2HCO3

2HCO3 denotes two bicarbonate ions (HCO3−). It is not a discrete neutral molecule, but a stoichiometric quantity that appears in chemical equations and in solution containing bicarbonate as part of the carbonate system.

Bicarbonate is a key component of the carbonic acid–bicarbonate buffering system in aqueous solutions. In water, carbon dioxide readily forms carbonic acid (H2CO3), which can dissociate to bicarbonate: H2CO3 ⇌ HCO3− + H+. Bicarbonate can further dissociate: HCO3− ⇌ CO3^2− + H+. At 25°C, the pKa values are about 6.3 for the H2CO3/HCO3− couple and about 10.3 for the HCO3−/CO3^2− couple. These equilibria allow two bicarbonate units to participate in buffering and pH regulation, particularly in blood and other bodily fluids.

In physiological contexts, bicarbonate helps maintain acid–base balance. In human blood, carbon dioxide is transported as bicarbonate through red blood cells and plasma, a process coupled with the chloride shift. In geology and environmental science, the dissolution of CO2 in water forming Ca(HCO3)2 can keep calcium carbonate in solution; when CO2 pressure or temperature changes, Ca(HCO3)2 may precipitate as CaCO3, contributing to sedimentary and cave formations.

Commonly encountered pairs or solutions include salts such as sodium bicarbonate (NaHCO3) and potassium bicarbonate (KHCO3). The concept of “2HCO3” is also used in stoichiometric equations for neutralization, gas absorption, and carbonate chemistry in natural waters and industrial processes. Safety and handling are typical for mild bicarbonate solutions, which are broadly regarded as low-toxicity.