pKa Calculator
Convert between Ka and pKa and find the pH of half-dissociation.
About pKa Calculator
At pH equal to pK_a an acid is exactly half ionised, which is why pK_a is read directly off a titration curve at half equivalence. Two units either side of pK_a corresponds to about 99 per cent in one form. The ionisation state at physiological pH governs drug absorption and distribution, which is why pK_a is central to pharmacology.
Formula
pKa
pK_a = −log₁₀K_a; ionised fraction = 10^(pH−pK_a) ÷ (1 + 10^(pH−pK_a))
- half equivalence
- pH equals pK_a when exactly half the acid is dissociated
Worked example
With the default values below, pKa Calculator returns pKa of 4.7447. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Acid dissociation constantminimum 0
- 0.000018
- Solution pHaccepts -2–16
- 4.5
Results
- pKa
- 4.7447
- pH at Half Equivalence
- 4.7447
- Ratio of Conjugate Base to Acid at That pH
- 0.56921
- Percentage Ionised at That pH
- 36.274%
- Acid Strength
- Weak acid
Step by step
- 1Values used
- 2pKa
- 3pKa= 4.7447
- 4pH at Half Equivalence= 4.7447
- 5Ratio of Conjugate Base to Acid at That pH= 0.56921
- 6Percentage Ionised at That pH= 36.274
- 7Acid Strength= Weak acid
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is pKa calculated?
pK_a = −log₁₀K_a; ionised fraction = 10^(pH−pK_a) ÷ (1 + 10^(pH−pK_a)) At pH equal to pK_a an acid is exactly half ionised, which is why pK_a is read directly off a titration curve at half equivalence. Two units either side of pK_a corresponds to about 99 per cent in one form.
Why does pKa matter?
The ionisation state at physiological pH governs drug absorption and distribution, which is why pK_a is central to pharmacology.
What values do I need to enter?
This calculator takes 2 inputs: Acid dissociation constant, Solution pH. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.