Buffer pH Calculator (Henderson–Hasselbalch)
Buffer pH calculator with the Henderson–Hasselbalch equation: pH from pKa and the base/acid ratio, or the ratio for a target pH.
pH = pKa + log10 [A−][HA]
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
A buffer is a weak acid HA mixed with its conjugate base A⁻. Its pH follows the Henderson–Hasselbalch equation, pH = pKa + log₁₀([A⁻]/[HA]).
A buffer works best within about one unit of its pKa. Solve for [A⁻] or [HA] to find how much salt or acid to add for a target pH.
Formula
pH = pKa + log10 [A−][HA]
| Symbol | Quantity | Units |
|---|---|---|
| pKa | pKa of the acid | — |
| [A−] | Conjugate base | mol/L, mmol/L, µmol/L, nmol/L, mol/m³ |
| [HA] | Weak acid | mol/L, mmol/L, µmol/L, nmol/L, mol/m³ |
| pH | Buffer pH | — |
Worked example
Given: pKa = 4.76 · [A−] = 0.2 mol/L · [HA] = 0.1 mol/L
pH = pKa + log10[A−][HA]
Result: pH = 5.06103
Frequently asked questions
What happens when [A⁻] = [HA]?
The log term is zero, so pH = pKa. That is the point of maximum buffer capacity.
How accurate is the equation?
Very good for concentrations above about 0.01 M and pH between 3 and 11. It ignores activity coefficients and water autoionization.