First pKa of carbonic acid

Value 3.45 unitless Range: ±0.15 unitless
Organism Generic
Reference Adamczyk, K., Prémont-Schwarz, M., Pines, D., Pines, E., and Nibbering, E. T. J. (2009). Real-time observation of carbonic acid formation in aqueous solution. Science 326, 1690–1694. abstract & p.1693 right column 2nd paragraphPubMed ID19965381
Method Kinetics of deuteron transfer from a photoacid to dueterate bicarbonate (DCO3-) were measured using spectroscopic methods. The difference in pKa was between the acid and base (bicarbonate) was fit to the kinetics using a Marcus plot.
Comments Abstract: "A Marcus free-energy correlation supports an associated pKa (Ka is the acid dissociation constant) of 3.45 +/- 0.15, which is substantially lower than the value of 6.35 that is commonly assumed on the basis of the overall carbon dioxide-to-bicarbonate equilibrium." P.1693 right column 2nd paragraph: "[Investigators] used the Marcus free-energy correlation for aqueous proton transfer (refs 19, 41) to plot the overall proton-transfer rate of the encounter complex as a function of ΔpKa, defined as the difference between pKa(photoacid in S1 state) and pKa(conjugate acid of accepting base). After correcting [their] measured rate for D/H isotope substitution (ref 19), [they] found that the correlation holds for a carbonic acid pKa of 3.45 ± 0.15, as opposed to the widely used effective pKa of 6.35 for CO2(aq)/H2O. This result follows naturally from the real-time observation of proton transfer to bicarbonate on the picosecond time scale, without interfering effects from slower reversible deprotonation or dehydration reactions of carbonic acid."
Entered by Avi Flamholz
ID 111761