3.13:
Buffers: Buffer Capacity
Buffer capacity, represented as beta, is the quantitative measure of a buffer's resistance to pH change upon adding an acid or base. It is expressed as the number of moles of acid (Ca) or base (Cb) added to a one-liter buffer solution before a unit pH alteration.
The higher the buffer capacity, the higher the buffer's ability to resist the pH change.
To an acid with pKa equal to 5, the number of moles of base added is plotted versus pH. The derivative of the curve obtained shows the highest buffer capacity when pH equals pKa, where the concentration of weak acid and its conjugate base are equal in the buffer solution. So, the effective pH range of a buffer is one unit more or less than the pKa.
Outside the 3-10 pH range, the existing high concentration of H+ or OH− ions resists the pH change to the acid or base added.
The concentration of the buffer species proportionally dictates buffer capacity.
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.