Buffer Solutions

• Buffer solutions are a special type of solution that are resistant to changes in pH upon addition of small amounts of acid or base. This useful property comes from their composition: a solution of a weak acid and its conjugate base or a weak base with its conjugate acid.
• The principle behind buffer action is the equilibrium that is established between the weak acid/base and its conjugate pair in water. When small amounts of acid or base are added, the equilibrium shifts to minimise the change - this is the application of Le Chatelier’s principle.
• The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] and [HA] are the concentrations of the conjugate base and the weak acid, respectively. Similarly for a basic buffer, pOH = pKb + log([B+]/[BOH]).
• Buffers can be categorised into two types: acidic buffers, made from a weak acid and its salt (conjugate base), and basic or alkaline buffers, made from a weak base and its salt (conjugate acid).
• An example of an acidic buffer is a mixture of ethanoic acid and sodium ethanoate. For a basic buffer, an example could be a mixture of ammonia and ammonium chloride.
• In laboratory environments, buffer solutions are used in many biochemical experiments where pH needs to be carefully controlled. Besides this, our blood also acts as a buffer to maintain our body’s pH around 7.4.
• Factors that influence the buffer capacity (how well the solution can resist changes in pH) include the concentration of the buffer components and the ratio between them. The maximum buffering capacity is when the concentrations of the acid and base are equal, i.e., pH = pKa for an acidic buffer and pOH = pKb for a basic buffer.
• Note that a buffer cannot maintain its pH indefinitely. The buffer capacity is exhausted when all of the weak acid or base has been converted to its conjugate partner.
• When a strong acid or base is added to a buffer, a neutralisation reaction occurs. For an acidic buffer, the A- ions would react with the H+ ions from the added acid to become HA and water, thereby preventing a significant pH decrease. The opposite reaction happens when base is added.
• Preparation of buffer solutions requires careful selection and measurement of the components. It often involves mixing a solution of the weak acid or base with a salt of the conjugate species, or by adding half-neutralising a solution of the weak acid or base with a strong base or acid, respectively.