Base (chemistry) In the Brensted- Lowry classification, any chemical species, ionic or molecular, capable of accepting or receiving a proton (hydrogen ion) from another substance. The other substance acts as an acid in giving up the proton. A substance may act as a base, then, only in the presence of an acid. The greater the tendency to accept a proton, the stronger the base. The hydroxyl ion acts as a strong base. Substances that ionize in aqueous solutions to produce the hydroxyl ion (OH), such as potassium hydroxide (KOH) and barium hydroxide [Ba(OH)z], are also conventionally called bases. Anions of weak acids such as acetic and formic, act as bases in reacting with solvent water to form the molecular acid and hydroxyl ion, for example, the acetate ion (CH3COO-). Ammonia (NH3) and amines react similarly in aqueous solutions. In these examples, the acetate ion and acetic acid (CH3COOH) and NH3 and the ammonium ion are conjugate base-acid pairs. The basicity constant, Kb, is the equilibrium constant for the proton transfer reaction, and it is a quantitative measure of base strength. The Lewis classification involves the concept of a base as a substance that donates an electron pair to an acid aceptor. In the gas phase, NH3 acts as a base contributing an electron pair to the formation of a convalent bond with the boron trifluoride (BF3) molecule.