Lab / Acids and bases
Buffer Calculator (Henderson–Hasselbalch Recipe)
Make a buffer at a target pH: choose a common buffer such as phosphate, Tris, acetate, or HEPES, or enter any pKa, and get the ratio and the concentrations and masses of the acid and base forms for your volume.
Buffer Calculator (Henderson–Hasselbalch Recipe): Henderson–Hasselbalch gives pH = pKa + log₁₀([base] ÷ [acid]), so the ratio is 10^(pH − pKa). For a 100 mM phosphate buffer at pH 7.4 (pKa 7.2), the ratio is 1.585: 38.7 mM sodium dihydrogen phosphate and 61.3 mM disodium hydrogen phosphate. Masses use the anhydrous molar masses listed with each buffer. Runs 100% locally in your browser with zero server file uploads.
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The Henderson–Hasselbalch equation, pH = pKa + log₁₀([base] ÷ [acid]), gives the ratio of the two forms for a target pH; at pH = pKa they are equal. The masses assume the anhydrous forms with the molar masses shown in the list's source formulas; check the label of what you weigh, as hydrates are heavier. pKa values are at 25 °C and shift with temperature and ionic strength, Tris strongly so, so adjust the final pH with a meter.
Worked example
500 mL of 50 mM Tris buffer at pH 8.0 (pKa 8.07): ratio 10^(−0.07) = 0.851, so 27.0 mM Tris-HCl and 23.0 mM Tris base: 2.128 g and 1.393 g.
To check the pH of a weak acid or base alone, use the pH calculator.
Hydrates and salts
Phosphate salts are often sold as hydrates: Na₂HPO₄·7H₂O is 268.07 g/mol, not 141.96, so scale the mass by the ratio of molar masses. The molar mass calculator gives the mass of any formula.
Make concentrated stocks of each form and mix them, then dilute with the dilution calculator.
How to use it
- Choose the buffer, or type its pKa.
- Enter the target pH, the total buffer concentration, and the volume.
- Read the base-to-acid ratio and the amount of each form to weigh.
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Frequently asked questions
Which buffer should I choose?
One whose pKa is within about 1 unit of the target pH, where it resists change best; for pH 7.4, phosphate or HEPES.
Why does my buffer read a different pH?
pKa values shift with temperature and concentration, Tris by about −0.03 per °C, so adjust the final pH with a meter and a little acid or base.
What if I only have the acid form?
Weigh the acid form for the total concentration and titrate with NaOH (or HCl for a base form) to the target pH on a meter.
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