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Properties of buffers

A buffer is a two-way sponge. It holds a reservoir of a weak acid and its conjugate base, so it can soak up acid from one side and base from the other. Take away either component and the sponge stops working.

§1

The two-component reservoir.

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A buffer resists pH change because it holds a reservoir of both a weak acid and its conjugate base. When base is added, the weak acid neutralizes it; when acid is added, the conjugate base neutralizes it.

This two-way capacity is why both components must be present in significant amounts. A weak acid alone can absorb added base but not added acid; the conjugate base is what handles acid.

A buffer works best near its pKa, where the two components are comparable in amount and the reservoir can respond to either kind of addition.

UNIT 8 TOPIC 8.8 • PROPERTIES OF BUFFERS CHOOSING A BUFFER A buffer needs appreciable amounts of BOTH a weak acid (HA) and its conjugate base (A⁻). USEFUL BUFFER RANGE (pKa ± 1) buffering possible here pKa − 2 pKa − 1 pKa pKa + 1 pKa + 2 Needs both forms HA is the acidic component; A⁻ is the basic component. A⁻ neutralizes added H⁺ A⁻ + H⁺ → HA HA neutralizes added OH⁻ HA + OH⁻ → A⁻ + H₂O Each form mops up one kind of added strong species. Best buffering capacity Maximum buffering when [HA] ≈ [A⁻] (pH = pKa) Higher total concentration → greater capacity. At the half-equivalence point [HA] = [A⁻], so pH = pKa — the buffer resists change most. Choosing the pair Pick a weak acid whose pKa is within ±1 of the target pH. useful range = pKa ± 1 Example weak acids: acetic acid pKa 4.7 dihydrogen phosphate pKa 7.2 ammonium pKa 9.2 Match the pair to your target pH. HENDERSON–HASSELBALCH pH = pKa + log ( [A⁻] / [HA] ) when [A⁻] = [HA], log(1) = 0, so pH = pKa AP Chemistry · Unit 8 · Acids and Bases
Fig. 8.8.1 A buffer resists pH change because it holds a reservoir of both a weak acid and its conjugate base: the acid neutralizes added base, and the conjugate base neutralizes added acid. Both components must be present for buffering to work.
§2

Why buffers are stable.

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Trace the stability to the two-way reservoir.

  1. Identify both components. A buffer holds a weak acid and its conjugate base together.
  2. Neutralize added base with the acid. The weak acid donates protons to consume added base.
  3. Neutralize added acid with the conjugate base. The conjugate base accepts protons to consume added acid.
  4. Require both to be present. Missing either component breaks the two-way buffering.
§3

The pieces you'll meet.

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Both components make the reservoir.

buffer
Buffer
Weak acid + conjugate base; resists pH change.
reservoir
Reservoir
The stock of both components that absorbs additions.
acid absorbs base
Weak acid
Neutralizes added base.
base absorbs acid
Conjugate base
Neutralizes added acid.
both required
Both required
Missing either component breaks the buffer.
near pKa
Works near pKa
Where the two components are comparable.
§4

Worked example: why a buffer is stable.

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Composition. A buffer holds, say, acetic acid (HA) and acetate (A⁻) together.

Add base. The added OH⁻ is neutralized by the weak acid HA, which donates protons — so the pH barely moves.

Add acid. The added H⁺ is neutralized by the conjugate base A⁻, which accepts protons — again the pH barely moves.

Why both. Remove A⁻ and the buffer cannot absorb acid; remove HA and it cannot absorb base. The stability comes from having both, not from any single component.

§5

Mistakes that cost real points.

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Pitfall · 01

"A buffer is stable because of its weak acid alone."

A buffer's stability comes from having both a weak acid and its conjugate base. The weak acid alone handles added base, but only the conjugate base can absorb added acid. Attributing buffering to one component misses half the mechanism.

Fix. Explain buffering as a two-way reservoir: the weak acid absorbs base and the conjugate base absorbs acid.

Pitfall · 02

"A buffer contains only the conjugate base (or only the acid)."

A buffer requires both the weak acid and its conjugate base present in significant amounts. If one is essentially absent, the mixture cannot neutralize additions from both directions and is not a buffer.

Fix. Confirm both components are present; a single component is not a buffer.

Pitfall · 03

"Adding a strong acid to make a buffer works as well as using a weak acid."

A buffer is built from a weak acid (and its conjugate base), not a strong acid. A strong acid dissociates completely and has no meaningful conjugate-base reservoir to resist pH change, so it cannot buffer.

Fix. Build buffers from weak acid/base conjugate pairs; strong acids and bases do not buffer.

§6

Skill Check.

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