Reactions of the ferryl ion with some compounds found in cloud water

Abstract

The lifetime of the ferryl ion FeO2+, (minutes at pH below 1) is reduced by a factor of 50 at pH above 3. This is rationalized in terms of an acid-base equilibrium between two different hydrolytic forms of this species (Fe(OH)++2 and Fe(OH)+3) with a pKa=2.0. The rate constants for reactions between FeO2+ and selected compounds found in cloud water were measured in acid solutions by stopped-flow technique. For inorganic reactants: kHNO2=1.1×104 M−1 s−1, kNO2−≤105 M−1 s−1, kCl−=1.0×102 M−1 s− 1, kHSO3−=2.5×105 M−1 s−1, kSO2=4.5×105 M−1 s−1, and kMn(II)=1.0×104 M−1 s−1 were obtained and for the organic reactants: kHCOOH=160 M−1 s−1, kHCOO−=3×105 M−1, kCH3COOH=3.1 M−1 s−1, kCH2(OH)2=400 M−1 s−1, kCH3COCH3=16 M−1 s−1, kCH3CH20H=2.5×103 M−1 s−1, kC6H5OH=4.0×103 M−1 s−1, and kC6H5COOH=80 M−1 s−1 were obtained. A good correlation between log(k) and the standard one electron reduction potential (E°) indicates that the reactions of inorganic compounds proceed by electron transfer. The reaction mechanisms in case of organic compounds are very similar to the reactions of the OH radical, i.e., H-abstraction, and a fairly good correlation between log(k) and the bond dissociation energy BDE was obtained. Activation parameters were measured for the reaction of FeO2+ with HNO2 (Ea=34.5 kJ/mol); Mn2+ (E a=21.3 kJ/mol); HCOOH (Ea=22.3 kJ/mol); CH2 (OH)2 (Ea=44.5 kJ/mol); and C6H5 OH (Ea=28.1 kJ/mol). © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 215–221, 1998.