Buffer capacity

The buffer capacity of a solution, β, is a measure of the solution’s resistance to changes in pH and is defined as the number of moles of a strong acid or a strong base needed to change one unit of pH of 1.00 L of the solution. It is mathematically expressed as:

\beta=\frac{dc_b}{dpH}\; \; \; \; \; \; \; \; (1)


\beta=-\frac{dc_a}{dpH}\; \; \; \; \; \; \; \; (2)

where cb is the number of moles per litre of a strong base and ca is the number of moles per litre of a strong acid.

Since buffer capacity is defined as a positive value, eq2 has a negative sign, as the change in pH is negative when a strong acid is added to the buffer solution.



Is a strong base versus strong acid system a buffer?


According to the above definition, a strong base versus strong acid system can also be considered a buffer, but with negligible buffer capacity. This is because a strong acid is fully dissociated in water and any addition of a strong base reduces the concentration of H+; whereas an aqueous weak acid is partially dissociated and any H+ that is removed by the base is in part replenished by further dissociation of the weak acid, thereby minimising the increase in the solution’s pH. When an acid is added to the strong acid solution, any molecular acid formed by the combination of H+ from the acid added and the conjugate base of the strong acid, immediately dissociates into the component ions. However, H+ of an acid added to a weak acid solution, combines with the conjugate base of the weak acid, and partially shifts the equilibrium of the dissociation of the weak acid to the left, minimising the decrease in the solution’s pH.


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