pKa Dissociation Calculator
The acid dissociation constant Ka measures how completely an acid HA dissociates in water: HA is in equilibrium with H+ and A-; Ka = [H+][A-]/[HA]. The pKa = -log10(Ka) scale makes it easier to compare acidities across a huge range. This calculator converts between Ka and pKa, and applies the Henderson-Hasselbalch equation to calculate the fraction of the compound that is ionized at a user-specified pH. This is essential for understanding solubility, membrane permeability, and reactivity of organic acids and bases as a function of pH.
pKa formula
pKa = -log10(Ka)
Ka = 10^(-pKa)
Fraction ionized = 1 / (1 + 10^(pKa - pH)) x 100
The Henderson-Hasselbalch equation: pH = pKa + log10([A-]/[HA]). At pH = pKa, [A-] = [HA] and the fraction ionized is 50%. Each unit of pH above pKa shifts the equilibrium 10-fold toward the ionized form.
pKa values for common organic functional groups
- Carboxylic acids (RCOOH): pKa 3 to 5 (formic 3.74, acetic 4.76, benzoic 4.20).
- Ammonium ions (RNH3+): pKa 9 to 11 (methylamine conjugate acid 10.64).
- Phenols (ArOH): pKa 8 to 10 (phenol 9.99).
- Thiols (RSH): pKa 10 to 11 (cysteine side chain 8.18).
- Imidazolium (histidine): pKa approximately 6.0, near physiological pH.
pKa dissociation: frequently asked questions
What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant Ka: pKa = -log10(Ka). A lower pKa means a stronger acid (larger Ka, more dissociation). A higher pKa means a weaker acid. For a base, the pKa refers to its conjugate acid.
How does pH relate to pKa?
The Henderson-Hasselbalch equation relates them: pH = pKa + log10([A-]/[HA]). At pH = pKa, the acid is exactly 50% ionized. At pH one unit above pKa, 91% is ionized; at pH one unit below, only 9% is ionized.
What are typical pKa values for organic functional groups?
Carboxylic acids: pKa 3 to 5 (acetic acid 4.76). Phenols: pKa 8 to 10. Ammonium ions: pKa 9 to 11 (ammonia 9.25). Alcohols: pKa 15 to 18 (ethanol 15.9). Alkanes C-H: pKa approximately 50. Water: pKa 15.7.
What is the fraction ionized formula?
For an acid HA, fraction ionized at pH = 1 / (1 + 10^(pKa - pH)). At pH below pKa, the compound is mostly protonated (HA form). At pH above pKa, mostly deprotonated (A- form). This applies to simple monoprotic acids.
How is pKa measured in the laboratory?
Potentiometric titration is the most common method: titrate the acid with NaOH and record the pH at the half-equivalence point, where pH = pKa. Spectrophotometric methods work for chromophoric acids where the ionized and neutral forms have different UV/Vis spectra.
Official sources
- NIST: NIST Standard Reference Database (acid dissociation constants).
- IUPAC: IUPAC Compendium of Chemical Terminology (Gold Book).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.