid is
consumed by X— of the buffer to produce HX; thus [HX] increases and X—
decreases.
When a strong base is added to the buffer, the added OH— is consumed by
HX to produce X— ; in this case [HX] decreases and [X—] increases.
Example:
Acetic Acid / Sodium Acetate Buffer
1. Write the equilibrium reaction for the above buffer system
2. Predict the buffer concentration changes when a strong acid is added to the
following buffer system.
CH3COOH (aq) + H2O(l) CH3COO- (aq) + H3O+(aq)
3. Predict the buffer concentration changes when a strong acid is added to the
following buffer system.
CH3COOH (aq) + H2O(l) CH3COO- (aq) + H3O+(aq)
May 06, 2013
Sample Question
Sample question: Coca cola and milk of magnesia, Mg(OH)2, are two substances that
need to be buffered by the cells in your body. This critically important buffer is a mixture
of dihydrogen phosphate ions and hydrogen phosphate ions. The buffer system is as
follows.
H2PO4- (aq) + H2O(l) ⇋ HPO42- (aq) + H3O+(aq)
When cola is added to the above buffer system, the equilibrium will shift to favour __i__ ;
when milk of magnesia is added to the buffer system, the concentration of __ii__ will
increase.
Row
A.
B.
C.
D.
i
reactants
reactants
products
products
ii
H2PO4- (aq)
HPO42- (aq)
H2PO4- (aq)
HPO42- (aq)
May 06, 2013
The ability of a buffer to resist drastic changes in pH can be
explained using Bronstead-Lowry acid base reactions
Example: Sodium Acetate/Acetic Acid Buffer
When a strong acid is added to the buffer:
The Bronstead-Lowry Acid Base Reaction:
(the strong acid gets consumed)
May 06, 2013
Example: Sodium Acetate/Acetic Acid Buffer
When a strong base is added to the buffer:
The Bronstead-Lowry Acid Base Reaction:
(the strong base gets used up)
http://www.mcgrawhill.ca/school/applets/abchem/animations/ch17/acm3s4_15.swf
May 06, 2013
Biological Functions of Buffers
• carbonic acid/hydrogen carbonate buffer in the blood
CO2(g) + H2O(l)
H2CO3(aq)
May 06, 2013
Buffer Capacity
-the maximum amount of either strong acid or strong
base that can be added before a significant change in
the pH will occur
-it is determined by the concentration of the weak acid
and weak base in the buffer