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A buffer solution is a solution the pH of which does not change significantly when a small amount of acid or base is added to it. There are four categories of buffers.
Strong acid buffers
A strong acid such as nitric acid can act as a buffer with a low pH. Strong acids are fully dissociated in aqueous solution and thus the concentration of oxonium ions is high.
The addition of a small amount of acid or base to the acid will thus havea negligible effect on the pH of the acid.
To calculate the pH use: pH = -log[H+]
Strong base buffers
A strong base can be used as a buffer with high pH. The addition of small amount of acid or base has negligible effect on the pH. This can be checked using the equation: pH + pOH = 14
Weak acid buffers
Buffer solutions with constant pH values of between 4 and 7 can be prepared from a weak acid and one of its salts. Ethanoic acid and sodium ethanoate are often used for this purpose. Sodium ethanoate in water is fully ionised:
On the other hand, ethanoic acid is only partially ionised:
If acid is added this equilibrium shifts to the left. The additional H3O+ ions are thus removed and the pH remains constant.
The presence of the sodium ethanoate in the buffer solution ensures that there is a large reservoir of CH3COO- ions to cope with this addition of acid.
If the base (OH-) is added, then the following happens:
Removal of H3O+ ions by this reaction results in the equilibrium shifting to the right. The concentration of H3O+ ions and thus the pH of the solution remains constant.
The presence of the ethanoic acid ensures that there is a large reservoir of undissociated CH3COOH molecules ready to dissociate in order to cope with the addition of base.
Weak base buffers
Buffer solutions with constant pH values between 7 and 10 can be prepared from a weak base and one of its salts. A solution of ammonia and ammonium chloride is typically used.
In aqueous solution the ammonium chloride is fully dissociated:
The ammonia is only partially dissociated:
If acid is added it is neutralised by the OH- ions. Equilibrium shifts to the right, thus maintaining the concentration of OH- ions and thus a constant pH.
If base is added the equilibrium shifts to the left, thus maintaining the concentration of the OH- ions constant.
The presence of ammonium chloride in the buffer solution ensures that there is a large reservoir of NH4+ ions to cope with the addition of base.