A Bicarbonate or, more properly, a hydrogen carbonate is a polyatomic ion whose formula is HCO₃^-. It is the intermediate form in the deprotonation of carbonic acid: removing the first proton from carbonic acid forms bicarbonate; removing the second proton leads to the carbonate ion.

The salts which contain the bicarbonate ion are also known as bicarbonates, such as sodium bicarbonate (baking soda) NaHCO₃. Bicarbonates, when exposed to an acid such as acetic acid of vinegar, release carbon dioxide. This is used to cause breads to rise in cooking and to propel toy rockets.

The bicarbonate/carbonate ionic system is also a buffer in blood.

Bicarbonates are more correctly named hydrogen carbonates in the chemical nomenclature system. Occasionally they are referred to as "acid carbonates".

Formation of Bicarbonates

Many carbonate compounds will absorb additional carbonate ions by the addition of carbonic acid into a solution of the carbonate. For example, sodium carbonate becomes sodium bicarbonate by the following reaction:

Na₂CO₃ + H₂CO₃ → 2(NaHCO₃)

Since carbonic acid is formed by dissolving carbon dioxide in water, conversion can be performed by simply bubbling the gas into a solution of the carbonate. Conversely, most bicarbonates will become carbonates by releasing the excess carbon dioxide from a solution of the bicarbonate.

The traditional compound therefore became called bicarbonate (literally "two-carbonate") because of the apparent doubling of the quantity of carbonate ions within the compound.

The simplicity of this reaction is very important in nature. For example, speleothems are formed in caves when carbon dioxide in the air reacts temporarily with limestone (calcium carbonate) to form soluble calcium bicarbonate. Later, within the cave, the carbon dioxide escapes the solution, redepositing the calcium carbonate in the form of a hard deposit.

See also

• carbon dioxide
• carbonic acid
• carbonate