ABSTRACT
Hydration of CO2 yields HCO3−via the reaction: 
Acid–base physiologists traditionally simplify the reaction by omitting the H2CO3 term and lumping all ionic CO2 species into the HCO3− term. The simplified reaction forms the basis for the familiar Henderson–Hasselbalch equation of the CO2–HCO3− buffer system: 
where αCO2 is the solubility coefficient relating [CO2] and 
(Henry’s Law). The apparent pK (pKa) in this equation lacks a rigorous thermodynamic definition. Instead, it is an empirical factor relating pH, the product of αCO2 and 
and the apparent [HCO3−] (i.e. the sum of all ionic CO2 species).
(Henry’s Law). The apparent pK (pKa) in this equation lacks a rigorous thermodynamic definition. Instead, it is an empirical factor relating pH, the product of αCO2 and and the apparent [HCO3−] (i.e. the sum of all ionic CO2 species).
