Last updated: 2026-06-14

Understanding the Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation predicts the pH of a buffer solution from the strength of its weak acid and the ratio of conjugate base to acid. It is a cornerstone of acid-base chemistry and biochemistry, used to design buffers that hold a reaction — or a living cell — at a stable pH.

The formula

Because only the ratio of base to acid appears, diluting a buffer does not change its pH — both concentrations fall together and the ratio stays the same.

Worked example

An acetate buffer uses acetic acid (pKa = 4.76). With 0.30 M acetate and 0.10 M acetic acid, the ratio is 3:1:

To raise the pH, add more conjugate base; to lower it, add more weak acid.

When pH equals pKa

When the conjugate base and weak acid have equal concentrations, the ratio is 1, log₁₀(1) = 0, and the pH equals the pKa exactly. This is the point of maximum buffer capacity — the buffer resists pH change most strongly — and it is the half-neutralization point used to measure pKa values experimentally.

Choosing a buffer

A buffer works well only within about ±1 pH unit of its pKa, where the base-to-acid ratio stays between 1:10 and 10:1. To buffer at a target pH, pick a weak acid whose pKa is close to it:

Assumptions and limits

The Henderson-Hasselbalch equation assumes the weak acid is genuinely weak and that the equilibrium concentrations are close to the amounts you mixed. It loses accuracy for very dilute buffers, very strong or very weak acids, and at extreme pH values where water's own ionization contributes. For typical bench buffers it is accurate and convenient.