Why is the #pH# of a buffer close to the #pK_a# of the acid used to make the buffer...?

1 Answer
anor277
Aug 23, 2016

The #pH# of a buffer is tolerably close to the #pK_a# of the acid because this gives the greatest buffer capacity.

Explanation:

Buffers are formed from a mixture of appreciable quantities of a weak acid and its conjugate base. The #pH# of the solution is tolerably close to the #pK_a# of the acid, which of course will be somewhat below #7#.

The buffer equation gives us:

#pH=pK_a + log_10([[A^-]]/[[HA]])#

Of course when #[A^-]=[HA]# (at the point of half equivalence in a titration), #pH=pK_a#, because #log_(10)1=0#.

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