pKavärden

pKavärden, commonly written as pKa values, are a standard way to express the acidity of a substance in solution. They are the negative base-10 logarithms of the acid dissociation constant (Ka) for an acid HA that dissociates in water: HA ⇌ H+ + A−, and pKa = −log10 Ka. A lower pKa indicates a stronger acid, while a higher pKa indicates a weaker acid. For a monoprotic acid, the pKa is the pH at which half of the acid molecules are deprotonated.

In practice, pKa values are used with the Henderson–Hasselbalch equation to relate pH, the acid’s pKa, and the ratio of deprotonated to protonated forms: pH = pKa + log([A−]/[HA]). This relation helps predict buffer behavior and the ionization state of molecules in different pH environments. Polyprotic acids have multiple pKa values corresponding to successive deprotonation steps.

pKa values depend on conditions. They are typically measured in water at 25°C and a defined ionic strength, but they shift with temperature, solvent, and ionic strength. In non-aqueous solvents, pKa scales differ and some compounds change their acid–base behavior. Common examples include acetic acid with pKa ≈ 4.76, ammonium (the conjugate acid of ammonia) with pKa ≈ 9.25, and carbonic acid with pKa1 ≈ 6.3 and pKa2 ≈ 10.3 under standard conditions. HCl is a very strong acid with a highly negative pKa, while many biological compounds have pKa values near physiological pH, influencing solubility, permeability, and enzyme interactions.

pKavärden are central in chemistry, biochemistry, and pharmacology for describing how ionization state changes with pH and how this affects molecular behavior.