Real-Time Observation of Carbonic Acid Formation in Aqueous Solution

Abstract

A Glimpse of Wet Carbonic Acid: Both carbon dioxide and bicarbonate play extraordinarily widespread roles in biochemical and geochemical reactions. It is surprising therefore that carbonic acid, the intermediate in the water-coupled interconversion of these two compounds, has never been directly characterized in aqueous solution. Adamczyk et al. (p. 1690 , published online 12 November) have succeeded in glimpsing the elusive acid by inducing an aqueous photoacid (a compound rendered transiently more acidic upon light absorption) to react with dissolved bicarbonate. Using infrared spectroscopy, they show that the carbonic acid product persists for nanoseconds. Analysis of its formation kinetics affords a direct p K _a value of 3.5, substantially lower than the effective value derived from observations of the net bicarbonate/carbon dioxide equilibrium.