Pulse radiolysis study of aqueous nitric acid solutions: formation mechanism, yield, and reactivity of NO3 radical

Abstract

A pulse radiolysis study of aqueous nitric acid and nitrate solutions has been carried out. The formation kinetics of NO{sub 3} radical was followed with nanosecond time resolution. Two formation processes of NO{sub 3} 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 + HNO{sub 3} {yields} H{sub 2}O + NO{sub 3}, with a rate constant of (1.4 {plus minus} 0.1) {times} 10{sup 8} M{sup {minus}1} s{sup {minus}1}. The fast formation process is most probably the direct action of radiation on nitrate anion or nitric acid: NO{sub 3}{sup {minus}} (gamma radiation) {yields} e{sup {minus}} + NO{sub 3} or HNO{sub 3} (gamma radiation) {yields} HNO{sub 3}{sup +} + e{sup {minus}} {yields} NO{sub 3} + H{sup +} + e{sup {minus}}. The molar absorption coefficient of NO{sub 3} was evaluated as 1,300 {plus minus} 100 M{sup {minus}1} cm{sup {minus}1} at 640 nm. On the basis of this result, G values of NO{sub 3} weremore » evaluated in nitric acid, lithium nitrate, and sodium nitrate solutions. The main decay reaction of NO{sub 3} was proposed to be NO{sub 3} + NO{sub 2} {yields} N{sub 2}O{sub 5} in nitric acid solutions and its rate constant was evaluated as (1.7 {plus minus} 0.3) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}. Rate constants of reactions of NO{sub 3} with radiolytic products and some other compounds were obtained.« less