Carbonic acid captured

Scientists from Germany and Israel have caught a fleeting glimpse of carbonic acid, the simple yet elusive molecule that plays a key role in nature, from regulating the pH of blood to mediating crucial events in the global carbon cycle. And it appears that the acid is not as weak as the textbooks would have us believe.

Carbonic acid, the hydrated form of carbon dioxide, is an important molecule that is involved in buffering biological fluids such as blood and is a key intermediate in the exchange of carbon dioxide between the atmosphere and the oceans. However, it is so short-lived in solution that its aqueous chemistry has been difficult to study directly.

Now, teams led by Erik Nibbering from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Germany and Ehud Pines of the Ben-Gurion University of the Negev in Israel have managed to take a snapshot of carbonic acid by using ultrafast lasers. 

When CO₂ dissolves in water it forms carbonic acid before rapidly dissociating to the bicarbonate anion. The researchers devised a way to generate the acid in a controlled way that would enable it to be observed. They did this by placing bicarbonate ions in the presence of a photoacid - a compound whose acidity can be triggered by a pulse of optical energy. By exciting the photoacid with a shot of laser light for a few tens of femtoseconds, protons are generated which associate with the bicarbonate to form carbonic acid. Femtosecond infrared spectroscopy can then be synchronised with the generation of the acid to get a look at the molecule before it disappears.

By measuring the rate of protonation of the bicarbonate, the researchers were able to gather that the textbook figure for the acidity of carbonic acid is probably significantly inaccurate. ’It is definitely much more acidic than people thought,’ says Nibbering. ’When we do the time-resolved experiment we see that its acidity lies somewhere between that of acetic and formic acid.’

Pines says that the study also demonstrates that carbonic acid is more stable than previously thought. This, he says, could have a profound effect on the acidification of the oceans by elevated levels of carbon dioxide in the atmosphere, and could mean the oceans are likely to be significantly more acidic than current models suggest. 

Eric Achterberg, a marine biogeochemist at the University of Southampton in the UK, says that the work could have important implications for modelling the sequestration of carbon dioxide under the seabed as part of carbon capture and storage concepts that are being studied. ’An improved knowledge of the acid dissociation constant for carbonic acid will be key in calculations on chemical reactivity of CO₂ towards host rocks, and the potential movement of CO₂  towards the overlying sea through cracks and faults,’ says Achterberg.

Simon Hadlington