Buffers:
-A buffer solution is that solution that allows solutions to resist
large changes in pH upon the addition of limited amounts of acid
Or base .
Chemically buffer solutions are made up of a mixture of two
substances , a conjugate base and a conjugate acid .
Buffers can be divided according to their chemical nature into two
Types : 1- Acidic buffers which contains a weak acid and its salt of
A strong base .
2- Basic buffers which contains a weak base and its salt of a
strong acid .
How do buffers resist changes in pH ?
Together the two species ,conjugate base and conjugate acid ( the
buffer components) will resist large changes in pH by partially
Absorbing the added H+ ions and OH- ions as seen in the following
Buffer example HA /AWhen an acid is added the added H+ will react with the conjugate
Base , H+ + A- → HA ,
When a base is added the OH- ions will react with the conjugate
Acid , OH- + HA → H2O + A- (producing water and the
Conjugate base )
So in both cases as seen the limited amount of H ions or OH- ions
Added (which could change significantly the pH of the solution)
Reacted with the two basic components of the buffer (the
Conjugate base and conjugate acid) producing products that do
Not have a significant effect on the pH .
Note :1- Buffered solutions do show a change in pH upon the
Additon Of acid or base, but the change is insignificant compared
to the Change if no buffer was present.
2- The amount of change recorded depends on the strength of the
Buffer and the [A-] / [HA] ratio .
Example: a) Describe the components of an acetate buffer.
b) show the mechanism by which it resists changes in pH.
Solution: The acetate buffer is composed of a mixture of CH3COOH
Representing the conjugate acid, and CH3COO- representing the
Conjugate base, so the acetate buffer is a solution of the following
Composition CH3 COOH/ CH3COO- .
b)When an acid is added it will react with the conjugate base
H+ + CH3COO- → CH3COOH
When a base is added it will react with the conjugate acid
Component of the buffer , OH- + CH3COOH→ H2O + CH3COO
Buffer Capacity :
The quantitative measure of the buffers resistance to changes in
PH upon the addition of acid or base. Buffer capacity can be
Defined as the number of moles of H+ or OH ions required
To cause a one unit change in pH in 1liter of buffer.
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Preparation of buffers
Example ; Describe the preparation of 2 liters of a 0.25M formate
Buffer , pH 4.5 , starting from 1M formic acid and solid sodium
Formate HCOONa . Pka: of HCOOH is 3.75.
Solution; First calculate the proportions of the two formate species
Present,
From Henderson – Hasselbalch equation ,
PH = pKa + log [A- ]
[HA]
The 0.25M formate buffer represents the sum concentration of the
Two components of the buffer ,
( 0.25M of formate buffer = [HCOOH] + [HCOONa])
Let y = [A-] , so [HA] = 0.25 – y , (A- is the conjugate base
HCOONa , while HA represents the conjugate acid HCOOH )
Thus pH = 3.75 + log
y
0.25 – y