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Buffer pH (Henderson-Hasselbalch)
Calculate the pH of a buffer from pKa and the ratio of conjugate base to acid concentrations.
pH
7.40
The Henderson-Hasselbalch equation
pH = pKa + log₁₀([A⁻] / [HA]), where [A⁻] is the conjugate base and [HA] the weak acid. When the two are equal, the log term is zero and pH = pKa — the point of greatest buffering capacity.
Worked example
An acetate buffer (pKa 4.76) with twice as much acetate as acetic acid: 4.76 + log₁₀(2) = 4.76 + 0.30 = 5.06.
Limits and common mistakes
- The equation is reliable within about ±1 pH unit of the pKa; outside that range the buffer has little capacity.
- pKa changes with temperature: Tris shifts about −0.028 pH units per °C, so a Tris buffer set to pH 8.0 at 25 °C reads about 8.6 at 4 °C.
- High ionic strength shifts the effective pKa — always confirm with a calibrated pH meter.
Learn the concept
FAQ
What is the Henderson-Hasselbalch equation?
pH = pKa + log10([A⁻]/[HA]). It relates buffer pH to the pKa of the weak acid and the ratio of conjugate base to acid concentrations.
When does Henderson-Hasselbalch break down?
At very high or very low ratios of base to acid (typically beyond ±1 pH unit from the pKa) and at extreme concentrations or ionic strengths, the simple equation deviates from reality.
How do I prepare a buffer at a target pH?
Pick a buffer whose pKa is within ±1 unit of your target pH (e.g. Tris pKa 8.1, HEPES pKa 7.5, PIPES pKa 6.8), then adjust the base/acid ratio per Henderson-Hasselbalch.