Effect of dilution on buffering capacity

Buffer solutions have the capacity to resist change of pH on dilution. Addition of H20(l) will dilute both the weak acid and its salt by the same amount and so the pH remains the same, e.g. in the case of the NH3/NH4CI buffer  

Addition of a strong acid or a strong base to a buffer solution has very little effect on the pH, this being one of the main properties of a buffer. This can be contrasted with the effect of addition of either to water where there is a dramatic change in pH. 

However, there is no such dramatic change when HCl(aq) is added to a buffer, as shown in the following analysis. Consider, for example, the weak acid/weak acid salt with a strong base buffer, i.e. HA/NaA. 

If HCl(aq) is added to this buffer solution it will neutralise an equivalent number of mol of the base, A (aq), present in the buffer solution and form an equivalent number of mol of HA(aq). 

As a result, there will be changes in the position of equilibrium in each of the equilibria (A), 

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