Work out buffer solution instantly with clear inputs, formula shown and shareable results.
The Henderson-Hasselbalch equation gives a buffer's pH as pKa plus the log of the conjugate base to acid ratio. Equal concentrations put the pH exactly at the pKa; a 1.5:1 ratio lifts it by 0.18. Buffering is only effective within about one pH unit of the pKa, because outside that window one component is nearly exhausted.
Henderson-Hasselbalch
pH = pKa + log10([A-] / [HA])
Useful window
pKa - 1 to pKa + 1
One to one. The log of 1 is zero, so equimolar acid and conjugate base sit exactly at the pKa - the point of maximum buffer capacity.
Barely, because dilution scales both concentrations equally and only the ratio matters. Capacity falls, though, so it resists less added acid or base.