Carbonic acid (H2CO3) is formed by the aqueous solvation of carbon dioxide and it has a vital role in fields as diverse as blood pH regulation, and CO2 absorption from the atmosphere into the oceans. In spite of its importance in these processes, however, direct observation of H2CO3 has proved difficult, and hence our understanding of its true acid–base chemistry has been limited.

Now, the groups of Erik Nibbering from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin and Ehud Pines from the Ben-Gurion University, Israel, have managed to capture and observe carbonic acid in solution1. They used a photoacid (a naphthol disulfonate) that can be optically triggered to protonate HCO3− on a very fast timescale. This means the team could effectively use a laser pulse to create H2CO3, which they could then study by infrared spectroscopy.

The intensity of the carbonyl stretching frequency was measured between one picosecond and one nanosecond after exciting the photoacid, and the kinetics were analysed using line-shape fitting. Nibbering and colleagues discovered that the pKa of carbonic acid is around 3.4, much lower than the value of 6.35 widely assumed by considering the carbon-dioxide-to-bicarbonate equilibrium.