Transient species produced in the photochemical decomposition of ceric salts in aqueous solution. Reactivity of nitrogen oxide and hydrogen compd. with oxygen and sulfur (HSO4) free radicals

Abstract

The flash photolysis of ceric sulfate and ceric nitrate in aqueous solutions was studied. Ceric sulfate produced a transient species with an optical absorption maximum at 4550 A which decays bimolecularly with k = 6.5 ± 1.3 X 10s M~l sec-1 and is assigned to the HSCh free radical. Ceric nitrate gave rise to an optical spectrum with maxima at 5950, 6400, and 6750 A. This intermediate was identified as the NO3 radical by its characteristic absorption spectrum in the visible region. It was found to decay by a first-order process with k = 9.5 X 102 3sec-1. In addition, a new ultraviolet band with maxima at 3350 A is reported. The main primary photolytic process in both sulfate and nitrate salts is explained on the basis of a charge transfer from the hydration sphere to the Ce(IV) ion: Ce(IV) -H20 -t Ce(III) + OH + H+, followed by reactions OH + HS04-HS04 + OHand OH + HNO3 -> NO3 + H20. Evidence for this mechanism was derived from the extrapolation to "zero" time of the pseudo-first-order decay plots in presence of various additives. The decrease of the "initial" concentration of HS04 and X03 produced was found to be proportional to the /c(OH + S) X Cs values of the solutes used. The rate constants for the reaction of HS04 and XO3 with added solutes-formate, acetate, methanol, ethanol, 2-propanol, thallous ions, and cerous ions-were obtained. HS04 is found to be a stronger oxidizing agent than X03 although HS04 is not as strong as OH radicals.

Rate constants for Hydrogen abstraction reactions of NO3 in aqueous solution

“…The NO, radical was produced by the pulse radiolysis of 6 M HNO, solutions. In our previous work [ 11, we found that the spectrum of the transient produced in this way matched that reported for NO, produced in the flash photolysis of ceric nitrate solutions [8,91 and was similar to the gas-phase spectrum of this radical. From the maximum absorption at 600 nm, and assuming an absorptivity [91 of 1000 M-' cm-', we calculate that under our typical operating conditions the NO, radical concentration was about 6-12 p M .…”

Possible role of NO3 in the nighttime chemistry of a cloud

“…Note in Table 2 that two patterns exist for those rate constants that have been measured, one for electron transfer reactions (Table 2a) and the other for hydrogen abstraction reactions (Table 2b); for each type of reaction the radicals react in a specific sequence. These patterns, which have been noted by a variety of investigators, including Dogliotti and Hayon [1967], Hasegawa and Neta [1978], and Maruthamuthu and Neta 1-1978], reflect the relative oxidizing power of the various radicals, with stronger oxidizers reacting more rapidly than weaker oxidizers. In the case of electron transfer reactions, SO4-and NO 3 appear to react at roughly equal rates, with C12-reacting more slowly by a factor of about 3-15, and H2PO 4 and HPO,•-generally reacting slower still.…”

Chemistry of Multiphase Atmospheric Systems

“…(a) the rate constants for the reaction of C1 2 -with HS03 -and for N03 with HS03 -and H202 were estimated by assuming these reactions to be about 1/10 as fast as the equivalent 504-' (Maruthamutu and Neta (1978), Dogliotti and Hayon (1967), and Hasegawa and Neta (1978». c.> where l is the mean free path, K is the universal gas constant, m(I) is the molecular weight of I, q.is the accommodation coefficient, [I] is the aqueous-phase concentration of species I in the cloud droplet in M(moles/liter), Heff(I) is the effective Henry's Law or solubility constant for I in M atm-1, and R=1.36 x 10-22 atm-sm3/molec/ o K. The term V(a) in Eqn.…”