Temperature Dependence of Carbonate Radical in NaHCO3 and Na2CO3 Solutions: Is the Radical a Single Anion?

The carbonate radical was investigated by pulse radiolysis in both NaHCO 3 and Na 2 CO 3 solutions over 15−400 °C. There was almost no change in the absorption spectrum of the carbonate radical within the temperature range studied. However, the absorbance of carbonate radical increased with temperature for NaHCO 3 solution at any concentration considered, whereas the absorbance for Na 2 CO 3 solution increased with temperature at high concentrations and decreased at low concentrations ≤ 2 × 10 -3 m (mol/kg). The second-order rate constant for the decay of the carbonate radical exhibited a complicated temperature dependence: it decreased with temperature up to about 120 °C, and then it slowly increased with temperature up to 300 °C followed by a rapid increase with temperature at 300−400 °C. The decay was also dependent on the concentration of NaHCO 3 or Na 2 CO 3 . If considering the carbonate radical as CO 3 •-, the experimentally measured yield was always smaller than the simulated one, and the discrepancy was enlarged at lower carbonate/bicarbonate concentrations. We propose that the carbonate radical is present in the form of a dimer anion, either (CO 3 ) 2 •3- or H(CO 3 ) 2 •2- . The equilibrium constants for the formation of (CO 3 ) 2 •3- and H(CO 3 ) 2 •2- were obtained for various temperatures. The measured radical yield can correlate well with the simulated result using the dimer model. We further estimated p K a of H(CO 3 ) 2 •2- at temperatures up to 250 °C. The p K a at 25 °C was 9.30±0.15, in good agreement with the literature value of 9.5−9.6 obtained by measuring the pH dependence of the reactivity.