title: How Acidic Is Carbonic Acid?
abstract: Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK(a) of carbonic acid H2CO3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors. The time-dependent diffusion-assisted PT rate was analyzed using the Szabo-Collins-Kimball equation with a radiation boundary condition. The on-contact PT rates were found to follow the acidity order of the carboxylic acids: the stronger was the acid, the slower was the PT reaction to its conjugate base. The pK(a) of carbonic acid was found to be 3.49 ± 0.05 using both the Marcus and Kiefer-Hynes free energy correlations. This establishes H2CO3 as being 0.37 pK(a) units stronger and about 1 pK(a) unit weaker, respectively, than the physiologically important lactic and pyruvic acids. The considerable acid strength of intact carbonic acid indicates that it is an important protonation agent under physiological conditions.
authors: Department of Chemistry, Ben-Gurion University of the Negev , P. O. Box 653, Beer-Sheva 84105, Israel.
authors: Dina
authors: D
authors: Pines
authors: Department of Chemistry, Ben-Gurion University of the Negev , P. O. Box 653, Beer-Sheva 84105, Israel.
authors: Julia
authors: J
authors: Ditkovich
authors: Department of Chemistry, Ben-Gurion University of the Negev , P. O. Box 653, Beer-Sheva 84105, Israel.
authors: Tzach
authors: T
authors: Mukra
authors: Department of Chemistry, Ben-Gurion University of the Negev , P. O. Box 653, Beer-Sheva 84105, Israel.
authors: Yifat
authors: Y
authors: Miller
authors: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.
authors: Philip M
authors: PM
authors: Kiefer
authors: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.
authors: Snehasis
authors: S
authors: Daschakraborty
authors: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.|Ecole Normale Supérieure-PSL Research University, Chemistry Department, Sorbonne Universités-UPMC University Paris 06, CNRS UMR 8640 Pasteur, 24 rue Lhomond, 75005 Paris, France.
authors: James T
authors: JT
authors: Hynes
authors: Department of Chemistry, Ben-Gurion University of the Negev , P. O. Box 653, Beer-Sheva 84105, Israel.
authors: Ehud
authors: E
authors: Pines
mesh_terms: D002255:Carbonic Acid / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013050:Spectrometry, Fluorescence
chemical_list: D002255:Carbonic Acid