O-2(-) DECAY CATALYZED BY CU2-SOLUTIONS - A PULSE-RADIOLYSIS STUDY FOR ATMOSPHERIC CHEMISTRY( AND CU+ IONS IN AQUEOUS)

The catalytic decay of O2- radicals due to Cu+ and Cu2+ ions in O2 sat urated solutions of low ionic strength was studied by pulse radiolysis and computer simulation of the entire reaction mechanism. Phosphate w as used as buffer and formate was used to convert OH radicals into O2- . At pH almost-equal-to 6, the very low solubility products of Cu2+ wi th OH- and PO43- limited the copper concentration to the order of muM. In experiments with [Cu2+] > [O2-], the rate constant k(Cu2+ + O2-) = (8.0 +/- 0.5).10(9) M-1 s-1 was determined. In systems with [O2-] muc h greater than [Cu]tot, steady state for the copper ions is attained w ith apparent first-order kinetics, which correlates k(Cu2+ + O2-) with k(Cu+ + O2-). The O2- decay rate decreased with increasing pH due to Cu2+ complexation with HPO42-. Correspondingly the pK(CuHPO4) was foun d to be -4.0 +/- 0.2 under the assumption that k(CuHPO4 + O2-) less-th an-or-similar-to 0.01.k(Cu2+ + O2-). With this value, k(Cu+ + O2-) cou ld be determined as (9.4 +/- 0.8). 10(9) M-1 s-1 by an iterative proce dure. The consequences for the chemistry of atmospheric waters are dis cussed.