How this calculator works
Choose a weak acid buffer or a weak base buffer. In base mode, enter the weak base pKb and the conjugate acid as the acid partner. Convert pKb to conjugate-acid pKa using pKw - pKb. 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
An acid buffer with pKa = 4.76 and equal partners has pH 4.76. A base buffer with pKb = 4.75, equal partners and pKw = 14 has pH 9.25.
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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.