What this calculates
This calculator estimates the pH of a buffer made from a weak acid and its conjugate base. It uses the ratio of their concentrations, so both values must be expressed on the same basis.
Henderson-Hasselbalch formula
pH = pKa + log10([A−]/[HA]). When the acid and base concentrations are equal, the estimate is pH ≈ pKa. A tenfold increase in the conjugate-base to acid ratio increases the estimate by one pH unit.
When the approximation breaks down
Very dilute buffers, ratios near zero, polyprotic species, large activity effects, and temperature-dependent pKa values need a fuller equilibrium treatment. Do not use this compact estimate as a substitute for a validated laboratory method. For a strong electrolyte check, use the Acid-Base pH Calculator.
Frequently asked questions
Is this an exact equilibrium solver?
No. It is the Henderson-Hasselbalch buffer approximation.
Can acid and base use different units?
Yes, when the units are equivalent and normalized; using the same unit makes the ratio easier to audit.