About this tool
Work out the pH of a buffer solution from the Henderson-Hasselbalch equation, or rearrange it to find the required base-to-acid ratio or the pKa. You can enter the acid strength as pKa directly or as Ka, and the tool converts it. Everything is computed locally in your browser.
The equation is pH = pKa + log10([A⁻] / [HA]), where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. Rearranged, the ratio [A⁻]/[HA] = 10^(pH − pKa) and pKa = pH − log10([A⁻]/[HA]). When Ka is supplied, pKa = −log10(Ka). Concentrations must be positive.
Pick what to solve for, enter the known values, and the buffer pH, the base/acid ratio and the pKa are all shown. This is useful when preparing biological buffers such as phosphate or acetate near their effective pH range.
Frequently asked questions
What is the Henderson-Hasselbalch equation?
It relates buffer pH to acid strength and composition: pH = pKa + log10([A⁻]/[HA]). [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. It is most accurate when the ratio is between about 0.1 and 10.
Can I enter Ka instead of pKa?
Yes. Switch the toggle to Ka and the tool converts it with pKa = −log10(Ka). Ka must be a positive number for the logarithm to be defined.
How do I find the ratio for a target pH?
Choose to solve for the ratio and enter the pH and pKa. The tool returns [A⁻]/[HA] = 10^(pH − pKa), which tells you the mole ratio of conjugate base to weak acid needed.
Is this a substitute for measuring pH?
It is a close estimate that assumes ideal behaviour and ignores activity effects and ionic strength. For precise work always confirm the final buffer with a calibrated pH meter. Calculations run only in your browser.
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