A buffer solution resists large changes in pH when small amounts of acid or alkali are added. It contains substantial amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid.
This is Additional Higher Level (AHL) content in Reactivity 3.1, proton transfer reactions.
Consider a buffer containing ethanoic acid, , and ethanoate ions, . The equilibrium is:
The buffer works because both components are present at concentrations much greater than the amount added. Added hydronium ions react with the conjugate base, while added hydroxide ions react with the weak acid. These reactions convert one buffer component into the other, so the ratio changes only slightly.
Both members of the conjugate acid-base pair remove added ions through proton transfer:
| Substance added | Buffer response | Effect on pH |
|---|---|---|
| Small amount of acid | Most added is consumed, so pH decreases only slightly. | |
| Small amount of alkali |
A buffer can be prepared by mixing a weak acid with a soluble salt of its conjugate base, or by partially neutralizing the weak acid using strong alkali. Buffering requires amounts of both components; dilution preserves their ratio but reduces buffer capacity.
For an acidic buffer, the pH depends on the acid's and the concentration ratio:
Buffering is most effective when the two concentrations are similar. A buffer has a limited buffer capacity: once one component is largely consumed, it can no longer resist pH change effectively.
A common misconception is that a buffer keeps pH completely constant. It does not; it makes the pH change smaller than it would be in an unbuffered solution.
In an IB exam, an explain question requires the proton-transfer equations and identification of which buffer component removes added acid or alkali. Do not answer only by referring generally to Le Châtelier's principle.
| Most added is consumed, so pH increases only slightly. |
