Buffers are solutions which resist changes in pH on addition of small amount of acid or alkali. Buffers usually consist of a weak acid and its conjugate base (acid buffer) or a weak base and its conjugate acid (basic buffer).
How does an acid buffer work?
As mentioned above, an acid buffer consists of a weak acid and its conjugate base. An example is ethanoic acid and its conjugate base ethanoate ion. The conjugate base is usually supplied as a salt. The buffer is most effective at resisting pH changes when the concentration of the undissociated acid molecule (ethanoic acid) is similar to the concentration of the conjugate base (ethanoate ion).
When a small amount of acid is added, the protons combine with the conjugate base (ethanoate ions) to form the weak acid (ethanoic acid). The H+ that is added is removed, hence the concentration of H+ in solution and thus the pH changes only slightly.
When a small amount of base is added, the base reacts with the weak acid to form salt and water. The hydroxide ions are removed from the solution in the form of water and the pH of the solution only changes slightly.
Calculating the pH of buffer solutionThe pH of a buffer solution can be calculated using the Handerson-Hasselbach equation.
Students should note that this equation can only be used for calculating the pH of buffer solutions. Also the equation above can only be used to calculate the pH of acid buffers. For base buffers, use the equation below
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