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

Abstract Pulse radiolysis and laser photolysis were utilized to determine the molar absorption coefficient of the (bi)carbonate radical (HCO 3 /CO − 3 ), converted from hydroxyl and sulfate radical. The analysis gave a molar absorption coefficient 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 (HCO 3 ) 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 reactions with other inorganic reactants such as I − , NO − 2 , ClO − 2 and H 2 O 2 was also studied at high ionic strength. Except for H 2 O 2 the bicarbonate radical (HCO 3 at pH 8.4) has higher reactivity toward these reactants than carbonate radical (CO − 3 at pH 11.8), due most probably to the pH dependence of the driving force for the reactions.