9780521516358book.pdf

1.3.3 Buffer solutions

A buffer solution is one that resists a change in pH on the addition of either acid or

base. They are of enormous importance in practical biochemical work as so many

biochemical molecules are weak electrolytes so that their ionic status varies with pH

so there is a need to stabilise this ionic status during the course of a practical experi-

ment. In practice, a buffer solution consists of an aqueous mixture of a weak acid and

its conjugate base. The conjugate base component would neutralise any hydrogen

ions generated during an experiment whilst the unionised acid would neutralise any

base generated. TheHenderson–Hasselbalchequation is of central importance in the

preparation of buffer solutions. It can be expressed in a variety of forms. For a buffer

based on a weak acid:

pH¼pKaþlog½conjugate baseŠ

½weak acidŠ

ð 1 : 7 Þ

or

pH¼pKaþlog

½ionised formŠ

½unionised formŠ

For a buffer based on the conjugate acid of a weak base:

pH¼pKaþlog

½weak baseŠ

½conjugate acidŠ

ð 1 : 8 Þ

or

pH¼pKaþlog½unionised formŠ

½ionised formŠ

Table 1.6 lists some weak acids and bases commonly used in the preparation of buffer

solutions. Phosphate, Hepes and Pipes are commonly used because of their optimum

pH being close to 7.4. The buffer action and pH of blood is illustrated in Example 2

and the preparation of a phosphate buffer is given in Example 3.

Buffer capacity

It can be seen from the Henderson–Hasselbalch equations that when the concentration

(or more strictly the activity) of the weak acid and base is equal, their ratio is one and

their logarithm zero so that pH¼pKa. The ability of a buffer solution to resist a change

in pH on the addition of strong acid or alkali is expressed by itsbuffer capacity(b).

This is defined as the amount (moles) of acid or base required to change the pH by one

unit i.e.

b¼ db

dpH

¼da

dpH

ð 1 : 9 Þ

where dband daare the amount of base and acid respectively anddpH is the resulting

change in pH. In practice,bis largest within the pH range pKa1.

11 1.3 Weak electrolytes