The concept of buffer action

In Lowry-Bronsted terms, the common feature of these mixtures is the presence of an acid and its conjugate base. This acid-base pair, together with appropriate counter ions, constitutes a buffer substance. 

A convenient definition (Van Slyke, 1922) of a buffer is a substance which by its presence in solution increases the amount of acid or alkali that must be added to cause unit change in pH. Addition of 1 ml of iMHCl to a litre of distilled water, pH 7, lowers the pH to 3. Alternatively, addition of 1 ml of 1M NaOH raises the pH to 11. Much less change (only about 0.02 pH units) is observed if the same amount of acid or alkali is added to a htre of solution, also pH 7, that is 0.05m in imidazolinium hydrochloride and 0.047m in imidazole. Thus imidazole/imidazolinium ion is a good buffer at pH 7. 

Another useful property of a buffer, at least within the range pH 4 to 10, is that its pH remains substantially unchanged upon dilution of the solution. The effectiveness of a buffer depends on its buffer capacity (resistance to pH change on addition of acid or alkali), the pH change on dilution, and the effects of adding neutral salts or changing the temperature. These are discussed in Chapter 2. 

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