3.13: Buffers: Buffer Capacity
Buffer capacity is the quantitative measure of a buffer to resist the change in pH. As shown in the following equation, the buffer capacity, denoted by 'beta', is expressed as the number of moles of acid or base needed to change the pH of a one-liter buffer solution by 1 unit. Here, Ca and Cb indicate the number of moles of acid and base, respectively. Note that dpH represents the change in pH.
In the graph, pH is plotted as a function of the number of moles of base (Cb) added to a weak acid with pKa equal to 5. The curve's derivative yields another plot that depicts the buffer capacity versus pH. This plot demonstrates that the buffer capacity is highest when pH equals pKa, where the solution contains an equal concentration of the weak acid and its conjugate base. For this weak acid, the buffer capacity is highest at pH 5, where the resistance to pH changes is the highest. Typically, the buffer of choice should have a pKa value that is within plus or minus 1 unit of the desired pH. In addition to the pKa value, buffer capacity also depends on the concentration of the weak acid and its conjugate base in the solution. The higher the buffer species' concentration, the higher the buffer capacity.
