Buffer pH Calculator

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Buffer pH Calculator

Estimate the pH of an acidic or basic buffer from its dissociation constant and conjugate-pair concentrations using the Henderson – Hasselbalch relationship.

Acid/base mode: Acidic buffer Ratio: 1.000 Effective range: 3.76 – 5.76
Example workbook is ready to download.

Buffer inputs

Choose the conjugate pair present in the solution.
Use pKa/Ka for acidic buffers and pKb/Kb for basic buffers.
Enter a positive pKa value, typically between 0 and 14 for aqueous work.
mol/L
Required; enter a finite concentration greater than zero.
mol/L
Required; use the concentration in the same units as the other component.

Live result

Estimated buffer pH
4.760
The conjugate pair is equimolar, so pH equals pKa.
pKa
4.760
Concentration ratio
1.000
Log ratio term
0.000
Suggested working range
3.76 – 5.76
pH = 4.760 + log₁₀(0.100 / 0.100) = 4.760
Estimated pH 4.760.

Calculation detail

Quantity Symbol Value Role
Weak acid concentration [HA] 0.100 mol/L Denominator of the ratio
Conjugate base concentration [A⁻][A⁻]/[HA][HA] = 0.100 mol/L, and [A⁻][A⁻]/[HA] equals 1.000. Since log₁₀(1) = 0, pH = 4.76 + 0 = 4.760. If the conjugate base is doubled to 0.200 mol/L while acid remains 0.100 mol/L, the ratio becomes 2 and the logarithmic term is about 0.301, so the estimated pH rises to about 5.061.

Formula, assumptions, and interpretation

For an acidic buffer, the calculator uses pH = pKa + log₁₀([A⁻]/[HA]). For a basic buffer at 25 °C, it first estimates pOH with pOH = pKb + log₁₀([BH⁺]/[B]), then applies pH = 14 – pOH. The 14 relationship is temperature-dependent because it comes from the ion product of water; this implementation follows the common 25 °C convention.

The approximation is most informative when both members of the conjugate pair are present in meaningful amounts and the target pH lies within roughly one unit of the relevant pK. Chemistry LibreTexts discusses the effective buffer range and limitations of the Henderson – Hasselbalch approximation. At very low concentrations, or where ionic strength is high, activity-based equilibrium calculations can differ materially from this estimate.

Practical check: changing both concentrations by the same factor leaves the estimated pH unchanged because only their ratio appears in the equation. However, the real buffer capacity does change, which is why equal pH does not imply equal resistance to added acid or base.

Common mistakes

  • Entering pKa in K mode or Ka in pK mode.
  • Reversing numerator and denominator, which shifts pH in the wrong direction.
  • Using a strong acid/base pair that does not form a conventional buffer.
  • Assuming pK ± 1 guarantees adequate capacity without considering total concentration.
  • Applying pH + pOH = 14 without noting that the value is specifically appropriate near 25 °C.