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 NaHCO3 and Na2CO3 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 NaHCO3 solution at any concentration considered, whereas the absorbance for Na2CO3 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 NaHCO3 or Na2CO3. If considering the carbonate radical as CO3•-, 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 (CO3)2•3- or H(CO3)2•2-. The equilibrium constants for the formation of (CO3)2•3- and H(CO3)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 pKa of H(CO3)2•2- at temperatures up to 250 °C. The pKa 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.