Calculate the pH of a buffer from the concentrations of a weak acid and its conjugate base, or find the ratio you need to hit a target pH. This free Henderson–Hasselbalch calculator also solves for pKa and lists common lab buffers.
Henderson–Hasselbalch equation
Worked example
An acetate buffer contains 0.15 M sodium acetate and 0.10 M acetic acid (pKa 4.76). pH = 4.76 + log(0.15/0.10) = 4.76 + 0.176 = 4.94. To make a pH 5.00 buffer instead, you need [A⁻]/[HA] = 10^(5.00 − 4.76) = 1.74.
Common buffers
| Buffer | pKa (25 °C) | Useful pH range |
|---|---|---|
| Citrate | 3.13, 4.76, 6.40 | 2.1 – 7.4 |
| Acetate | 4.76 | 3.8 – 5.8 |
| MES | 6.15 | 5.5 – 6.7 |
| Phosphate (H₂PO₄⁻/HPO₄²⁻) | 7.21 | 5.8 – 8.0 |
| HEPES | 7.55 | 6.8 – 8.2 |
| Tris | 8.07 | 7.0 – 9.0 |
| Ammonia/ammonium | 9.25 | 8.3 – 10.3 |
The equation assumes dilute solutions; at high ionic strength or for precise work, check the final pH with a calibrated meter.
Frequently asked questions
What is the Henderson–Hasselbalch equation?
pH = pKa + log([A⁻]/[HA]). It gives the pH of a buffer made from a weak acid (HA) and its conjugate base (A⁻).
How do I make a buffer of a certain pH?
Pick an acid whose pKa is within 1 unit of your target, enter the target pH and pKa, and leave one concentration empty. The calculator gives the concentration needed for the ratio.
Why is a buffer best near its pKa?
At pH = pKa the acid and base are equal, so the buffer can neutralise added acid or base equally well. Beyond pKa ± 1 its capacity drops sharply.
Can I use moles instead of concentrations?
Yes. Both species are in the same volume, so the ratio of moles equals the ratio of concentrations.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.