Buffer Solutions

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:

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On the other hand, ethanoic acid is only partially ionised:

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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:

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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:

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The ammonia is only partially dissociated:

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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.

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Therefore:

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