Bicarbonate serves a crucial biochemical role in the physiological pH buffering system.
The bicarbonate ion (hydrogen carbonate) is an anion with the empirical formula HCO3− and a molecular mass of 61.01 daltons; it consists of one central carbon atom surrounded by three oxygen atoms in a trigonal planar arrangement, with a hydrogen atom attached to one of the oxygens. The bicarbonate ion carries a negative one formal charge and is the conjugate base of carbonic acid, H2CO3; it is the conjugate acid of CO32−, the carbonate ion as shown by these equilibrium reactions.
CO32− +2 H2O ⇋ HCO3− + H2O + OH− ⇋ H2CO3 +2 OH−
H2CO3 +2 H2O ⇋ HCO3− + H3O+ + H2O ⇋ CO32− +2 H3O+
A bicarbonate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound. Many bicarbonates are soluble in water at standard temperature and pressure, particularly sodium bicarbonate and magnesium bicarbonate; both of these substances contribute to total dissolved solids, a common parameter for assessing water quality.
Bicarbonate is an alkaline, and a vital component of the pH buffering system of the body (maintaining acid-base homeostasis). 86%-90% of CO2 in the body is converted into carbonic acid (H2CO3), which can quickly turn into bicarbonate (HCO3−).
With carbonic acid as the central intermediate species, bicarbonate, in conjunction with water, hydrogen ions, and carbon dioxide forms this buffering system which is maintained at the volatile equilibrium required to provide prompt resistance to drastic pH changes in both the acidic and basic directions. This is especially important for protecting tissues of the central nervous system, where pH changes too far outside of the normal range in either direction could prove disastrous. (See acidosis, or alkalosis.)
The flow of bicarbonate ions from rocks weathered by the carbonic acid in rainwater is an important part of the carbon cycle.
Bicarbonate also serves in the digestive system. It raises the internal pH of the stomach, after highly acidic digestive juices have finished in their digestion of food. Ammonium bicarbonate is used in digestive biscuit manufacture.