8.8: Properties of Buffers

What is a Buffer?

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

How Buffers Work

Buffers work by neutralizing added acid or base through the following equilibrium: $$ HA(aq) \rightleftharpoons H^+(aq) + A^-(aq) $$ * If $ H^+ $ is added, the equilibrium shifts to the left, consuming the added $ H^+ $ and minimizing the pH change.

• If $ OH^- $ is added, it reacts with $ H^+ $ in the solution to form water, which causes the equilibrium to shift to the right, producing more $ H^+ $ and minimizing the pH change.

Buffer Solutions

Buffer Capacity

Buffer capacity is the amount of acid or base a buffer can neutralize before the pH begins to change appreciably. Buffer capacity depends on the concentration of the weak acid and its conjugate base. The higher the concentrations of the buffer components, the greater the buffer capacity.

Buffer Range

The buffer range is the pH range over which the buffer can effectively resist changes in pH. The effective buffer range is generally considered to be $ pH = pK_a \pm 1 $ , where $ pK_a = -log(K_a) $ .

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution:

$$ pH = pK_a + log\frac{[A^-]}{[HA]} $$
where:

• $ pH $ is the pH of the buffer solution.
• $ pK_a $ is the negative logarithm of the acid dissociation constant ( $ K_a $ ) of the weak acid.
• $ [A^-] $ is the concentration of the conjugate base.
• $ [HA] $ is the concentration of the weak acid.

A similar equation can be used for basic buffers:

$$ pOH = pK_b + log\frac{[HB^+]}{[B]} $$
where:

• $ pOH $ is the pOH of the buffer solution.
• $ pK_b $ is the negative logarithm of the base dissociation constant ( $ K_b $ ) of the weak base.
• $ [HB^+] $ is the concentration of the conjugate acid.
• $ [B] $ is the concentration of the weak base.

Remember that $ pH + pOH = 14 $ .

Preparing Buffers

Buffers are usually prepared by:

1. Mixing a weak acid with its conjugate base. This can be achieved by mixing the weak acid and a salt of its conjugate base (e.g., acetic acid and sodium acetate).

2. Mixing a weak base with its conjugate acid. This can be achieved by mixing the weak base and a salt of its conjugate acid (e.g., ammonia and ammonium chloride).

3. Partially neutralizing a weak acid with a strong base, or a weak base with a strong acid. The goal is to have significant amounts of both the weak acid/base and its conjugate present in the solution.