Reinvestigation of the acid-base equilibrium of the (bi)carbonate radical and pH dependence of its reactivity with inorganic reactants

Pulse radiolysis and laser photolysis were utilized to determine the molar absorption coefficient of the (bi)carbonate radical (HCO3./CO3-.), converte d from hydroxyl and sulfate radical. The analysis gave a molar absorption c oefficient of 2000 +/- 100 dm(3) mol(-1) cm(-1) at 600 nm, being virtually independent of pH between 7 and 13. The acid-base equilibrium constant, pKa , of the bicarbonate radical (HCO3.) was determined to be 9.5 +/- 0.2 from the pH dependence of rate constants for the reactions with [Fe(CN)(6)](4-) and SCN- at high ionic strength of 0.5 mol dm(-3). The pH dependence for re actions with other inorganic reactants such as I-, NO2-, ClO2- and H2O2 was also studied at high ionic strength. Except for H2O2 the bicarbonate radic al (HCO3. at pH 8.4) has higher reactivity toward these reactants than carb onate radical (CO3-. at pH 11.8), due most probably to the pH dependence of the driving force for the reactions. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.