Chapter 14 · Acids and Bases
14.10Buffers: Solutions That Resist pH Change
11 min · two checks
Predict
What two things must a buffer contain?
The idea
- Explain how a buffer resists pH change.
- Recognize a buffer pair.
A buffer is a weak acid plus its conjugate base, for example CH₃COOH and CH₃COO⁻, or a weak base plus its conjugate acid. Dump a little strong acid in and the conjugate base takes the proton, becoming the weak acid. Dump a little strong base in and the weak acid donates a proton, becoming the conjugate base. In both cases the pH hardly moves, until you add so much that one member of the pair is used up. That limit is the buffer capacity.
Water and solutions of only a strong acid are not buffers. They have nothing present in both forms to absorb both directions of insult. Blood is buffered, notably by carbonic acid and hydrogen carbonate, because enzyme chemistry cannot tolerate a wandering pH. The Henderson–Hasselbalch equation, pH = pK_a + log([base]/[acid]), tells you the pH sits near the acid’s pK_a when the two concentrations are similar. You can use the idea even before you use the equation: equal amounts of the pair, pH near pK_a.
Keep these
- A buffer is a weak acid and its conjugate base, both plentiful.
- Added strong acid or base is converted into the other member of the pair.
- Capacity runs out when one member is consumed.