What is a buffer?
A buffer is a solution that resists a change in pH when small amounts of acid or base are added. It is composed of either a weak acid or a weak base with its respective salt.
What's going on in the buffer?
For clarity, let's use the example ethanoic acid and sodium ethanoate.
CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)
CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)
The acid and its salt partially and reversibly dissociate, creating the above equilibriums.
What is the concentration of the buffer?
[H+] = (Ka[CH3COOH]) / [CH3COO-Na+]
pH = - log([H+])
Note:
[acid] = mol. acid / V
[salt] = mol.salt / V
V = the total volume and so:
[acid] / [salt] = mol. acid / mol. salt
Therefore, [H+] = (Ka(mol. CH3COOH)) / (mol. CH3COO-Na+)
Adding acid
The H+ ions react with the CH3COO- ions.
CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)
Equilibrium pushed ⟵, increasing [CH3COOH].
CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)
Equilibrium pushed ⟶, decreasing [CH3COO-Na+]
As all stoichiometric ratios are 1:1 and total volume is constant:
[H+] = (Ka(mol. CH3COOH + x)) / (mol. CH3COO-Na+ - x)
where x is the number of moles of acid added. As this is a small number, there is only a negligible change in pH = - log ([H+])
Adding a base
The alkali neutralises the acid.
CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)
Equilibrium pushed ⟶, decreasing [CH3COOH].
CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)
Equilibrium pushed ⟵, increasing [CH3COO-Na+]
As all stoichiometric ratios are 1:1 and total volume is constant:
[H+] = (Ka(mol. CH3COOH - x)) / (mol. CH3COO-Na+ + x)
where x is the number of moles of acid added. As this is a small number, there is only a negligible change in pH = - log ([H+])
Note:
Ka = acid dissociation constant
[X] = concentration of X
mol. x = moles of x
and log = logarithm base 10