HCO3

HCO3−, commonly called bicarbonate or hydrogen carbonate, is an anion formed when carbonic acid loses a proton. In aqueous solutions it is a central component of the carbonic acid–bicarbonate buffering system, which helps regulate pH in many biological and environmental contexts.

In water, carbon dioxide from the atmosphere or respiration forms carbonic acid: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−. The pKa of this system is around 6.3, making bicarbonate a major pH buffer near physiological pH. In human blood, the bicarbonate concentration and the partial pressure of CO2 jointly determine pH, commonly expressed by the Henderson–Hasselbalch equation: pH = pKa + log([HCO3−]/(0.03 × PCO2)).

Physiologically, the bicarbonate system is maintained by lungs and kidneys. The lungs modulate CO2 levels, while the kidneys reabsorb and generate HCO3−. Normal plasma HCO3− is about 22–28 mEq/L. Disruptions in bicarbonate balance contribute to metabolic acidosis or alkalosis; respiratory adjustments can compensate for acid–base disorders.

In nature and industry, bicarbonate occurs as dissolved inorganic carbon in water and is abundant in the ocean’s carbonate system. Sodium bicarbonate (NaHCO3) and other bicarbonates are widely used as baking soda, antacids, and buffering agents; they can be produced by reacting CO2 with bases such as NaOH.