Chemical Equilibrium, Law Of Mass-action : Buffer Solutions

A solution can be prepared which contains a definite concentration of hydrogen ions which is not altered by dilution. This is called a buffer solution. An example is a solution containing sodium acetate and acetic acid (a weak acid). For the acid we have

[H^۰] x [Ac']/[HAc] = const. = K_a.

Now add Ac' ions in the form of sodium acetate, NaAc. This salt is practically completely ionised and the ionisation of the weak acetic acid is almost entirely repressed by the addition of the acetate, since the equilibrium HAc <=> H^۰ + Ac' is shifted to the left almost completely by the mass-action of the Ac' of the sodium acetate. Hence we can put:

[HAc] = original total acid concentration = [acid], [Ac'] = NaAc total concentration = [salt];

[H^۰] = K_a[HAc]/[Ac'] = K_a[acid/[salt],

which is independent of dilution, since [acid]/[salt] remains practically unchanged. In a solution containing equivalent amounts of acid and salt, [H^۰] = K_a.

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Chemical Equilibrium, Law Of Mass-action
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