Henderson–Hasselbalch Calculator
Find buffer pH from pKa and composition, or find the conjugate-base-to-acid ratio required for a target pH.
01 Set up your calculation
- Choose whether to solve for pH or for the concentration ratio.
- Enter the pKa of the relevant acid dissociation step.
- Supply both concentrations or a target pH, using values appropriate to the solution temperature.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Buffer pH and pKa.
Follow the numbers
With pKa 4.76, [HA] = 0.1 mol/L and [A⁻] = 0.2 mol/L, the ratio is 2. The estimated pH is 4.76 + log₁₀(2) = 5.06103.
03 From equilibrium to a logarithmic relationship
For an acid HA in equilibrium with its conjugate base A⁻, the acid dissociation expression links their ratio to hydrogen-ion activity. Rearranging and taking a base-ten logarithm produces the Henderson–Hasselbalch form. This calculator uses concentrations as an approximation to activities.
A change of one pH unit relative to pKa corresponds to a tenfold change in the base-to-acid ratio. The relationship is logarithmic, so doubling the ratio shifts the estimate by about 0.301 pH units, not by one unit.
04 A ratio is not a complete preparation recipe
The same ratio can be achieved at many total concentrations. Those solutions can have a similar predicted pH but very different capacities to absorb an added acid or base. A target ratio therefore does not specify how much buffer is needed for an experiment.
The usual practical buffer region is roughly pKa ± 1. Outside that region, one partner is relatively scarce. Very dilute, highly concentrated, or multi-equilibrium solutions may require activity corrections and a more complete model. The tool does not select a chemically compatible buffer for you.
Questions from the notebook
Why use the conjugate base rather than a strong base concentration?
The equation applies to the equilibrium pair A⁻ and HA after any neutralization reaction has occurred.
Can pH equal pKa?
Yes. That occurs when the two partner concentrations are equal in this approximation.
Can I enter Ka directly?
Convert it to pKa = −log₁₀(Ka) first, or use the pKa calculator.
References & reading
- OpenStax, Rice University — Buffers ↗
Explains conjugate pairs, buffer behavior, and the Henderson–Hasselbalch relationship.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.