Range: ±0.15 unitless
||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
||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.
||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."