dicarbonate

Dicarbonate is not a standardly defined chemical species in mainstream chemistry. In most references, the term is used informally to describe one of two concepts that are not widely established as isolable compounds: (a) a hypothetical dianion formed from two carbonate units, C2O6^4−, which would require two carbonate fragments to be joined in a manner not observed under ordinary conditions; or (b) a dimeric form of carbonic acid, H2(CO3)2, discussed only in theoretical or transient-assembly contexts. Because there is no broadly accepted, stable compound named dicarbonate, the term is seldom used in modern databases or standard textbooks.

In established carbon chemistry, the principal species are carbonic acid (H2CO3), bicarbonate (HCO3−), and carbonate (CO3^2−). These relate through a series of acid–base equilibria in aqueous solution: CO2 dissolves in water to form H2CO3, which equilibrates with HCO3−, and HCO3− further equilibrates to CO3^2−. The system underpins the bicarbonate/carbonate buffering of natural waters and biological fluids, with pKa values around 6.35 for H2CO3/HCO3− and around 10.33 for HCO3−/CO3^2− at 25 °C.

If encountered, the term dicarbonate should be interpreted cautiously, as it does not denote a commonly recognized, stable species. See also carbonic acid, bicarbonate, carbonate, and carbon dioxide.