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

• Published in: Magnetic resonance in chemistry 2024-02, Vol.62 (2), p.114
• Main Authors: Oliveira, Daniel Rossado, da Costa, Eric Tavares, Schenberg, Leonardo Araujo, Ducati, Lucas Colucci, do Lago, Claudimir Lucio
• Format: Article
• Language: English
• ISSN: 1097-458X

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO and another one for the species H CO , HCO , and CO -herein collectively named H CO . 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 and CO . The detection of H CO at low pH solutions is hindered, because of the concurrent liberation of CO when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the H CO peak across a wide pH range, even at pH 1.8 where the dominant species is H CO . The method employed the formation of frozen layers of NaH CO 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 to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H CO 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 CO pK value determined by the present method was 3.41 ± 0.03, for 15% D 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.