About the Henderson-Hasselbalch Calculator
This Henderson-Hasselbalch calculator works out the pH of a buffer made from a weak acid and its conjugate base, and runs the equation backwards to tell you what ratio — and what concentrations of each component — you need to hit a target pH. Pick a common buffer such as acetate, phosphate, Tris or HEPES, or type in any pKa.
It is designed for students preparing buffers in a teaching lab, researchers making up working solutions and anyone checking a textbook buffer problem. Because only the ratio of base to acid matters, you can enter concentrations, moles or millimoles — as long as both use the same unit.
The equation is an approximation that works best when both components are reasonably concentrated (above about 0.01 M) and the pH is within one unit of the pKa. The built-in pKa values are typical figures at 25 °C; real pKa shifts with temperature and ionic strength (Tris drops by about 0.03 per °C), so always confirm the final pH with a meter.
How to use the henderson-hasselbalch calculator
- 1Choose whether you want the pH of a buffer or a recipe for a target pH.
- 2Pick a buffer system, or select Custom and type its pKa.
- 3Enter the acid and conjugate base amounts in the same unit — or the target pH and total concentration.
- 4Read the pH or the base-to-acid ratio and the amount of each component.
- 5Check the note: keep the pH within ±1 of the pKa for good buffering.
Formula and method
The Henderson-Hasselbalch equation is the acid dissociation expression Ka = [H⁺][A⁻]/[HA] rearranged in logarithmic form. It says the pH of a buffer depends on the pKa of the weak acid and the ratio of conjugate base to acid. When the two are equal, the log term is zero and pH = pKa.
For a recipe, the ratio r = 10^(pH − pKa) is split over the total buffer concentration C: [HA] = C/(1 + r) and [A⁻] = C − [HA]. The approximation assumes the equilibrium concentrations equal the amounts added, which is valid when both are much larger than [H⁺] and [OH⁻].
- pKa
- −log₁₀ of the acid dissociation constant Ka
- [A⁻]
- Conjugate base concentration (or moles)
- [HA]
- Weak acid concentration (or moles)
- C
- Total buffer concentration, [HA] + [A⁻]
Worked examples
Acetate buffer, 0.10 M acid + 0.15 M acetate
pH = 4.76 + log₁₀(0.15 ÷ 0.10) = 4.76 + 0.176 = 4.94. More base than acid pushes the pH above the pKa.
Phosphate buffer at pH 7.4, 0.1 M total
The ratio HPO₄²⁻ : H₂PO₄⁻ = 10^(7.4 − 7.21) = 1.549. Splitting 0.1 M in that ratio gives 0.0392 M H₂PO₄⁻ and 0.0608 M HPO₄²⁻.
Ammonia buffer from moles
With 0.05 mol NH₄⁺ (the acid) and 0.02 mol NH₃ (the base), pH = 9.25 + log₁₀(0.4) = 9.25 − 0.398 = 8.85. Only 28.6% of the pair is in the base form.
Frequently asked questions
What is the Henderson-Hasselbalch equation?+
It is pH = pKa + log₁₀([A⁻]/[HA]), relating the pH of a buffer to the pKa of its weak acid and the ratio of conjugate base to acid. It is derived by taking the negative log of the Ka expression.
Can I use moles instead of concentrations?+
Yes. Both components are in the same final volume, so the volume cancels in the ratio. Moles, millimoles or molarities all give the same pH as long as acid and base use the same unit.
What is the effective range of a buffer?+
A buffer resists pH change best within about ±1 pH unit of its pKa, where the base-to-acid ratio lies between 1:10 and 10:1. Outside that range one component runs low and capacity drops sharply.
Why does my measured pH differ from the calculation?+
pKa depends on temperature and ionic strength, and the equation uses concentrations rather than activities. Tris, for example, shifts about −0.03 pH units per °C, so adjust with a pH meter at the working temperature.
Which pKa should I use for phosphate buffer?+
Phosphoric acid has three pKa values (about 2.15, 7.21 and 12.3). For physiological buffers around pH 7 use pKa₂ ≈ 7.21, the H₂PO₄⁻/HPO₄²⁻ pair.