13 C NMR as an analytical tool for the detection of carbonic acid and pK a determination.

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. 13 C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO 2(aq) and another one for the species H 2 CO 3 , HCO 3 - , and CO 3 2- -herein collectively named H x CO 3 x-2 . The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO 3 - and CO 3 2- . The detection of H 2 CO 3 at low pH solutions is hindered, because of the concurrent liberation of CO 2 when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the H x CO 3 x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H 2 CO 3 . The method employed the formation of frozen layers of NaH 13 CO 3 and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO 2 to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H 2 CO 3 was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car-Parrinello molecular dynamics. The H 2 CO 3 pK a value determined by the present method was 3.41 ± 0.03, for 15% D 2 O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.