pKa

pKa is the negative base-ten logarithm of an acid’s equilibrium dissociation constant, Ka. Lower pKa values indicate stronger acids in a given solvent and standard state.

Acid dissociation constant: An equilibrium constant describing an acid’s dissociation into its conjugate base and a proton. pKa is the negative logarithm of this equilibrium constant.
Chemical equilibrium: A dynamic state in which forward and reverse reactions proceed at equal rates. An acid’s Ka describes the equilibrium reached during dissociation.
Acid–base titration: A quantitative method that determines an analyte’s concentration by adding a reagent of known concentration. A titration curve can reveal an acid’s pKa at its half-equivalence point.
Acid strength: A measure of how readily an acid donates a proton under specified conditions. pKa quantifies acid strength through an equilibrium constant, rather than defining the broader property.
Henderson–Hasselbalch equation: An equation relating solution pH, an acid’s pKa, and the ratio of conjugate base to acid concentrations. It uses pKa to estimate acid–base composition in buffer solutions.
Equilibrium constant: A ratio of product to reactant activities at equilibrium, each raised to its stoichiometric power. Ka is an equilibrium constant, and pKa transforms its scale.
Isoelectric point: The pH at which a molecule has zero net electrical charge. pKa values of ionizable groups help determine this point for amino acids and proteins.
pKb: The negative base-ten logarithm of a base’s equilibrium constant for reaction with water. It expresses base equilibria, whereas pKa expresses acid dissociation.
Conjugate acid–base pair: Two species that differ by one proton, with one acting as an acid and the other as its base. The equilibrium behind pKa converts an acid into its conjugate base.
Logarithm: The exponent to which a base must be raised to produce a given number. The base-ten logarithm turns widely varying Ka values into manageable pKa values.

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