Acid and base dissociation constants
pKa is the negative base-10 logarithm of Ka, and pKb is defined analogously. For a conjugate acid–base pair in water, KaKb = Kw and pKa + pKb = pKw.
This calculator uses the common educational approximation pKw = 14 at 25 °C. Rigorous equilibrium constants depend on temperature, standard state, activity conventions, and ionic medium.
How to convert Ka, Kb, pKa, and pKb
- Choose the known quantity: Select Ka, Kb, pKa, or pKb.
- Enter the value: K values must be positive; pK values may be positive or negative.
- Calculate: Review all four values for the same conjugate pair.
- Check conditions: Use a temperature-specific pKw for work requiring more than the stated 25 °C approximation.
Formula and variables
Enter one value. The calculator converts the logarithmic form and uses Kw ≈ 1 × 10⁻¹⁴ for the conjugate partner.
pKa = −log10(Ka); pKb = −log10(Kb); pKa + pKb ≈ 14 at 25 °C- Ka — Acid dissociation constant
- Equilibrium constant for acid dissociation (dimensionless under thermodynamic convention)
- Kb — Base dissociation constant
- Equilibrium constant for conjugate-base hydrolysis (dimensionless under thermodynamic convention)
- pKw — Water ion-product exponent
- Negative logarithm of Kw (dimensionless)
Acetic-acid pKa example
Use pKa = 4.76 under the calculator’s 25 °C approximation.
- pKa
- 4.76
- Ka = 10⁻⁴·⁷⁶
- pKb = 14 − 4.76 = 9.24
- Kb = 10⁻⁹·²⁴
Result: Ka ≈ 1.74 × 10⁻⁵ and Kb ≈ 5.75 × 10⁻¹⁰.
The conjugate base is weak under these specified aqueous conditions.
Understanding your results
Smaller pKa means larger Ka
The logarithmic scale reverses direction: every one-unit pK decrease multiplies K by ten.
- Ka and Kb must refer to a true conjugate acid–base pair.
- The sum equals pKw, not universally the number 14.
- Conditional concentration constants can vary with ionic strength.
- Polyprotic species have separate constants for each dissociation step.
Assumptions
- The acid and base form one aqueous conjugate pair.
- Temperature is 25 °C and pKw is approximated as 14.
- Entered constants follow compatible equilibrium and standard-state conventions.
Limitations
- Does not adjust pKw for temperature or calculate activity coefficients.
- Does not solve pH, equilibrium composition, buffer capacity, or polyprotic speciation.
- The pKw = 14 relation is an educational approximation rather than an exact universal identity.
Common mistakes
- Pairing constants from species that are not conjugates.
- Using Ka + Kb instead of Ka × Kb.
- Assuming pKa + pKb is 14 at every temperature and solvent.
- Entering zero or a negative dissociation constant.
Practical use cases
Acid–base coursework
Convert logarithmic and ordinary dissociation constants.
Conjugate-pair comparison
Relate acid strength to conjugate-base strength under stated conditions.
Frequently asked questions
Can pKa be negative?
Yes. A sufficiently large Ka corresponds to a negative pKa.
Is Ka times Kb always 10⁻¹⁴?
For a conjugate pair it equals Kw, whose value depends on temperature and conventions; 10⁻¹⁴ is the common 25 °C approximation used here.
Does this calculate solution pH?
No. pH also depends on analytical concentration, equilibria, activities, and other species.
Sources and review
- Acid Dissociation Constant — IUPAC Compendium of Chemical Terminology. Accessed 2026-07-13.
Reviewed 2026-07-13.