🧴 Buffer Solutions

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

How a mix of a weak acid and its conjugate base resists changes in pH.

A buffer is a solution that resists changing pH when a small amount of acid or base is added to it — useful anywhere a process needs to stay within a narrow pH range (biological systems are full of them; blood itself is buffered). It works by combining a weak acid with its own conjugate base (or a weak base with its conjugate acid) in the same solution, so there's a reserve of both an acid-neutralizer and a base-neutralizer on hand at once.

The Henderson-Hasselbalch equation is a shortcut derived from the weak acid's own equilibrium expression, letting the buffer's pH be read directly off the ratio of its two components rather than solving the full equilibrium each time.

Key formula

pH = pKa + log₁₀([A⁻] ÷ [HA])

Variables

pKa

the weak acid's own equilibrium constant, expressed on a log scale

[A⁻]

concentration of the conjugate base

[HA]

concentration of the weak acid

Notes & limitations

• A buffer works best — resists pH change most effectively — when the acid and conjugate base concentrations are close to equal, which happens when the solution's pH is close to the acid's pKa. That's why choosing a buffer means picking a weak acid whose pKa is near the pH you actually need to hold.
• A buffer has a limited capacity: enough strong acid or base added will eventually use up one of the two components entirely, at which point the solution stops buffering and the pH starts changing rapidly, same as an unbuffered solution.