All calculatorsBuffer pH Calculator
Compute buffer pH using the Henderson–Hasselbalch equation for acid (HA/A⁻) or base (B/BH⁺) buffers. Enter pK (or K) and component amounts.
Background
For an acid buffer, pH = pKa + log₁₀([A⁻]/[HA]). For a base buffer, pOH = pKb + log₁₀([BH⁺]/[B]), then pH = 14 − pOH. At typical lab concentrations, activities ≈ concentrations.
How to use this calculator
- Pick buffer type: acid (HA/A⁻) or base (B/BH⁺).
- Provide pK (or K): use pKa for acid buffers, pKb for base buffers.
- Enter amounts: either concentrations, or masses + molar masses + volume.
- Calculate: we compute pH (or pOH → pH) and show steps.
Formula & Equation Used
Acid buffer: pH = pKa + log₁₀([A⁻]/[HA])
Base buffer: pOH = pKb + log₁₀([BH⁺]/[B]), then pH = 14 − pOH
Concentration from mass: C = (m/M)/V
Example Problems & Step-by-Step Solutions
Example 1 — Acetate: pKa=4.76; [A⁻]=0.10 M, [HA]=0.10 M
pH = 4.76 + log(0.10/0.10) = 4.76.
Example 2 — Acetate: pKa=4.76; [A⁻]=0.050 M, [HA]=0.200 M
pH = 4.76 + log(0.050/0.200) = 4.16.
Example 3 — Ammonia: pKb=4.75; [B]=0.120 M, [BH⁺]=0.080 M
pOH = 4.75 + log(0.080/0.120) = 4.57 → pH = 9.43.
Frequently Asked Questions
Q: pKa or Ka? pKb or Kb?
Either works—enter pK or K and we handle the conversion.
Q: Activities vs concentrations?
At typical lab strengths, activity ≈ concentration. For high ionic strength, use activity coefficients.
Q: Temperature?
We assume 25 °C and pH + pOH ≈ 14. pK values are temperature-dependent.
