Buffer pH Calculator (Henderson–Hasselbalch)
Enter concentrations (or moles in the same final volume) for a weak acid and its conjugate base.
What this pH of buffer solution calculator does
This calculator estimates the pH of a buffer made from a weak acid (HA) and its conjugate base (A−). It uses the Henderson–Hasselbalch equation, which is the most common quick method in chemistry labs, classwork, and formulation work.
A buffer resists changes in pH when small amounts of acid or base are added. That is why buffers are essential in biology, pharmaceuticals, analytical chemistry, and environmental testing.
Formula used
The calculator uses:
pH = pKa + log10([A−] / [HA])
- pH = acidity/basicity of the solution
- pKa = acid strength parameter for the weak acid
- [A−] = conjugate base concentration (or moles in same volume)
- [HA] = weak acid concentration (or moles in same volume)
If you enter Ka instead of pKa, the calculator converts using pKa = -log10(Ka).
How to use this calculator
1) Enter acid and base amounts
Input positive values for both HA and A−. You can use concentrations (M) or moles, as long as both are in consistent terms.
2) Enter pKa (or Ka)
You can provide either pKa directly, or Ka if that is what your source gives. pKa is most common in textbook and lab-table usage.
3) Click “Calculate pH”
The tool returns pH, pOH, and a short interpretation. It also shows whether your base-to-acid ratio is in the practical buffer zone.
Worked examples
Example 1: Acetate buffer
Suppose you have acetic acid/acetate with pKa = 4.76, [A−] = 0.10 M, [HA] = 0.10 M. Ratio = 1, so log10(1) = 0, and pH = 4.76.
Example 2: Base-rich mixture
If pKa = 6.35, [A−] = 0.20 M, [HA] = 0.05 M, ratio = 4. Then pH = 6.35 + log10(4) ≈ 6.35 + 0.60 = 6.95.
When this method is accurate
- Weak acid/conjugate base pair is correctly identified
- Solution is not extremely dilute
- Ionic strength effects are modest (activities close to concentrations)
- Temperature is near the pKa reference value
For very high precision work, use activity corrections and full equilibrium calculations.
How to design a buffer for target pH
Rearranging the equation gives:
[A−]/[HA] = 10^(pH - pKa)
This lets you choose the needed base/acid ratio for a target pH. In practice, buffers work best when pH is within about ±1 of pKa.
Common mistakes to avoid
- Using a strong acid/strong base pair as if it were a buffer pair
- Mixing units (for example, moles for one input and molarity for the other)
- Entering zero or negative values
- Ignoring that pKa changes with temperature and medium
Quick FAQ
Can I use moles instead of concentration?
Yes, as long as both acid and base are in the same final volume. The ratio is what matters.
What if my ratio is far above 10 or below 0.1?
The mixture may have weak buffering capacity and can deviate from ideal behavior.
Does this include strong acid/base additions?
No. This specific calculator estimates pH from existing buffer composition. Additional stoichiometric steps are needed when strong acid or base is added.
Bottom line
This pH of buffer solution calculator provides a fast, reliable estimate for everyday chemistry problems and lab preparation. Use it for planning, checking homework, and quick formulation decisions—then move to full equilibrium methods when precision is critical.