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Buffer pH, Acid–Base Ratio & pKa

Solve the Henderson–Hasselbalch relationship between weak-acid buffer pH, pKa and the conjugate-base-to-acid concentration ratio.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

With pKa 5 and equal acid and conjugate-base concentrations, estimated pH is 5. Increasing the base-to-acid ratio to 10 raises the estimate to 6.

pH = pKa + log₁₀([conjugate base]/[weak acid]); pKa = −log₁₀(Ka)

Worked example

Enter these known values and leave the other values blank.

Conjugate-base concentration
0.1 M
Weak-acid concentration
0.1 M
Acid dissociation constant
0.00001
Estimated buffer pH
5
Acid pKa
5

Assumptions and limitations

  • Both concentrations and Ka must be positive. Use an acid and its conjugate base at equilibrium under common conditions.
  • The displayed concentration formula approximates activity ratios. Its suitability depends on the solution; entering arbitrary concentrations does not establish a valid buffer model.
  • This tool does not perform neutralization stoichiometry, determine buffer capacity or account for activity coefficients. Calculate any prior reaction and dilution before entering concentrations.

Common questions

Can I calculate the base concentration for a target pH?

Yes. Enter target pH, acid concentration and pKa; leave the conjugate-base concentration blank.

Can I enter pKa instead of Ka?

Yes. They are linked by pKa = −log₁₀(Ka), so either value can determine the other.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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