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Acid pKa from Ka or Buffer Measurements

Calculate pKa from a positive acid dissociation constant or estimate it separately from buffer pH and component concentrations.

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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

Ka = 0.00001 gives pKa = 5. A buffer at pH 5 with equal acid and conjugate-base concentrations also gives an estimated pKa of 5.

pKa = −log₁₀(Ka); buffer estimate = pH − log₁₀([base]/[acid])

Worked example

Enter these known values and leave the other values blank.

Measured buffer pH
5
Conjugate-base concentration
0.1 M
Weak-acid concentration
0.1 M
Acid dissociation constant
0.00001
pKa estimated from the buffer
5
pKa calculated from Ka
5

Assumptions and limitations

  • The two sections are independent. Entered Ka does not automatically replace the estimate calculated from buffer pH and concentrations.
  • Ka and both concentrations must be positive; the buffer-estimate section retains a positive-pH domain. A valid pKa can be negative when Ka is greater than one.
  • The concentration-ratio approximation applies to an appropriate weak-acid/conjugate-base buffer with suitable activity behavior. Use equilibrium concentrations at the same temperature.

Common questions

Why can the two pKa results disagree?

The sections use independently entered data. Measurement conditions, activity effects, temperature or inconsistent inputs can make a buffer estimate differ from a supplied equilibrium constant.

Does lower pKa mean a stronger acid?

For comparable acid dissociation reactions under the same conditions, lower pKa corresponds to larger Ka and stronger dissociation.

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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