AP Chemistry · Unit 8 · Topic 8.8
Properties of Buffers
Two solutions can start at the same pH and respond very differently to one drop of acid. Their starting hydronium concentrations do not tell you how much chemical reserve each contains.
Two components provide two responses
A weak acid HA consumes added hydroxide, while its conjugate base A⁻ consumes added hydronium. Both components must remain in appreciable amounts for useful two-way buffering. The added acid or base changes their relative amounts, so the pH changes somewhat. Resistance to change is not the same as perfect constancy.
Separate ratio from total concentration
For a suitable buffer, pH depends mainly on the A⁻/HA ratio and pKa. Resistance to a specified added amount also depends on the total moles available. This distinction is why two equal-ratio buffers can share a starting pH yet have different capacities. Their volumes matter whenever the disturbance is specified in moles rather than concentration.
Compare two original buffers
| Sample | Initial HA / mmol | Initial A⁻ / mmol | Initial pH |
|---|---|---|---|
| Concentrated pair | 10.0 | 10.0 | 5.00 |
| Dilute pair | 1.00 | 1.00 | 5.00 |
The second sample is not merely a smaller copy of the first response: the same added moles use a much larger fraction of its reserve.
Work the same acid challenge twice
Add 0.50 mmol strong acid to each sample, with negligible volume change. It converts A⁻ into HA. The concentrated pair becomes 9.50 mmol A⁻ and 10.50 mmol HA, giving
The dilute pair becomes 0.50 mmol A⁻ and 1.50 mmol HA, giving pH = \(5.00+\log(0.50/1.50)=4.523\). The concentrated sample changes by about 0.043 pH unit, compared with 0.477 for the dilute sample.
Understand dilution without overclaiming
Adding water reduces both conjugate-pair concentrations by approximately the same factor, leaving their ratio nearly unchanged. Its pH is therefore approximately preserved in the ordinary buffer regime, but the capacity per liter decreases. If the whole sample is retained, its total acid/base reserve is unchanged by dilution alone. At extreme dilution, water ionization and activity changes can matter.
Choose a relevant pKa
A conjugate pair works most effectively for two-way buffering when the desired pH is reasonably close to pKa, often within about one unit in introductory problems. Far from pKa, one form is scarce and can be depleted by a small challenge. This is a practical operating range, not an abrupt boundary at exactly one unit.
Use a fair experimental comparison
Measure equal-volume samples before and after identical delivered acid or base moles. Include an unbuffered comparison and replicate the additions. A claim that a buffer “worked better” should report its initial composition, volume, temperature, and actual pH change; otherwise differences in the challenge may explain the result.
Practice with answers
- Which component is consumed by added strong base? HA.
- Will doubling both concentrations change the ideal starting pH? No, but it increases capacity per volume.
- Does diluting a retained buffer sample destroy its total moles of HA and A⁻? No; it lowers concentrations and capacity per liter.
Factual reference: OpenStax Chemistry 2e, 14.6. The paired challenge experiment, calculations, and explanations are independently authored.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.