Henderson-Hasselbalch Calculator
Enter a pKa and the conjugate base and weak acid concentrations to get the pH of a buffer solution — and see why equal concentrations make the pH equal the pKa.
Buffer pH in one step
Enter the pKa, the conjugate base [A⁻], and the weak acid [HA] in mol/L and the calculator returns the buffer pH from pH = pKa + log₁₀([A⁻]/[HA]).
Use the same unit
Both concentrations must be in the same unit (mol/L). Only their ratio matters, so equal concentrations give a pH equal to the pKa.
What is the Henderson-Hasselbalch calculator?
The pH of a buffer
The Henderson-Hasselbalch calculator works out the pH of a buffer solution from three numbers: the pKa of the weak acid, the concentration of the conjugate base [A⁻], and the concentration of the weak acid [HA]. A buffer is a mixture of a weak acid and its conjugate base that resists changes in pH when small amounts of acid or base are added, and the equation pH = pKa + log₁₀([A⁻]/[HA]) predicts exactly where its pH sits. It is the number behind blood chemistry, fermentation, and the buffers chemists prepare every day to hold a reaction at a steady pH.
Enter a pKa and the base and acid concentrations in mol/L to get the buffer pH instantly — equal concentrations return the pKa.
The buffer pH is the pKa plus the base-10 logarithm of the conjugate base divided by the weak acid.
pH = pKa + log₁₀([A⁻] / [HA])Because only the ratio of the two species appears, doubling both concentrations leaves the pH unchanged while making the buffer stronger. When the conjugate base outweighs the acid the log term is positive and the pH rises above the pKa; when the acid dominates it falls below. Use mol/L for both and the pH comes back on the usual scale.
Suppose you mix acetate and acetic acid, each at 0.1 mol/L, with a pKa of 4.76.
Find the ratio
0.1 / 0.1 = 1 — the conjugate base and the acid are present in equal amounts.
Take the base-10 logarithm
log₁₀(1) = 0 — a ratio of one contributes nothing to the pH.
Add the pKa
pH = 4.76 + 0 = 4.76 — the buffer sits exactly at its pKa, where it works best.
The equation is widely used, but it is an approximation worth applying with care.
An approximation for moderate weak-acid buffers
The Henderson-Hasselbalch equation assumes a weak acid that is only partly dissociated and concentrations that are neither very dilute nor very concentrated. It loses accuracy for strong acids and bases, for extreme buffer ratios far from 1, and when ion activity differs from concentration. Both concentrations must be greater than zero — the logarithm of zero or a negative ratio is undefined — while the pKa itself may be negative for strong acids.