pH & Buffer Designer
Henderson-Hasselbalch in real time. Pick a buffer system, set the conjugate-base and weak-acid concentrations, and watch the pH strip track. Switch to weak-acid mode for monoprotic pH from pKa alone.
How to use this tool
Predict the pH of a buffer before you make it, and see how the acid-to-base ratio sets it. The Buffer tab uses the Henderson-Hasselbalch equation; the Weak acid tab estimates the pH of a plain weak acid from its pKa.
What to enter
- Buffer system: choose a pairing (acetate, phosphate, Tris…); it loads the matching pKa.
- [Conjugate base]: molarity of the salt / deprotonated form (A⁻).
- [Weak acid]: molarity of the protonated form (HA).
- Weak-acid tab: instead enter a pKa and an initial concentration for a single weak acid.
Reading the result
The headline is the solution pH, drawn on a colour pH strip so you can see where it lands. The useful buffer range (pKa ± 1) tells you whether the buffer actually has capacity at your target, outside it, buffering is weak.
Worked example
With equal concentrations of conjugate base and weak acid (0.1 M each) the log term is zero, so pH = pKa, an acetate buffer sits at pH ≈ 4.76.
Result
The colour strip places your pH on the 0–14 scale at a glance. A buffer only resists pH change within about pKa ± 1, if your target sits outside the useful range shown, switch to a buffer system whose pKa is closer.
Methodology
Buffer pH uses the Henderson-Hasselbalch equation, pH = pKa + log₁₀([A⁻] / [HA]). Weak-acid pH uses the simplified pH ≈ ½(pKa − log C), valid when [H⁺] is small relative to the initial acid concentration (typically C ≫ 10·Ka).
Sources
- Henderson, L. J. (1908) J. Am. Chem. Soc. 30, 954.
- Hasselbalch, K. A. (1917) Biochem. Z. 78, 112.
- NIST pKa data for biological buffers.
Known limits
- Henderson-Hasselbalch breaks down outside pKa ± 2; outside that range the buffer has negligible capacity.
- The weak-acid approximation fails for very dilute (< 10⁻³ M) or very strong acids.