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Buffer capacity

Two buffers can sit at the exact same pH and behave completely differently under stress. What separates them is capacity — how much acid or base they can swallow before the pH finally gives way, and that grows with concentration.

§1

How much a buffer can take.

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Buffer capacity is how much added acid or base a buffer can absorb before its pH changes significantly. It is a measure of the size of the reservoir, not of the pH.

A more concentrated buffer has more of both components, so it can neutralize more added acid or base — it holds its pH longer. Two buffers at the same pH can have very different capacities if their concentrations differ.

Capacity is finite: once one component is largely used up, the buffer is overwhelmed and the pH changes sharply. Capacity depends on the amount of buffer, not on the pH it happens to sit at.

UNIT 8 TOPIC 8.10 • BUFFER CAPACITY BUFFER CAPACITY Buffer capacity = how much strong acid or base a buffer absorbs before pH changes sharply. MORE CONCENTRATION → HOLDS pH LONGER both start at pH = pKa strong acid / base added → pH change LOW conc. small capacity HIGH conc. large capacity SAME pKa — DIFFERENT CAPACITY LOW TOTAL 0.10 M HA 0.10 M A⁻ HIGH TOTAL 1.0 M HA 1.0 M A⁻ Equal [HA] = [A⁻] ratio → both start at the same pH (pH = pKa), but capacity differs. WHY THE CONCENTRATED ONE WINS Higher total concentration provides more HA and A⁻ particles to neutralize the added strong acid or base. So it holds its pH far longer before the sharp rise — greater buffer capacity. CED ANCHOR The ratio [A⁻]/[HA] sets the pH; the absolute amounts of the conjugate pair set the buffer capacity. AP Chemistry · Unit 8 · Acids and Bases
Fig. 8.10.1 Buffer capacity is how much acid or base a buffer can absorb before its pH changes significantly. A more concentrated buffer has a larger reservoir, so it holds its pH longer. Capacity is finite: enough added acid or base eventually overwhelms it.
§2

Judging buffer capacity.

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Look at the amount of buffer, not the pH.

  1. Recognize capacity is about amount. It measures how much acid/base can be absorbed, set by the reservoir size.
  2. Tie capacity to concentration. A more concentrated buffer has a larger capacity.
  3. Compare at equal pH. Two buffers at the same pH can differ in capacity if concentrations differ.
  4. Remember it is finite. Enough added acid or base exhausts a component and overwhelms the buffer.
§3

The pieces you'll meet.

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Capacity = reservoir size.

capacity
Buffer capacity
How much acid/base absorbed before pH shifts.
concentration
Concentration
Larger concentration → larger capacity.
finite
Finite
Enough addition overwhelms the buffer.
not pH
Not the pH
Capacity is about amount, not the pH value.
reservoir size
Reservoir size
The stock of both components sets capacity.
same pH
Same pH, diff capacity
Two buffers at one pH can differ in capacity.
§4

Worked example: two buffers, same pH.

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Setup. Buffer A is 1.0 M in each component; buffer B is 0.010 M in each. Both have equal components, so both sit at pH = pKa.

Same pH. Their pH values are identical, because pH depends on the ratio, not the concentration.

Different capacity. Buffer A (concentrated) can absorb far more added acid or base before its pH breaks — it has a much larger capacity.

Lesson. Capacity is set by the amount of buffer, not by its pH; concentrated buffers hold their pH longer.

§5

Mistakes that cost real points.

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Pitfall · 01

"Buffer capacity is unlimited."

Capacity is finite. A buffer resists pH change only until one of its components is largely consumed; enough added acid or base overwhelms it and the pH changes sharply. It cannot absorb an unlimited amount.

Fix. Treat buffer capacity as finite: enough acid or base will exhaust a component and break the buffer.

Pitfall · 02

"Buffer capacity is determined by the buffer's pH."

Capacity is set by the amount (concentration) of buffer, not by its pH. Two buffers at the same pH can have very different capacities if their concentrations differ. The pH tells you the ratio; capacity tells you the reservoir size.

Fix. Judge capacity by concentration (reservoir size), not by the pH value.

Pitfall · 03

"A more dilute buffer resists pH change just as well."

A more dilute buffer has a smaller reservoir, so it is overwhelmed by less added acid or base — it has a lower capacity. Concentration matters for how much a buffer can take, even if the pH is the same.

Fix. Expect a more concentrated buffer to have a higher capacity and hold its pH longer than a dilute one.

§6

Skill Check.

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