Why do solutions that contain buffers tend to resist pH changes?

1 Answer
anor277
Sep 2, 2016

Because #[H_3O^+]# or #[HO^-]# is moderated by the equilibrium expression.

Explanation:

We can represent the acid base equilibrium of a weak acid as:

#HA +H_2O rightleftharpoons H_3O^+ + A^-#

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

Because the ratio #([A^-])/[[HA]# is moderated by the logarithmic term, #pH# remains tolerably close to #pK_a#, the negative logarithm of the acid dissociation constant. Where #[A^-]=[HA]#, #pH=pK_a#, because #log_10(1)=0#.

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