Buffer Calculator
Design a buffer using the Henderson-Hasselbalch equation.
Molar masses are optional — fill in to also compute the mass to weigh.
Learn more
What it does
The buffer calculator designs a buffer using the Henderson-Hasselbalch equation. Given a target pH, pKa, total concentration, and volume, it returns the required acid and conjugate base amounts, and optionally the masses to weigh. It is used by biochemists and lab workers preparing buffer solutions.
How it works
The Henderson-Hasselbalch equation is pH = pKa + log10([A-]/[HA]). Solving for the ratio gives [A-]/[HA] = 10^(pH - pKa). With a total buffer concentration C = [HA] + [A-], the individual concentrations are [HA] = C / (1 + ratio) and [A-] = C × ratio / (1 + ratio). Multiplying by the total volume gives the moles of each form, and with the molar masses the tool also gives the mass of acid form and conjugate base form to weigh. Buffer capacity is highest when pH equals pKa.
Worked example
Design a 0.1 M acetate buffer at pH 4.50 using acetic acid (pKa 4.76) in 500 mL. The ratio is 10^(4.50 - 4.76) = 10^(-0.26) = 0.55. Then [HA] = 0.1 / (1 + 0.55) = 0.0645 M and [A-] = 0.1 × 0.55 / 1.55 = 0.0355 M. Moles of HA = 0.0645 × 0.500 = 0.0323 mol, and moles of A- = 0.0177 mol. With acetic acid MW 60.05 g/mol and sodium acetate MW 82.03 g/mol, weigh 1.94 g acetic acid and 1.45 g sodium acetate, dilute to 500 mL.
When to use it
Use it to prepare a buffer at an exact pH for an enzymatic or cell-culture assay. Use it to scale a buffer recipe to a different final volume. Use it to convert a desired concentration and pH into actual weighing masses.
FAQ
- How do I design a buffer at a specific pH?
- Enter the weak acid pKa, the target pH, the total buffer concentration, and the volume. The tool returns the acid and conjugate base concentrations and moles from [A-]/[HA] = 10^(pH - pKa).
- How do I calculate how much acid and base to use?
- Find the ratio [A-]/[HA] = 10^(pH - pKa). Then [HA] = C / (1 + ratio) and [A-] = C × ratio / (1 + ratio), where C is the total concentration. Multiply by volume for moles.
- Why enter molar masses in the buffer calculator?
- Molar masses are optional but let the tool also compute the actual mass of the acid form and conjugate base form to weigh, not just the moles.
- Within what pH range does a buffer work best?
- A buffer is effective within about one pH unit above or below its pKa, with maximum capacity when pH equals pKa and [A-] equals [HA].