What is the Henderson-Hasselbalch equation?

It is pH = pKa + log([A⁻]/[HA]), where pKa is the negative log of the acid dissociation constant, [A⁻] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. It accurately predicts buffer pH when both species are present in non-trivial amounts.

What is a buffer, and why does this equation apply?

A buffer is a solution that resists pH changes. It contains a weak acid and its conjugate base in equilibrium. The Henderson-Hasselbalch equation describes this equilibrium and shows that pH depends on the ratio [A⁻]/[HA], not the absolute concentrations.

What pKa value should I use?

Use the pKa of the specific weak acid at the temperature of your experiment. Common values: acetic acid 4.74, carbonic acid (first) 6.35, phosphoric acid (second) 7.20, ammonium 9.25. Many references list pKa values at 25 °C.

What are the limitations of this equation?

The Henderson-Hasselbalch equation assumes ideal behavior and is most accurate when both [A⁻] and [HA] concentrations are between 0.1× and 10× each other (ratio 0.1–10). It becomes less accurate for very dilute solutions, strong acids/bases, or extreme pH values far from pKa.

Why does the ratio matter more than absolute concentrations?

Because the logarithm in the equation only takes the ratio [A⁻]/[HA]. Doubling both concentrations keeps pH identical. This is why you can specify concentrations in any consistent unit (mol/L, mmol/L) as long as both use the same unit.

What does a ratio of 1 mean?

When [A⁻] = [HA] the ratio is 1, log(1) = 0, so pH = pKa. This is the half-equivalence point, where buffering capacity is maximized. Effective buffering typically extends ±1 pH unit around pKa.

Henderson-Hasselbalch Calculator

Enter pKa and the concentrations of your conjugate base and weak acid to instantly calculate buffer pH.

Henderson-Hasselbalch Inputs

pH = pKa + log([A⁻] / [HA])

pKa of the weak acid

[A⁻] — conjugate base concentration

[HA] — weak acid concentration

Common buffers (click to load)

Enter values and click Calculate to see results

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Summary

The Henderson-Hasselbalch Calculator applies the equation pH = pKa + log([A⁻]/[HA]) to find the pH of a buffer solution from the acid dissociation constant and component concentrations. It also supports the reverse calculation — finding the required base-to-acid ratio for a target pH. All math runs client-side with no data sent to any server.

How it works

Select the calculation mode: forward (compute pH) or reverse (compute the [A-]/[HA] ratio needed for a target pH).
Enter the pKa of the weak acid (e.g., 4.74 for acetic acid).
For forward mode: enter the concentration of the conjugate base [A-] and the weak acid [HA] in any consistent units (mol/L).
For reverse mode: enter the desired target pH instead of concentrations.
The result appears instantly — buffer pH, base-to-acid ratio, and buffer classification.
Use the Copy button to copy the result to your clipboard.

Use cases

Calculate the pH of an acetate buffer for a biochemistry experiment.
Design a phosphate buffer at a specific pH for cell culture media.
Verify buffer pH from measured component concentrations in the lab.
Find the required base-to-acid ratio to hit a target pH.
Understand how changing component ratios shifts buffer pH.
Prepare for chemistry exams by checking Henderson-Hasselbalch problems.
Cross-check buffer calculations from textbook examples.
Optimize buffer conditions for enzyme assays or protein purification.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu