How this calculation works
First react the added strong reagent with the available conjugate member. Track unreacted strong excess separately. Only apply the Henderson–Hasselbalch ratio when positive amounts of both weak partners remain.
pH ≈ pKa + log10(remaining A⁻ / remaining HA), after stoichiometric strong-acid or strong-base neutralizationA worked example
Starting with 10 mmol HA and 10 mmol A⁻, 2 mmol strong acid leaves 12 mmol HA and 8 mmol A⁻. At pKa 4.76, the approximate pH is 4.583909.
Good to know
Single weak monoprotic pair, not full equilibrium, chemical-handling advice or a validity certificate. No activity, temperature, polyprotic or dilution correction. pH is suppressed after pair exhaustion.
Reference: OpenStax: buffer stoichiometry and pH (opens in a new tab) ↗
Neutralization comes before the logarithm
Strong acid consumes A⁻ and produces HA; strong base consumes HA and produces A⁻. Applying the initial ratio after adding reagent would ignore that stoichiometric change.
The amount reacting cannot exceed the available conjugate member. Any remaining strong reagent is excess, not an additional source of weak conjugate product.
An exhausted pair needs a different model
When either weak partner reaches zero, the finite buffer-ratio formula is no longer applicable. This page labels the state outside the buffer model rather than reporting log zero or an invented full-equilibrium pH.
The strong excess is still shown as a stoichiometric amount. It is not used to guess a complete pH without the appropriate equilibrium and activity treatment.
A ratio window is only a heuristic
The conventional 0.1–10 partner ratio is displayed as a buffering heuristic. Being inside that window does not prove that concentration, activity or equilibrium assumptions are accurate.
Final volume allows remaining concentrations to be inspected and must include additions. The ratio itself cancels a common volume, but that does not make physical dilution irrelevant to the approximation.
Use the result with the next measurement
The Nernst cell potential calculator also uses logarithmic activity ratios, but the electrochemical reaction quotient and standard-state assumptions differ from this approximate acid–base buffer ratio.
Single weak monoprotic pair, not full equilibrium, chemical-handling advice or a validity certificate. No activity, temperature, polyprotic or dilution correction. pH is suppressed after pair exhaustion.
A few questions, answered
Do I apply the initial ratio after adding acid?
No, neutralize first and use remaining partners.
Can the tool predict pH after exhaustion?
No, it deliberately suppresses the buffer approximation.
Does the ratio window certify accuracy?
No, it is a conventional heuristic.