When is the Henderson–Hasselbalch equation not reliable?

When the ratio is far outside 0.1–10, when either component is very dilute, at high ionic strength, or when strong acid or base has been added without doing the stoichiometry first. In those cases solve the full equilibrium or measure.

Does this calculate buffer capacity?

No. Two buffers with the same ratio have the same estimated pH but can resist added acid very differently. Capacity grows with total concentration and is largest when pH is close to pKa.

Which pKa should I use?

Use a pKa for your temperature and solvent. Values change with temperature and ionic strength, and some biological buffers are notably temperature-sensitive.

Tools

Buffer pH calculator

Estimate buffer pH from pKa and concentrations, or find the base-to-acid ratio for a target pH, with Henderson–Hasselbalch.

Buffer pH (Henderson–Hasselbalch)

pKa of the acid

[HA] weak acid

mol/L

[A⁻] conjugate base

mol/L

Estimated pH: 4.76
[A⁻] / [HA]: 1

Ratio for a target pH

Target pH

Required [A⁻] / [HA]: 1.738

The Henderson–Hasselbalch relationship, pH = pKa + log₁₀([A⁻]/[HA]), estimates the pH of a weak acid and its conjugate base. It assumes the equilibrium concentrations are close to the amounts added and that activity can be replaced by concentration. The calculator warns you when the ratio falls outside 0.1–10 or a component is below 1 mM, where those assumptions become weak. If strong acid or base is added, do that stoichiometry first and enter the new amounts. This calculator estimates pH; it does not measure buffer capacity.

How to estimate buffer pH

1.Enter the pKa of the weak acid at your working temperature.
2.Enter the concentrations of the weak acid [HA] and the conjugate base [A⁻] in mol/L.
3.Read the estimated pH and the base-to-acid ratio, and note any range warning.
4.To design a buffer, enter a target pH to get the ratio [A⁻]/[HA] you need.

Worked example: acetate buffer

1.Acetic acid has pKa ≈ 4.76 at 25 °C.
2.With 0.100 M acetic acid and 0.100 M acetate the ratio is 1, so pH ≈ 4.76.
3.After adding 0.010 mol HCl to 1.00 L, the stoichiometry gives 0.110 M acid and 0.090 M acetate: pH ≈ 4.76 + log₁₀(0.090/0.110) ≈ 4.67.
4.For a target of pH 5.00 you need [A⁻]/[HA] = 10^(5.00 − 4.76) ≈ 1.74.

Frequently asked questions

When is the Henderson–Hasselbalch equation not reliable?: When the ratio is far outside 0.1–10, when either component is very dilute, at high ionic strength, or when strong acid or base has been added without doing the stoichiometry first. In those cases solve the full equilibrium or measure.
Does this calculate buffer capacity?: No. Two buffers with the same ratio have the same estimated pH but can resist added acid very differently. Capacity grows with total concentration and is largest when pH is close to pKa.
Which pKa should I use?: Use a pKa for your temperature and solvent. Values change with temperature and ionic strength, and some biological buffers are notably temperature-sensitive.