In the Brønsted-Lowry theory, an acid is defined as a substance that can donate a proton, and a base is a substance that can accept a proton.
The theory was introduced independently by the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry in 1923.
The formula for an acid-base reaction, according to the Brønsted-Lowry theory, goes as such:
HA + B ⇌ BH+ + A-
In this generalization, HA represents the Brønsted acid, B represents the Brønsted base, BH+ represents the conjugate acid, and A- represents the conjugate base. Note that the formula is written as an equilibrium rather than as a reaction.
While the Brønsted-Lowry theory does give an explanation more concise and accurate than the Arrhenius theory, it limits the bases that can fit the definition in accordance with its reaction summarization. Note that the base is merely generalized as “B”, thus defining it as a single component. Thus, this means that it can't be directly substituted with ionic compounds such as sodium hydroxide, as those are made up of two components, a cation and anion. Furthermore, those alkaline ionic compounds function by donating a hydroxide ion rather than directly accepting a hydrogen ion. However, the sole hydroxide ion does function as a Brønsted base.
Almost all acids can fit under the category of Brønsted acids, as most dissociate in water to release hydrogen ions, forming hydronium ions and conjugate bases. This includes both strong acids, such as sulfuric acid, nitric acid, and hydrochloric acid, in addition to weak acids, such as citric acid, acetic acid, and phosphoric acid.
Compared to Brønsted acids, there are a much more limited number of bases that can function as Brønsted bases. As explained above, all ionic compounds are eliminated from this category, as they do not accept a hydrogen ion in an acid-base reaction. Thus, bases like alkali metal hydroxides, by definition, aren't technically Brønsted bases as undissociated compounds. However, molecules like ammonia and pyridine are Brønsted bases, as they function by receiving a hydrogen ion to from a conjugate acid.
Some compounds are amphiprotic, meaning that they can function as both a Brønsted acid and a Brønsted base, depending on the reaction. For instance, water has such a property, as it self-dissociates to form H3O+ and OH- ions. Some ions also have such a property, including conjugate bases of polyprotic acids. For instance, hydrogen sulfate (HSO4-) can either accept a proton to form sulfuric acid or lose a proton to form sulfate (SO42-).