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Henderson-Hasselbalch Buffer pH Calculator

Calculate a buffer solution's pH from its pKa and conjugate base/acid concentrations, or solve for the base:acid ratio needed to hit a target pH.

Buffer pH
4.76
[A-] / [HA] ratio
1

How it works

The Henderson-Hasselbalch equation estimates a buffer's pH from a weak acid's dissociation constant and how much of it has been converted to its conjugate base: pH = pKa + log₁₀([A⁻] ÷ [HA]). When the conjugate base and acid concentrations are equal, the ratio is 1, log₁₀(1) = 0, and pH reduces exactly to pKa — the reason a buffer is chosen with a pKa close to the pH it needs to hold.

Solving in the opposite direction — what ratio of conjugate base to acid is needed to reach a specific target pH — rearranges the same equation to ratio = 10^(pH − pKa). A ratio above 1 means more conjugate base than acid is needed; below 1, more acid than base.

This is an approximation, not an exact equilibrium calculation: it assumes the buffer's own concentrations are far larger than the solution's H⁺/OH⁻ concentrations, and is most reliable when the base:acid ratio stays within roughly 0.1–10 (within one pH unit of pKa) — outside that range, the buffer has little capacity left and the approximation degrades.

FAQ

Why does this only work well within one pH unit of pKa?

Once the base:acid ratio moves far from 1 (roughly outside 0.1–10), one component is nearly used up relative to the other, and the solution has little buffering capacity left to resist further pH changes — the equation still computes a number, but the solution no longer behaves like an effective buffer at that point.

Where do I find a weak acid's pKa?

pKa is a published property of the specific acid (e.g. acetic acid ≈4.76, phosphoric acid's second dissociation ≈7.2) found in chemistry reference tables — it isn't something this calculator can look up for you, since it varies by compound and this site only computes from values you supply.

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