Online Tool Store
Buffer pH Calculator
Calculate buffer pH from pKa and concentrations, or solve the base-to-acid ratio needed for a target pH, with common buffer presets.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
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Approximate 25°C values — verify against your reagent's certificate for precise work.
- Base : acid ratio
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- Effective range (pKa ± 1)
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How the Buffer pH Calculator works
- Pick a common buffer from the preset list to fill in its pKa, or enter your own.
- Choose whether to solve for pH from known concentrations, or for the base-to-acid ratio needed to hit a target pH.
- For pH mode, add the molar concentrations of the acid and its conjugate base.
- Check the result sits within the shown effective range (pKa ± 1); outside that range the buffer has little capacity left.
The method
Henderson-Hasselbalch relates buffer pH to the pKa and the ratio of conjugate base to acid, which is why dilution barely changes pH.
pH = pKa + log₁₀([A⁻] / [HA])
4.76 + log₁₀(0.15 / 0.10) = 4.76 + 0.176 = 4.94. Halving both concentrations leaves the ratio, and therefore the pH, unchanged.
FAQ
Why does dilution not change buffer pH?
Because the equation depends on the ratio of base to acid, and dilution changes both equally. Capacity falls, but pH holds — until the buffer is too dilute for activity effects to be ignored.
How far from the pKa can a buffer work?
About one pH unit either side. Beyond that the ratio becomes extreme and a small addition of acid or base swings the pH sharply.
Is this accurate for real solutions?
It assumes ideal behaviour and ignores ionic strength and activity coefficients. For precise work at higher concentrations, measure the pH rather than trusting the calculation.
Can it tell me what ratio I need for a target pH?
Yes — switch "Solve for" to the ratio mode, enter your target pH, and it returns the base-to-acid ratio via the rearranged equation [A⁻]/[HA] = 10^(pH − pKa).
Where do the common buffer pKa presets come from?
They are standard approximate values at 25°C for buffers used widely in biology and chemistry (acetate, MES, phosphate, HEPES, Tris, ammonium). Tris in particular shifts noticeably with temperature, so check your reagent's certificate of analysis for precise work.
How we compare
| Feature | Online Tool Store | Henderson-Hasselbalch calculator sites | A stats package |
|---|---|---|---|
| Shows the equation | ✓ | ✓ | ✓ |
| Solves for the needed ratio | ✓ | ✓ | ✓ |
| Common buffer pKa presets | ✓ | ✓ | Manual lookup |
| No software licence | ✓ | ✓ | ✗ |
Buffer pH Calculator uses the ideal Henderson-Hasselbalch form — for accurate work at real ionic strengths, the meter beats the equation.