Chemistry · Buffers
Buffer calculator.
Henderson-Hasselbalch both ways: pH from the ratio, or the base-to-acid ratio and moles needed for a target pH.
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Henderson-Hasselbalch equation
For a weak acid HA and its conjugate base A-: pH = pKa + log10([A-] / [HA]). Rearranged, [A-] / [HA] = 10(pH - pKa). When pH equals pKa the two forms are equal.
Worked example. Target pH 7.40 with a weak acid of pKa 7.00 (a made-up value for illustration): ratio = 100.40 = 2.512, so base is 71.53% and acid 28.47% of the total. For 500 mL of buffer with 0.10 mol/L total, that is 0.03576 mol base and 0.01424 mol acid. In the other direction, a ratio of 0.5 with the same pKa gives pH = 7.00 + log10(0.5) = 6.7.
The equation is an approximation. As a common rule of thumb, not a published standard, it is most reliable when the base:acid ratio is between about 0.1 and 10 (pH within one unit of the pKa), and for moderate concentrations. Always check the final pH with a calibrated meter and adjust, since temperature, ionic strength and the pKa you use all move the real value.
Frequently asked questions
What is the Henderson-Hasselbalch equation?
pH = pKa + log10([A-]/[HA]). It links the pH of a buffer to the pKa of its weak acid and the ratio of conjugate base to acid.
How do I choose a buffer?
Pick a weak acid whose pKa is close to the pH you want, so the ratio ends up near 1 and the buffer resists change in both directions.
How accurate is it?
It is an approximation. It ignores activity and assumes the amounts you mix are the equilibrium amounts, so verify and adjust with a pH meter.
Does the total concentration change the pH?
In this equation only the ratio matters, but a higher total concentration gives more buffer capacity. The optional boxes split a total into moles of base and acid.
Sources
Checked
- OpenStax Chemistry 2e, 14.6 Buffers (Henderson-Hasselbalch equation)
- OpenStax Chemistry 2e, 14.2 pH and pOH
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.