pKa Calculator
Ka must be greater than 0.
What is pKa?
The pKa value is the negative base-10 logarithm of the acid dissociation constant Ka.
Definition:
pKa = -log10(Ka)
pKa = -log10(Ka)
The smaller the pKa, the stronger the acid.
- pKa < 0: very strong acids
- pKa 0–4: medium acids
- pKa > 4: weak acids
Essential for buffers and equilibria
Formulas
pKa from Ka:
pKa = -log10(Ka)
pKa = -log10(Ka)
Ka from pKa:
Ka = 10-pKa
Ka = 10-pKa
Henderson-Hasselbalch:
pH = pKa + log([A⁻]/[HA])
pH = pKa + log([A⁻]/[HA])
Rearranged:
pKa = pH - log([A⁻]/[HA])
pKa = pH - log([A⁻]/[HA])
Examples
Acetic acid
Ka = 1.8·10-5
pKa = -log(1.8·10-5) ≈ 4.74
Ka = 1.8·10-5
pKa = -log(1.8·10-5) ≈ 4.74
Formic acid
pKa = 3.75
Ka = 10-3.75 ≈ 1.78·10-4
pKa = 3.75
Ka = 10-3.75 ≈ 1.78·10-4
1:1 buffer
[A⁻]/[HA] = 1 → log(1)=0
Then: pH = pKa
[A⁻]/[HA] = 1 → log(1)=0
Then: pH = pKa
Buffer example
pH = 5.20; [A⁻]/[HA] = 2
pKa = 5.20 - log(2) = 4.90
pH = 5.20; [A⁻]/[HA] = 2
pKa = 5.20 - log(2) = 4.90
Technical Background
Acid strength and equilibrium
Ka describes the protolysis equilibrium: HA ⇌ H⁺ + A⁻. Larger Ka values mean stronger dissociation and therefore stronger acids.
Role in buffer systems
- Buffers work best at pH ≈ pKa
- Best buffer range is pKa ± 1
- pKa is central for choosing suitable buffer systems
Typical pKa values (25°C)
- HCl: about -6 (very strong acid)
- Formic acid: 3.75
- Acetic acid: 4.76
- Ammonium: 9.25 (conjugate acid of NH₃)
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