Henderson-Hasselbalch Calculator (pH from pKa)

This calculator applies the Henderson-Hasselbalch equation to a buffer. Enter the pKa of the weak acid and the concentrations of the conjugate base [A⁻] and the acid [HA]; you get the pH and the base-to-acid ratio.

Formula

pH = pKa + log₁₀([A⁻] ÷ [HA]).

• pKa: of the weak acid at the working temperature (acetic acid ≈ 4.76 at 25 °C)
• [A⁻], [HA]: concentrations in the same unit

Worked Example

An acetate buffer, pKa 4.76, 0.1 M acetate and 0.05 M acetic acid.

1. Ratio = 0.1 ÷ 0.05 = 2.000
2. log₁₀ 2 = 0.301
3. pH = 4.76 + 0.301 = 5.06

How to read the result

When the two concentrations are equal, pH = pKa. A buffer works best within about one pH unit of its pKa, where the ratio is between 0.1 and 10.

Limits and common mistakes

• The equation assumes dilute solutions; very concentrated or very dilute buffers deviate.
• pKa changes with temperature and ionic strength.
• A zero concentration gives Cannot compute.

Frequently Asked Questions

What if [A⁻] equals [HA]?

pH equals the pKa.

What ratio gives pH = pKa + 1?

A base-to-acid ratio of 10.

Can I use moles instead of concentration?

Yes, if both are in the same volume.

Does it work for bases?

Use the pKa of the conjugate acid (for ammonia, ammonium ≈ 9.25).

in pka | pKa | - | 4.76 | -
in a | Conjugate base [A⁻] | mol/L | 0.1 | 0
in ha | Weak acid [HA] | mol/L | 0.05 | 0
out ph | Buffer pH | - | pka + log10(a / ha) | 2
out ratio | Ratio [A⁻] / [HA] | - | a / ha | 3