13C NMR as an analytical tool for the detection of carbonic acid and pKa determination

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. ¹³C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO_2(aq) and another one for the species H₂CO₃, HCO₃⁻, and CO₃²⁻—herein collectively named H_xCO₃^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 HCO₃⁻ and CO₃²⁻. The detection of H₂CO₃ at low pH solutions is hindered, because of the concurrent liberation of CO₂ when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the H_xCO₃^x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H₂CO₃. The method employed the formation of frozen layers of NaH¹³CO₃ 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 CO₂ to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H₂CO₃ 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 H₂CO₃ pK_a value determined by the present method was 3.41 ± 0.03, for 15% D₂O 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.