Recall that a Bronsted-Lowry acid donates a proton. From the equation, we can see that becomes , which it lost an , or a proton. Thus, water must be the Bronsted-Lowry acid.

There are three main definitions for acids and bases: the Lewis acids and bases, Bronsted-Lowry acids and bases, and Arrhenius acids and bases.

A Lewis acid is a species which can accept an electron pair from a donor. An example of a Lewis acid is ; as a cation, it can accept an electron pair. A Lewis base is a species which can donate an electron pair. An example of a Lewis base is ; as an anion, it can donate an electron pair.

A Bronsted-Lowry acid is a species which can donate a proton. An example of a Bronsted-Lowry acid is , which can donate its hydrogen ion. A Bronsted-Lowry base is a species which can accept a proton from a donor. An example of a Bronsted-Lowry base is , which can accept a hydrogen ion in order to become water.

An Arrhenius acid is a species which releases hydrogen ions into solution. An example of an Arrhenius acid is . When this compound dissolves in aqueous solution, it produces and . An Arrhenius base is a species which releases hydroxide ions into solution. An example of an Arrhenius base is . When this compound dissolves in aqueous solution, it produces and .

Recall that a Lewis base accepts a proton, while a Lewis acid donates a proton. Looking at the equation, we can see that becomes . Since accepted a proton, it must be the Lewis base.

PH is defined as , where is hydronium concentration). In order to do this we simply take :

=

Remember a Brønsted-Lowry acid is defined as a proton donor.

In the forward reaction, is a proton donor. In the backward reaction, is the proton donor.

The acids in this reaction are and .