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What does this calculator estimate?
A buffer solution calculator applies the Henderson–Hasselbalch equation. Enter the weak acid's pKa and the acid and conjugate base concentrations to get the buffer pH.
- pH = pKa + log₁₀([A⁻]/[HA])
- Buffer resists pH change on acid/base addition
- Most effective within ±1 pH of pKa
How buffers hold pH steady
A buffer pairs a weak acid with its conjugate base. Added acid is consumed by the base, added base by the acid, so pH changes only slightly. The Henderson–Hasselbalch equation links the ratio of the pair to the resulting pH.
Limitations to watch for
The equation assumes the weak acid approximation holds — dilute solutions and very strong acids/bases deviate. It gives pH, not capacity: a buffer with tiny concentrations resists less than a concentrated one.
How to use it in practice
Choose a weak acid with pKa within ±1 of your target pH. Adjust the ratio [A⁻]/[HA] to fine-tune (equal concentrations give pH = pKa). For real experiments, verify with a calibrated pH meter.
["Enter the weak acid's pKa.", 'Enter [HA] (acid) and [A⁻] (conjugate base) concentrations.', 'The tool computes pH = pKa + log([A⁻]/[HA]).']
What a buffer does
A buffer resists pH change when acid or base is added. The Henderson-Hasselbalch equation relates pH to the acid/base ratio: pH = pKa + log([A−]/[HA]). The calculator applies it to prepare buffers at a target pH.
The Henderson-Hasselbalch equation
For a weak acid and its conjugate base, pH = pKa + log(base/acid). To make a pH 4.5 buffer from acetic acid (pKa 4.76), the base/acid ratio is 10^(4.5 − 4.76) ≈ 0.55. The calculator returns the ratio and the volumes for your molarity.
Buffer capacity matters
Buffers work best within ±1 pH of the pKa, where the acid/base ratio is near 1. Outside that range, capacity drops sharply. The calculator flags when a target pH is too far from the chosen buffer's pKa.
A worked example
Preparing 500 mL of 0.1 M acetate buffer at pH 4.5: with pKa 4.76, the ratio [acetate]/[acetic acid] ≈ 0.55, so about 0.035 M acetate and 0.065 M acetic acid. The calculator returns the masses or volumes for your reagents.
Practical buffer prep
Weigh the acid and salt, dissolve, adjust pH with strong acid/base, then bring to final volume. Temperature shifts pKa slightly, so prepare at the working temperature when precision matters.
How this calculator works
Formula
Henderson–Hasselbalch: pH = pKa + log₁₀([A⁻]/[HA]), where [A⁻] is the conjugate base concentration and [HA] the weak acid concentration.
Worked example
An acetate buffer with pKa 4.76, [A⁻] = 0.10 M and [HA] = 0.05 M: pH = 4.76 + log₁₀(2.0) ≈ 5.06.
Assumptions to verify
- The weak acid approximation applies.
- The buffer is in aqueous solution at ~25°C.
- Concentrations are the equilibrium values.
Frequently asked questions
What is a buffer?
A solution of a weak acid and its conjugate base that resists pH change when small amounts of acid or base are added.
What is the Henderson–Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/[HA]). It links the buffer's pH to the ratio of base to acid forms.
How do I choose a buffer for a target pH?
Pick a weak acid whose pKa is within about 1 unit of the target. At equal acid/base concentrations, pH = pKa.
What is pKa?
The pH at which the weak acid is half-dissociated — the acid's strength constant expressed as a log. Acetic acid has pKa 4.76.
Does the equation work for any acid?
It's most accurate for weak acids near their pKa. Very dilute or very concentrated solutions deviate.
What's the difference between pH and buffer capacity?
pH is the current acidity; capacity is how much acid/base the buffer can absorb before pH shifts significantly.
Why is pH = pKa at equal concentrations?
When [A⁻] = [HA], the log term is log(1) = 0, so pH = pKa. This is the buffer's midpoint.
What is a buffer solution?
A solution that resists pH change — a weak acid/base pair near its pKa.
What is the Henderson-Hasselbalch equation?
pH = pKa + log([conjugate base]/[acid]) — the buffer ratio formula.
How do I make a buffer at a specific pH?
Choose a weak acid with pKa within ±1 of the target, then mix to the ratio the equation gives.
Why does buffer capacity drop?
Far from the pKa, the acid/base ratio is lopsided and the buffer can't absorb much added acid or base.
Cite this tool
BoringToolsKit. “Buffer Solution Calculator.” boringtoolskit.com/buffer-solution-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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