# Buffer solution

A buffer resists pH changes. It holds a weak acid and its conjugate base together. Added acid is used up by the base, and added base is used up by the acid. Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]).

Chemistry (university) · Part 08: Acids, bases and electrochemistry · Card 4 of 9 · Last updated 2026-10-19

## Analogy

A pair of shock absorbers: one soaks up bumps from above and one from below.

## Example

**Acetate buffer, pKa = 4.74**

[A⁻] = 0.20 M, [HA] = 0.10 M
pH = 4.74 + log(2)
pH = 4.74 + 0.30 = 5.04

## In practice

Blood uses carbonic acid and bicarbonate as a buffer, keeping pH steady at about 7.4.

## Practice

### Flashcards

- **Henderson-Hasselbalch equation?** pH = pKa + log([A⁻]/[HA]).
- **How does a buffer resist added acid?** Its conjugate base reacts with the added H⁺.

### Interview questions

**Explain how a buffer works and calculate the pH of a solution with 0.20 M acetate and 0.10 M acetic acid (pKa 4.74).**

- Weak acid plus conjugate base
- Base reacts with added H⁺, acid with added OH⁻
- pH = pKa + log(0.20/0.10)
- pH = 5.04

## Questions people ask

### What makes a good buffer?

A good buffer has similar amounts of weak acid and conjugate base, and a pKa within about 1 unit of the target pH. It should also be fairly concentrated. A more concentrated buffer can absorb more added acid or base before its pH shifts.

### When does a buffer stop working?

A buffer fails once one of its two components is used up. If you add more strong acid than the amount of base present, the base is exhausted, and the pH falls sharply. The buffer capacity is the amount of acid or base it can absorb.

## Related concepts

- [Acid-base titration](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-5)

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