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Weak-Acid or Weak-Base Buffer pH

Relate buffer pH, conjugate-pair concentrations and acid/base equilibrium constants.

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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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Formula and method

Equal weak-acid and conjugate-base concentrations with pKa 5 give pH 5.

Acid pH = pKa + log10(salt/acid); base pH = 14 − pKb + log10(base/salt)

Worked example

Enter these known values and leave the other values blank.

Buffer type
Acid
pKa
5
Acid concentration
.1 M
Salt concentration
.1 M
Acid dissociation constant (Ka)
0.00001
pH
5

Assumptions and limitations

  • Use a weak acid/base and its conjugate partner at equilibrium. The concentration-ratio approximation neglects activity corrections and cannot describe strong-acid additions or buffer capacity by itself.
  • The base branch assumes pKw=14, appropriate approximately for dilute aqueous solution at 25 °C. Both members of the pair must have positive concentrations.

Common questions

What does this model assume?

Use a weak acid/base and its conjugate partner at equilibrium. The concentration-ratio approximation neglects activity corrections and cannot describe strong-acid additions or buffer capacity by itself.

Can I solve backward?

Clear the unknown value and enter compatible known values. Choices, one-way outputs and singular boundary cases cannot always be inverted.

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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