8.10 Buffer Capacity

Buffer Capacity

Adjust concentration and HA/A⁻ ratio to shift the β vs. pH capacity curve. A stacked chart compares buffered vs. pure water pH response to added acid or base.

Henderson-Hasselbalchβ = f(C, Ka)pKa ± 1 Range5 Acids
Topic 8.10

Buffer Capacity

Explain the relationship between the buffer capacity of a solution and the relative concentrations of the conjugate acid and conjugate base components of the solution.

EK 8.10.A.1 states the central result precisely: increasing the concentration of the buffer components, while keeping the ratio of these concentrations constant, keeps the pH of the buffer the same but increases the capacity of the buffer to neutralize added acid or base.

This separates two independent properties:

  • pH is set by the ratio [A⁻]/[HA] (via Henderson–Hasselbalch) and by the pKa.
  • Capacity is set by the absolute amounts of the two components.

A buffer of 0.10 M HA / 0.10 M A⁻ and one of 1.0 M HA / 1.0 M A⁻ have identical pH, because the ratio is 1 in both. But the second can absorb ten times as much added acid or base before it is exhausted.

Capacity is greatest when the two components are present in high concentration and in roughly equal amounts — that is, when pH ≈ pKa. Skewing the ratio far in one direction leaves little of the minority component, so the buffer is quickly overwhelmed from that side.

When a buffer fails. Add enough strong acid to consume all the A⁻, or enough strong base to consume all the HA, and the buffer is destroyed. Beyond that point the pH changes as sharply as it would in unbuffered water.

Designing a buffer therefore involves two independent decisions:

  1. Choose the conjugate pair so pKa is close to the target pH → determines the achievable pH range.
  2. Choose the concentrations → determines the capacity.

Dilution. Diluting a buffer does not change its pH (the ratio is preserved), but it does reduce its capacity proportionally — a distinction the AP Exam has tested directly.

Key points

  • Ratio sets pH; absolute concentration sets capacity.
  • Diluting a buffer leaves pH unchanged but reduces capacity.
  • Capacity is maximized when pH ≈ pKa and both components are concentrated.
  • A buffer fails once one component is fully consumed.

Common mistakes

  • Thinking a more concentrated buffer has a different pH. Same ratio, same pH.
  • Thinking dilution changes buffer pH. It does not, but it does lower capacity.
  • Assuming a buffer works at any pH. It is effective only within about ±1 unit of pKa.
  • Ignoring that capacity is directional. A buffer heavily skewed toward HA can absorb a lot of base but little acid.

Worked example

Buffer A is 0.10 M CH₃COOH / 0.10 M CH₃COONa. Buffer B is 1.00 M CH₃COOH / 1.00 M CH₃COONa. pKa = 4.74. (a) Compare their pH values. (b) Compare their capacities. (c) Buffer A is diluted with an equal volume of water. What happens to its pH and its capacity?

(a) pH. Both have a 1:1 ratio of conjugate base to acid, so
pH = 4.74 + log(1) = 4.74 for both. Concentration does not affect buffer pH.

(b) Capacity. Buffer B has ten times the capacity of buffer A. Per litre, buffer B contains 1.00 mol of acetate available to neutralize added acid and 1.00 mol of acetic acid available to neutralize added base, while buffer A contains only 0.10 mol of each. Buffer B can therefore absorb ten times as much added strong acid or strong base before either component is exhausted and the buffer fails.

(c) Diluting buffer A by half.
pH: unchanged at 4.74. Both concentrations drop to 0.050 M, so the ratio remains 1:1 and the log term remains 0.
Capacity: halved. There are now only 0.050 mol of each component per litre, so the diluted buffer can neutralize only half as much added acid or base.

This is the cleanest demonstration that pH and capacity are independent properties of a buffer.

Full notes for topic 8.10 →