Buffer solutionsEdexcel International A Level Chemistry: Revision notes

A buffer solution is one that resists changes in pH when small amounts of acid or alkali are added. An acidic buffer contains a weak acid (HA) and its conjugate base (A⁻), usually as a salt, for example ethanoic acid with sodium ethanoate.

The weak acid is only slightly dissociated, but the salt supplies plenty of A⁻, so both are present in large amounts:

• Added acid: H⁺ + A⁻ → HA. The conjugate base removes the H⁺.
• Added alkali: OH⁻ + HA → A⁻ + H₂O. The weak acid removes the OH⁻.

The ratio [HA]/[A⁻] changes only slightly, so [H⁺], and hence pH, stays almost constant.

From Ka = [H⁺][A⁻]/[HA], rearrange for [H⁺]:

[H⁺] = Ka × [HA]/[A⁻]

Then pH = −log₁₀[H⁺]. Because the volume is the same for both species, you can use amounts of HA and A⁻ in place of concentrations.

Worked example: ethanoic acid 0.200 mol dm⁻³, sodium ethanoate 0.100 mol dm⁻³, Ka = 1.74 × 10⁻⁵ mol dm⁻³. [H⁺] = 1.74 × 10⁻⁵ × 0.200/0.100 = 3.48 × 10⁻⁵ mol dm⁻³ so pH = 4.46.

If [HA] = [A⁻], then [H⁺] = Ka and pH = pKa.

To make a buffer of a required pH, find [H⁺] = 10⁻ᵖᴴ, then use the rearranged expression to get the ratio [A⁻]/[HA] = Ka/[H⁺].

Worked example: pH 5.00 with ethanoic acid at 0.500 mol dm⁻³ in 500 cm³. [H⁺] = 1.00 × 10⁻⁵, so [A⁻]/[HA] = 1.74 × 10⁻⁵ ÷ 1.00 × 10⁻⁵ = 1.74. [A⁻] = 0.870 mol dm⁻³, which is 0.435 mol in 500 cm³, so the mass of CH₃COONa (M = 82.0) is 35.7 g.

Adding a small amount of acid changes the amounts, not the pH much: adding 5.0 × 10⁻³ mol H⁺ gives HA 0.255 mol and A⁻ 0.430 mol, pH 4.99, a change of only 0.01.

When a weak acid is titrated with a strong base (or a weak base with a strong acid), the region before equivalence is a buffer: the curve rises slowly and flattens.

At the half-neutralisation point (half the volume needed for equivalence), half of the acid has been converted to A⁻, so [HA] = [A⁻] and pH = pKa. Hence Ka can be found from the pH at this volume: if the pH is 4.20, Ka = 10⁻⁴·²⁰ = 6.31 × 10⁻⁵ mol dm⁻³.

The buffer is most effective when [HA] and [A⁻] are similar, around pH = pKa.

Enzymes work only in a narrow pH range, so cells and body fluids must keep a nearly constant pH. In blood (pH about 7.4) the buffer is H₂CO₃/HCO₃⁻:

• Added H⁺ is removed: HCO₃⁻ + H⁺ → H₂CO₃
• Added OH⁻ is removed: H₂CO₃ + OH⁻ → HCO₃⁻ + H₂O

In foods, buffers prevent deterioration: as bacteria and fungi grow they produce acids or alkalis, and a buffer resists the resulting pH change, keeping the food stable.