Buffer pH Calculator (Henderson–Hasselbalch)
Find the pH of an acetate, phosphate or other buffer.
Your numbers
Your results
pH
pH
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Ratio [A⁻] ÷ [HA]
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Effective buffering range
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Buffer pH Calculator (Henderson–Hasselbalch) by TheCalcTools
How the Buffer pH Calculator (Henderson–Hasselbalch) works
Enter the acid’s pKa and the concentrations of the base and acid forms.
The pH equals the pKa when the two are equal.
Each tenfold change in the ratio shifts the pH by one unit.
The formula
pH = pKa + log₁₀([A⁻] ÷ [HA])
Worked example
Using the default values shown in the calculator:
| pKa of the weak acid | 4.76 |
|---|---|
| Conjugate base [A⁻] (mol/L) | 0.15 |
| Weak acid [HA] (mol/L) | 0.1 |
- Ratio [A⁻] ÷ [HA] = 0.15 ÷ 0.10 = 1.50.
- pH = 4.76 + log₁₀(1.5) = 4.936.
- The buffer works best from 3.76 to 5.76.
Result:
| pH | 4.936 |
|---|---|
| Ratio [A⁻] ÷ [HA] | 1.500 |
| Effective buffering range | 3.76 to 5.76 |
Frequently asked questions
When is a buffer most effective?
When the pH is within about one unit of the pKa, where both acid and base forms are present in useful amounts.
Does the equation work for dilute or very strong solutions?
It is an approximation. It works best for concentrations around 0.001–1 mol/L and ignores activity effects.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.