16.2: Buffers

A solution containing appreciable amounts of a weak conjugate acid-base pair is called a buffer solution, or a buffer. Buffer solutions resist a change in pH when small amounts of a strong acid or a strong base are added. A solution of acetic acid and sodium acetate is an example of a buffer that consists of a weak acid and its salt: CH₃COOH (aq) + CH₃COONa (aq). An example of a buffer that consists of a weak base and its salt is a solution of ammonia and ammonium chloride: NH₃ (aq) + NH₄Cl (aq).

How Buffers Work

To illustrate the function of a buffer solution, consider a mixture of roughly equal amounts of acetic acid and sodium acetate. The presence of a weak conjugate acid-base pair in the solution imparts the ability to neutralize modest amounts of added strong acid or base. For example, adding a strong base to this solution will neutralize hydronium ion and shift the acetic acid ionization equilibrium to the right, partially restoring the decreased H₃O⁺ concentration:

Likewise, adding a strong acid to this buffer solution will neutralize acetate ion, shifting the above ionization equilibrium right and returning [H₃O⁺] to near its original value. Figure 1 provides a graphical illustration of the changes to the buffer solution when strong acid and base are added. The buffering action of the solution is essentially a result of the added strong acid and base being converted to the weak acid and base that make up the buffer's conjugate pair. The weaker acid and base undergo only slight ionization, as compared to the complete ionization of the strong acid and base. The solution pH, therefore, changes much less drastically than it would in an unbuffered solution.

Figure 1. Buffering action in a mixture of acetic acid and acetate salt.

This text is adapted from Openstax, Chemistry 2e, Section 14.6: Buffers.

Adding a small amount of acid or base to a solution can cause a significant decrease or increase in the pH. However, many chemical and biochemical processes need a stable pH to function. Buffers can prevent a drastic change in the pH of a solution when their buffering capacity is not exceeded.

Buffers contain a weak acid and its conjugate base or a weak base and its conjugate acid. For example, human blood maintains its pH near 7.4 with a buffer composed of carbonic acid, a weak acid, and bicarbonate ions, its conjugate base.

Conjugate acid-base pairs form buffers as they do not neutralize their conjugate acid or base. For example, acetic acid and acetate cannot react. However, if acetic acid, a weak acid, and ammonia, a weak base, are added together, they will react to form a salt—ammonium acetate.

In a buffer, the weak acid neutralizes any added base by reacting with the hydroxide ions produced, whereas its conjugate base neutralizes any added acid by reacting with any hydronium ions. A similar mechanism works in the case of a weak base and its conjugate acid.

Two beakers, X and Y, contain the same volume of different solutions, each with a pH of 7.2. The solution in beaker X is not buffered. In contrast, the solution in beaker Y contains an acetic acid-acetate buffer.

If hydrochloric acid is added to beaker X, the pH of the solution suddenly drops due to the increased concentration of hydronium ions.

In contrast, the solution in beaker Y shows a nearly constant pH when hydrochloric acid is added to it because one of the buffer components, acetate, reacts with hydrochloric acid to produce a chloride ion and acetic acid.

Similarly, if sodium hydroxide is added to beaker X, the pH of the solution will suddenly increase due to the increased concentration of hydroxide ions.

On the other hand, the solution in beaker Y shows a minimal change in pH when sodium hydroxide is added to it, as one of the buffer components, acetic acid, reacts with sodium hydroxide and produces sodium acetate and a water molecule.  

The buffer can prevent a drastic change in the pH of a solution as long as the concentration of the conjugate acid-base pair in a solution is higher than the added strong acid or base.