AP Chemistry · Unit 8 · Topic 8.10
Buffer Capacity
“How much acid can this buffer absorb?” needs a second question: how much pH change is acceptable? Capacity becomes meaningful when the chemical challenge and the allowed response are both specified.
Define the limit before calculating
A buffer gradually loses effectiveness as one conjugate component is consumed. There is no universal amount it can tolerate without a specified criterion. We might ask how many moles of strong acid can be added before pH drops by 0.30, or compare how much base produces a one-unit rise. State the buffer volume and initial composition alongside the criterion.
Recognize directional reserves
HA is the reserve against added base; A⁻ is the reserve against added acid. A mixture rich in A⁻ has more stoichiometric room for an acid challenge than for a base challenge. Equal amounts near pH = pKa generally provide balanced two-way performance for a fixed total concentration, rather than maximizing resistance in only one direction.
Set up an original capacity question
A 100.0 mL sample contains 5.00 mmol HA and 5.00 mmol A⁻, with pKa = 5.00. Its initial pH is 5.00. Find the strong-acid amount x that lowers pH to 4.70, neglecting volume and activity changes. Acid converts A⁻ to HA, so the new amounts are 5.00 − x and 5.00 + x mmol.
Solve the stated threshold
Thus \(5.00-x=2.506+0.5012x\), giving x = 1.66 mmol. The mixture reaches the chosen 0.30-unit limit long before all 5.00 mmol A⁻ is consumed. Calling 5.00 mmol its capacity would confuse stoichiometric exhaustion with the specified acceptable pH change.
Compare scale and concentration
| Sample | Total pair amount | Acid to threshold |
|---|---|---|
| 100 mL, 0.100 M total pair | 10.0 mmol | 1.66 mmol |
| 200 mL, 0.100 M total pair | 20.0 mmol | 3.32 mmol |
| 100 mL, 0.0500 M total pair | 5.00 mmol | 0.831 mmol |
Doubling sample volume at fixed composition doubles total capacity, while the capacity per liter remains unchanged.
Read a capacity experiment
Add standardized acid in small known increments and plot pH against added millimoles per liter. A flatter local curve indicates greater capacity in that region. Replicate the curve and record temperature. If you compare equal drops instead of equal moles, different drop volumes or titrant concentrations can create a false capacity ranking.
Understand why the response accelerates
As A⁻ becomes scarce during acid addition, the same further acid amount changes the A⁻/HA ratio by a larger relative factor. The logarithmic pH therefore moves more rapidly. Near exhaustion, the ordinary buffer approximation breaks down and excess strong acid may dominate. A buffer does not fail suddenly at a magical concentration boundary.
Practice with answers
- Which initial mixture better tolerates acid: 8 mmol A⁻/2 mmol HA or 2 mmol A⁻/8 mmol HA? The base-rich first mixture has greater acid-neutralizing reserve, though their starting pH differs.
- If all sample amounts double, what happens to moles required for the same pH change? They double in this model.
- Why state an allowed pH change? Because capacity depends on the chosen performance threshold.
Factual reference: OpenStax Chemistry 2e, 14.6. The threshold definition, numerical experiment, and comparisons are independently written.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.