Buffer pH and Henderson–Hasselbalch calculations
A buffer contains a weak acid and its conjugate base, or a weak base and its conjugate acid, and resists modest pH changes. The Henderson–Hasselbalch equation relates pH to pKa and the component concentration ratio.
The equation is most useful when both buffer components are present in meaningful amounts and activities are reasonably approximated by concentrations. Final laboratory pH should be measured and adjusted under the actual temperature and ionic conditions.
How to use the buffer solution calculator
- Select the unknown: Choose pH, pKa, conjugate-base concentration, or weak-acid concentration.
- Enter three values: Use post-mixing concentrations in the same units.
- Calculate: Generate the missing value with the Henderson–Hasselbalch relationship.
- Verify experimentally: Prepare safely, equilibrate at the working temperature, and measure final pH with a calibrated meter.
Formula and variables
Buffer pH equals acid pKa plus the base-10 logarithm of conjugate-base concentration divided by weak-acid concentration.
pH = pKa + log₁₀([A⁻]/[HA])- pH — Solution pH
- Negative base-10 logarithm of hydrogen-ion activity
- pKa — Acid dissociation pKa
- Negative base-10 logarithm of Ka
- [A⁻] — Conjugate-base concentration
- Analytical concentration after mixing (mol/L)
- [HA] — Weak-acid concentration
- Analytical concentration after mixing (mol/L)
Equal-component acetate buffer
Acetic acid and acetate are each present at 0.10 mol/L, and pKa is 4.76.
- pKa
- 4.76
- [A⁻]
- 0.10 mol/L
- [HA]
- 0.10 mol/L
- pH = 4.76 + log₁₀(0.10/0.10)
- log₁₀(1) = 0
Result: The estimated pH is 4.76.
When acid and conjugate base concentrations are equal, pH equals pKa.
Understanding your results
Buffer ratio and range
A common useful buffer region is roughly pKa ± 1, corresponding to base-to-acid ratios from about 0.1 to 10.
- A larger [A⁻]/[HA] ratio raises pH.
- Equal component concentrations give pH = pKa.
- Buffer capacity also depends on total concentration, not only the ratio.
Assumptions
- Both conjugate components are present and concentrations are positive.
- Concentrations approximate activities sufficiently for the intended estimate.
- The entered pKa applies at the relevant temperature and medium.
Limitations
- Does not calculate ionic-strength activity corrections, temperature shifts, dilution volumes, or buffer capacity.
- Less accurate for very dilute solutions or ratios far outside the effective buffer range.
Common mistakes
- Using initial stock concentrations instead of concentrations after mixing.
- Reversing the base-to-acid ratio.
- Using natural logarithm instead of log base 10.
- Assuming calculated pH removes the need for measurement.
Practical use cases
Laboratory buffer planning
Estimate component ratios for biochemical and analytical procedures.
Acid-base education
Explore how pKa and conjugate-pair ratio determine buffer pH.
Frequently asked questions
When does pH equal pKa?
When conjugate-base and weak-acid concentrations are equal under the model assumptions.
Does total buffer concentration affect pH?
The ideal Henderson–Hasselbalch ratio controls pH, while total concentration strongly affects buffer capacity and non-ideal behavior.
Can I use different concentration units?
Yes, if both component concentrations use the same unit so their ratio is dimensionless.
Sources and review
- Buffer solution — IUPAC Compendium of Chemical Terminology. Accessed 2026-07-13.
- Chemistry 2e: Buffers — OpenStax. Accessed 2026-07-13.
Reviewed 2026-07-13.