For bases, the concentration of OH – must exceed the concentration of H 3O + in the solution. This imbalance can be created in two different ways.
First, the base can be a hydroxide, which merely dissociates to yield hydroxide ions:
where M represents the cation, usually a metal. The most familiar bases are such hydroxides. (See Table 1.)
The second type of base acts by extracting a hydrogen ion from a water molecule, leaving a hydroxide ion:
An example of this second type of base that is not a hydroxide can be an ammonia molecule in water (aqueous ammonia):
Ammonia acts as a base by stripping a proton from a water molecule, leaving an increased OH – concentration. Notice in the equilibrium reaction that 
and NH 3 are a conjugate acid‐base pair, related by transferring a single proton. Similarly, water acts as an acid by donating a proton to ammonia. H 2O and OH – are a conjugate acid‐base pair, related by the loss of a single proton.
Alternatively, the base may be a particular kind of negative ion with a high attraction for a hydrogen ion:
In 1923, the English chemist Thomas Lowry and the Danish chemist Johannes Br??nsted defined an acid and base in another way. An acid is a substance that can donate a proton, and a base is a substance that can accept a proton.
- The bicarbonate ion
may serve as either a Br??nsted‐Lowry acid or base. When it acts as an acid, what is its conjugate base? When it behaves as a base, what is its conjugate acid?