What this calculator does
pKa is the negative base-10 logarithm of Ka. Acetic acid, with a Ka of 1.8×10⁻⁵, has a pKa of 4.745.
The log scale exists because Ka values span an enormous range, from around 10⁻² for the strongest weak acids to 10⁻¹⁶ for the weakest. Comparing 1.8×10⁻⁵ against 6.3×10⁻¹⁰ is awkward; comparing 4.745 against 9.2 is not. Note that the direction reverses: a smaller pKa means a stronger acid.
The formula
The negative base-10 logarithm of Ka is taken. Each whole unit of pKa corresponds to a factor of ten in Ka, so an acid with a pKa two units lower is a hundred times more dissociated at the same concentration.
| Term | Meaning |
|---|---|
| Ka | The acid dissociation constant, the equilibrium constant for proton loss. |
| pKa | The negative log of Ka. Lower means a stronger acid. |
| Half-dissociation | At pH equal to pKa, an acid is exactly half dissociated. |
| pKb | The base equivalent. For a conjugate pair, pKa plus pKb is 14 at 25 °C. |
The inputs explained
| Field | What to enter |
|---|---|
| Acid dissociation constant (Ka) | The acid dissociation constant, usually a very small number. Enter in scientific notation, such as 1.8e-5. |
When to use it
Comparing acid strengths
Ranking acids on a linear scale is far easier than comparing exponents.
Choosing a buffer
A buffer works within about one pH unit of its pKa, so the pKa determines which acid to pick.
Predicting ionisation at a given pH
A molecule is mostly ionised above its pKa and mostly neutral below it, which governs drug absorption and chromatographic behaviour.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What pKa does each Ka give?
A range of dissociation constants across the weak acid spectrum.
| Ka | pKa | Ka used | Note |
|---|---|---|---|
| 1.8e-3 | 2.745 | 0.0018 | A smaller pKa means a stronger acid (more dissociation at a given pH). |
| 1.8e-5 | 4.745 | 0.000018 | A smaller pKa means a stronger acid (more dissociation at a given pH). |
| 1e-7 | 7.000 | 1.0000e-7 | A smaller pKa means a stronger acid (more dissociation at a given pH). |
| 1e-10 | 10.000 | 1.0000e-10 | A smaller pKa means a stronger acid (more dissociation at a given pH). |
Questions
Does a lower pKa mean a stronger or weaker acid?
Stronger. A lower pKa means a larger Ka, which means more dissociation. This reversal catches people out constantly, because the smaller number indicates the stronger acid. Hydrochloric acid has a pKa around −6; water has one around 15.7.
What happens at pH equal to pKa?
The acid is exactly half dissociated, with equal concentrations of the acid and its conjugate base. This is the definition of pKa and follows directly from the Henderson-Hasselbalch equation, where the log term becomes zero.
How does pKa relate to pKb?
For a conjugate acid-base pair, pKa plus pKb equals 14 at 25 °C, because Ka times Kb equals the ion product of water. A weak acid therefore has a relatively strong conjugate base, and vice versa.
Can pKa be negative?
Yes, for strong acids. Hydrochloric acid has a pKa around −6 and sulfuric acid around −3 for its first proton. A negative pKa means Ka exceeds 1, so the acid is essentially completely dissociated in water and the value is difficult to measure precisely.
For buffer pH, see the Henderson-Hasselbalch calculator. For the pH of a weak acid solution, see the weak acid and base pH calculator.