henderson hasselbalch calculator

What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation is a compact way to estimate the pH of a buffer system made from a weak acid and its conjugate base. It connects three useful quantities: pH, pKa, and the concentration ratio of base to acid.

In practice, this equation helps with tasks such as preparing lab buffers, understanding titration regions, and estimating how biological systems resist pH changes.

How to use this calculator

1) Calculate pH from pKa and concentrations

• Enter pKa, [A^-], and [HA].
• Click Calculate pH.
• The tool computes: pH = pKa + log10([A^-]/[HA]).

2) Calculate the required base/acid ratio for a target pH

• Enter pKa and target pH.
• Click Calculate [A^-]/[HA] Ratio.
• The tool computes: [A^-]/[HA] = 10^(pH - pKa).

3) Calculate pKa from pH and concentrations

• Enter pH, [A^-], and [HA].
• Click Calculate pKa.
• The tool computes: pKa = pH - log10([A^-]/[HA]).

Worked examples

Example A: Acetate buffer pH

Suppose pKa = 4.76, [A^-] = 0.20 M, and [HA] = 0.10 M. Ratio = 2. Therefore, pH = 4.76 + log10(2) ≈ 5.06.

Example B: Find ratio for pH 7.40 with phosphate-like pKa 7.21

Ratio = 10^(7.40 - 7.21) = 10^0.19 ≈ 1.55. So you need about 1.55 times as much conjugate base as weak acid.

Interpreting results in real buffer design

A useful rule of thumb: the Henderson-Hasselbalch approximation is most reliable when the ratio [A^-]/[HA] is between about 0.1 and 10, which corresponds to pH within roughly ±1 unit of pKa.

• Ratio near 1: strongest buffering around pH ≈ pKa.
• Very high ratio: solution is base-heavy; buffering against added base weakens.
• Very low ratio: solution is acid-heavy; buffering against added acid weakens.

Assumptions and limitations

The equation is an approximation. It works best for dilute solutions where activity effects are limited. In concentrated or high-ionic-strength systems, activity coefficients can shift apparent behavior.

• Assumes weak-acid/conjugate-base equilibrium dominates.
• Less accurate at extreme concentrations or very low/high pH.
• Temperature changes can alter pKa values.

Common mistakes to avoid

• Using a strong acid/strong base pair and expecting classic buffer behavior.
• Forgetting that concentration units must be consistent before taking a ratio.
• Entering zero or negative concentrations.
• Confusing natural log (ln) with base-10 log (log10).

Quick FAQ

Can I use millimolar instead of molar?

Yes. Because the formula uses a ratio, any consistent units work.

Does dilution change pH predicted by Henderson-Hasselbalch?

If both [A^-] and [HA] dilute equally, the ratio stays constant, so predicted pH stays about the same. Buffer capacity, however, decreases.

What pKa should I choose for a target pH buffer?

Choose a weak acid with pKa close to your target pH (typically within ±1) for better buffering performance.