Definitions
Three different definitions of acids and bases exist. The Brønsted-Lowry definition is the most
common, although you should be aware of the other two as well.
ARRHENIUS DEFINITION
The first definitions of acids and bases were formulated by Svante Arrhenius toward the end of the
19th century. Arrhenius defined an acid as a species that produces H+ (protons) in an aqueous
solution, and a base as a species that produces OH− (hydroxide ions) in an aqueous solution. These
definitions, though useful, fail to describe acidic and basic behavior in nonaqueous media.
BASIC CONCEPT
Arrhenius defined an acid as a species that produces H+ in an aqueous solution, and a base as
a species that produces OH− in aqueous solution.BRØNSTED-LOWRY DEFINITION
A more general definition of acids and bases was proposed independently by Johannes Brønsted
and Thomas Lowry in 1923. A Brønsted-Lowry acid is a species that donates protons, while a
Brønsted-Lowry base is a species that accepts protons. For example, NH 3 and Cl− are both Brønsted-
Lowry bases because they accept protons. However, they cannot be called Arrhenius bases since in
aqueous solution they do not dissociate to form OH−. Another advantage of the Brønsted-Lowry
concept of acids and bases is that it is not limited to aqueous solutions.