The buffer capacity β of a solution is a measure of the solution’s resistance to changes in pH. It 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. Mathematically,
or
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 because the change in pH is negative when a strong acid is added to the buffer solution.
Question
Is a strong base versus strong acid system a buffer?
Answer
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+ removed by the base is partially 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 added acid and the conjugate base of the strong acid immediately dissociates into the component ions. However, when H+ from an acid is added to a weak acid solution, it combines with the conjugate base of the weak acid, partially shifts the equilibrium of the weak acid’s dissociation to the left, thus minimising the decrease in the solution’s pH.