Buffer pH (Henderson-Hasselbalch) Calculator
Calculate buffer solution pH using the Henderson-Hasselbalch equation from pKa and acid/base concentrations.
Results
What is it?
The Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) is the fundamental formula for buffer chemistry. It relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to acid concentrations.
How to use
Enter the pKa of your weak acid (find it in the literature or on reagent datasheets), and the molar concentrations of the acid and conjugate base forms. Concentrations can be in any units as long as both use the same unit, since only the ratio matters.
Example scenario
Acetate buffer at pH 5.0: pKa = 4.76, need [A-]/[HA] = 10^(5.0-4.76) = 10^0.24 = 1.74. So mix 174 mM sodium acetate with 100 mM acetic acid. Result: pH = 4.76 + log(1.74) = 5.0.
Pro tip
A buffer is most effective within pKa +- 1 pH unit. Outside this range, buffer capacity drops sharply. Buffer capacity is highest (best resistance to pH change) when [A-] = [HA] (ratio = 1:1, pH = pKa exactly). For physiological buffers: PBS uses phosphate (pKa 7.20), HEPES (pKa 7.55) and MOPS (pKa 7.20) are common in cell biology. Temperature affects pKa -- Tris shifts ~0.028 pH units per degree Celsius.