Buffer pH Calculator (Henderson–Hasselbalch)
pH = pKa + log([A⁻]/[HA]). Pick a buffer, then enter a target pH and total concentration to get the acid and base amounts.
pKa of common biological buffers (25 °C)
| Buffer | pKa | Useful pH range |
|---|---|---|
| Acetate | 4.76 | 3.8 – 5.8 |
| MES | 6.15 | 5.2 – 7.2 |
| Phosphate (H₂PO₄⁻/HPO₄²⁻) | 7.21 | 6.2 – 8.2 |
| PIPES | 6.76 | 5.8 – 7.8 |
| MOPS | 7.2 | 6.2 – 8.2 |
| HEPES | 7.5 | 6.5 – 8.5 |
| Tris | 8.06 | 7.1 – 9.1 |
| Bicarbonate | 6.35 | 5.3 – 7.3 |
| Citrate (pKa3) | 6.4 | 5.4 – 7.4 |
| Glycine (pKa2) | 9.6 | 8.6 – 10.6 |
Need concentrations in grams? Use the molarity calculator.
FAQ
What is the Henderson–Hasselbalch equation?
pH = pKa + log₁₀([A⁻] ÷ [HA]), where [A⁻] is the conjugate base and [HA] the weak acid concentration.
How do I find the base-to-acid ratio for a target pH?
Ratio [A⁻]/[HA] = 10^(pH − pKa). At pH = pKa the ratio is 1:1.
What pH range does a buffer work in?
A buffer is most effective within about ±1 pH unit of its pKa.
Why does my measured pH differ?
pKa shifts with temperature and ionic strength (Tris changes about −0.03 per °C). Always check with a calibrated pH meter and adjust.
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