PULSE-RADIOLYSIS STUDY OF AQUEOUS NITRIC-ACID SOLUTIONS - FORMATION MECHANISM, YIELD, AND REACTIVITY OF NO3 RADICAL

A pulse radiolysis study of aqueous nitric acid and nitrate solutions has been carried out. The formation kinetics of NO3 radical was followed with nanosecond time resolution. Two formation processes of NO3 in nitric acid solutions were directly demonstrated: a fast one completed during the electron pulse and a slow one occurring in a microsecond time range. In nitrate solutions, only the fast formation process was observed. The slow formation process in nitric acid solutions was confirmed to be the reaction of OH radicals with undissociated nitric acid molecules, OH + HNO3 --> H2O + NO3, with a rate constant of (1.4 +/- 0.1) x 10(8) M-1 s-1. The fast formation process is most probably the direct action of radiation on nitrate anion or nitric acid: NO3- squiggly-arrow-pointing-right e- + NO3 or HNO3 squiggly-arrow-pointing-right HNO3+ + e- --> NO3 + H+ + e-. The molar absorption coefficient of NO3 was evaluated as 1300 +/- 100 M-1 cm-1 at 640 nm. On the basis of this result, G values of NO3 were evaluated in nitric acid, lithium nitrate, and sodium nitrate solutions. The main decay reaction of NO3 was proposed to be NO3 + NO2 --> N2O5 in nitric acid solutions and its rate constant was evaluated as (1.7 +/- 0.3) x 10(9) M-1 s-1. Rate constants of reactions of NO3 with radiolytic products and some other compounds were obtained.