Bronsted's definitions of acid and base:
Bronsted's definition is based on the chemical reactions that occur when both acids and bases are added with each other. In Bronsted's theory acid donates proton, while base accepts proton from acid resulting in the formation of water.
Example: Consider the following reaction.
Hydrogen chloride donates a proton, and hence it is a Bronsted acid. Ammonia accepts a proton, and hence it is a Bronsted base.
All the species that is present in a phosphoric acid solution
Which species can act as a Brønsted acid, which as a Brønsted base, and which as both a Brønsted acid and a Brønsted base.