Buffer pH Calculator
Free buffer pH calculator using the Henderson-Hasselbalch equation. Enter pKa and the acid and conjugate base concentrations to find your buffer's pH instantly.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
Uses the Henderson-Hasselbalch approximation, valid when both species are present in comparable, non-trivial amounts. You may enter concentrations or moles (the ratio is what matters).
How the Buffer pH Calculator Works
A buffer resists changes in pH because it contains both a weak acid and its conjugate base (or a weak base and its conjugate acid). This calculator uses the Henderson-Hasselbalch equation to find the buffer’s pH from the pKa and the concentrations of the two species.
The Formula
For an acidic buffer:
pH = pKa + log₁₀([A⁻] / [HA])
where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. For a basic buffer:
pOH = pKb + log₁₀([BH⁺] / [B]), then pH = 14 − pOH
Because the equation depends only on the ratio of the two species, you can enter concentrations or moles interchangeably.
Worked Example
For an acetate buffer with pKa = 4.74, [A⁻] = 0.10 M acetate and [HA] = 0.10 M acetic acid:
- pH = 4.74 + log(0.10 / 0.10)
- = 4.74 + log(1)
- = 4.74 + 0 = 4.74
When the two concentrations are equal, the pH equals the pKa exactly — the center of the buffer’s range.
Effect of the Ratio on pH
| [A⁻] : [HA] | log of ratio | pH (pKa = 4.74) |
|---|---|---|
| 1 : 10 | −1 | 3.74 |
| 1 : 1 | 0 | 4.74 |
| 10 : 1 | +1 | 5.74 |
A buffer works best within about one pH unit of its pKa, which corresponds to ratios between 1:10 and 10:1.
Practical Tips
- Choose an acid whose pKa is close to your target pH for maximum buffering capacity.
- The buffer ratio, not the absolute concentration, sets the pH — but higher total concentration gives more capacity.
- Temperature shifts pKa slightly, so prepare and measure buffers at the temperature you will use them.
Frequently asked questions
What is the Henderson-Hasselbalch equation?+
The Henderson-Hasselbalch equation relates a buffer's pH to the ratio of its conjugate base to its weak acid: pH = pKa + log([A⁻]/[HA]). It lets you predict the pH of a buffer or work out the ratio of components needed to hit a target pH, as long as both species are present in meaningful amounts.
How do you calculate the pH of a buffer solution?+
Take the pKa of the weak acid and add the base-10 logarithm of the conjugate base concentration divided by the weak acid concentration. Because the formula uses a ratio, you can plug in either concentrations or moles. The result is the buffer's pH; subtract from 14 to get pOH.
What is the difference between pKa and pKb?+
pKa measures the strength of an acid (lower pKa means a stronger acid), while pKb measures the strength of a base. They are related by pKa + pKb = 14 for a conjugate acid-base pair at 25 °C. For an acidic buffer use pKa; for a basic buffer enter pKb and the calculator finds pOH first.
When does the Henderson-Hasselbalch equation break down?+
It is an approximation that assumes the weak acid and its conjugate base are present in comparable, non-negligible amounts and that the solution is reasonably dilute. It loses accuracy at very high or very low ratios, in extremely dilute buffers, or when activity effects in concentrated solutions become significant.
What is the pH when acid and base concentrations are equal?+
When the conjugate base and weak acid concentrations are equal, the ratio is 1 and its logarithm is 0, so pH equals pKa exactly. This is the point of maximum buffering capacity, which is why buffers are usually prepared near the pKa of the chosen acid.