Buffer pH Calculator
Calculate the pH of a buffer from the pK_a of the weak acid and the concentrations of its conjugate base and acid — the Henderson–Hasselbalch equation.
The pH a buffer settles at, from its pK_a and the acid-to-base ratio.
How the buffer ph calculator works
A buffer is a weak acid with its conjugate base. Its pH is pK_a + log₁₀([A⁻] ÷ [HA]): equal amounts give pH = pK_a; ten times more base adds one pH unit. The equation assumes the concentrations are moderate and the ratio is between about 0.1 and 10, which is also where a buffer works well.
Common pK_a values: acetic acid 4.76, phosphate (H₂PO₄⁻/HPO₄²⁻) 7.21, ammonium 9.25, carbonic acid 6.35, Tris 8.07.
Formula: pH = pK_a + log₁₀([A⁻] ÷ [HA])
Worked examples
| Inputs | Buffer pH | Note |
|---|---|---|
| Acetate buffer, equal concentrations | 4.76 | pH = pK_a = 4.76 |
| Acetate, base 0.2 M, acid 0.05 M | 5.362 | +0.60 → pH 5.36 |
| Phosphate buffer, 0.15 M / 0.05 M | 7.687 | pH 7.69 |
FAQFrequently asked questions
Which acid should I choose for a target pH?
One with a pK_a within about one unit of the target — buffers work best between pK_a − 1 and pK_a + 1.
Does the total concentration matter?
Not for the pH (only the ratio does) but it sets the buffer capacity: a 0.5 M buffer absorbs five times as much acid or base as a 0.1 M one at the same pH.
Why does my measured pH differ slightly?
Activity effects and temperature shift pK_a; the equation is an approximation that is excellent for planning and usually within 0.1 pH of a meter.
Can I use moles instead of concentrations?
Yes — both species are in the same volume, so the ratio is the same.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.