Buffer pH (Henderson–Hasselbalch) calculator with steps
Track every Henderson-Hasselbalch step for buffer pH. Convert HA/A⁻ inputs to moles, design target-pH recipes, and simulate strong acid or base additions.
Use it for class prep or lab prep. Review molar conversions, ratio r, and H-H assumptions in a shareable How it’s calculated log.
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The Henderson-Hasselbalch approximation requires both HA and A⁻ to remain present. Very small ratios (r < 0.01), very large ratios (r > 100), or extreme pH can break this assumption. If one component is depleted, use a full equilibrium solver.
β is computed as 2.303·CT·(Ka[H⁺])/(Ka+[H⁺])². You can optionally add the water term [H⁺]+Kw/[H⁺]. Set CT to the buffer’s total analytical concentration.
FAQ
What should I enter before solving buffer pH?
Enter the acid pKa and the conjugate base-to-acid amount ratio first. Use molarity or moles consistently so the Henderson-Hasselbalch ratio represents the same buffer mixture.
When is the Henderson-Hasselbalch estimate valid?
It works best for a conjugate acid-base pair with appreciable amounts of both forms, usually within about one pH unit of the pKa and away from extreme dilution or strong titration endpoints.
What if I know acid and base volumes instead of concentrations?
Convert each component to moles first, or use concentration times volume when both solutions are prepared separately. The ratio should compare total conjugate base amount with total weak acid amount.
Why can lab pH differ from the calculator?
Activity coefficients, temperature, ionic strength, electrode calibration, dilution, and added strong acid or base can move the measured pH away from the simple model.
Can I use this for final lab validation?
Use it for planning and checking calculations. Confirm final buffer pH with a calibrated meter when the solution is used for experiments, quality control, or regulated work.
Buffer pH calculation notes
Identify the conjugate pair
Choose the acid pKa that matches the weak acid and its conjugate base. A mismatched pKa is the most common reason a buffer estimate is off by a large amount.
Use amount ratios
The Henderson-Hasselbalch equation uses the ratio of conjugate base to weak acid. Concentrations work when final volumes are comparable; otherwise convert to moles before comparing.
Check buffer range
A buffer is most useful near its pKa. If the target pH is far away, a different buffer system or a full acid-base calculation may be more appropriate.
Common lab mistakes
Do not ignore dilution after mixing stock solutions. Do not combine pKa data from a different temperature or solvent system without checking whether the shift matters.
Result interpretation
Treat the displayed pH as a planning estimate. For real solutions, measure and adjust carefully because ionic strength and activity effects can matter.