The reaction of •OH with O2, the decay of O3•- and the pKa of HO3• - interrelated questions in aqueous free-radical chemistry

In the reactions of ozone with organic compounds in aqueous solution, O-3(center dot-) is an abundant intermediate. A basic aspect of its conversion into (OH)-O-center dot is addressed here. The reactions O center dot- + O-2 reversible arrow O-3(center dot-) (1), H+ + O center dot- reversible arrow (OH)-O-center dot (8), (OH)-O-center dot + O-2 reversible arrow HO3 center dot (6), and H+ + O3(center dot-) reversible arrow HO3 center dot (5) are interconnected by a thermodynamic cycle. For equilibria (1) and (8) reliable equilibrium constants, and hence Gibbs energies are available (Delta G(0)(1) = 32 kJ mol(-1), Delta G(0)(8)= 67 kJ mol(-1)). For reaction (6), a Gibbs energy of Delta G(0)(6) = 47 kJ mol(-1) (K-6 = 10(-8.2)M) has now been calculated by G1. From the thermodynamic cycle one hence arrives at Delta G(0)(5) = -12 kJ mol(-1). This relates to PKa(HO3 center dot) = 2.1. Thus, the HO; radical is a very strong acid. This value agrees with a value of -2.0 obtained from the Bielski and Schwarz relationship for pK(a) values of OxHy compounds. Reaction (6) must be very slow, 0.1 < k(6) < 10(4)M(-1)s(-1). Copyright (C) 2010 John Wiley & Sons, Ltd.