How this calculator works
Choose the unknown in the acid-buffer equilibrium relation. Use concentrations of both equilibrium partners in the same solution. The ideal concentration approximation is most useful when both are appreciable and neither is overwhelmed by added strong reagent. Activities, temperature, ionic strength and polyprotic overlap can change actual pH. The ratio mode needs no absolute concentration.
Formula
pH = pKa + log10([base partner]/[acid partner]). For a weak-base buffer, conjugate-acid pKa = pKw - pKb.
Worked example
Equal acid and base concentrations give pH = pKa = 5. With pKa = 4 and pH = 5, the base concentration must be ten times the acid concentration.
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Frequently asked questions
When does pH equal pKa?
When the conjugate base and acid activities are equal; the concentration approximation uses equal concentrations.
Can this handle a buffer after neutralization?
First subtract the strong reagent stoichiometrically, then use the remaining conjugate pair.
Can either partner be absent?
No. The logarithmic relation requires both partners and fails near complete exhaustion.