Ionizable

Ionizable is an adjective used in chemistry to describe a substance that can form ions under certain conditions, typically by losing or gaining protons (protonation or deprotonation) or, less commonly, by undergoing redox ionization. In aqueous solution, ionizable species participate in acid-base equilibria of the form HA ⇌ H+ + A−. The position of this equilibrium is characterized by the pKa, the pH at which half of the species are ionized. Substances with low pKa acids ionize readily at physiological pH; substances with high pKa remain largely unionized until more basic conditions.

Common examples include carboxylic acids (R-COOH) with pKa around 4–5, which deprotonate to R-COO−; and amines (R-NH3+) with pKa around 9–11, which can be protonated to R-NH3+. Phenols and some thiols are also ionizable but with higher pKa values. In biology, many amino acid side chains (such as aspartate, glutamate, lysine, histidine, cysteine) are ionizable, influencing protein charge, structure, and function. The overall charge at a given pH affects solubility, binding, and membrane permeability, a central consideration in drug design and pharmacokinetics.

Ionizable groups also occur on materials and surfaces, such as silanol groups on oxides, which can deprotonate with increasing pH. Ionization is typically assessed by titration or spectroscopic methods and is described using Henderson-Hasselbalch or similar equations.