Laser-Based Studies of Reactions of the Nitrate Radical in Aqueous Solution

A new experimental technique for temperature dependent studies of the kinetics of the nitrate radical (NO3) in aqueous solution has been developed. In this technique NO3 is produced by excimer laser photolysis of peroxodisulfate anions at 351 nm in the presence of excess nitrate and detected by time-resolved long-path laser absorption using the 632.8 nm line of a HeNe-laser. — For the generation reaction of NO3, viz. (1) SO4− + NO3− SO42- + NO3 a rate constant of k1 = (5.0 ± 0.5) · 104 l mol · s at T = 298 K and zero ionic strength was found. For the reactions (2) NO3 + OH− NO3− OH and (3) NO3 + Cl− NO3− + Cl rate coefficients of k2 = (8.2 ± 0.9) · 107 and k3. = (1.0 ± 0.2) · 107 l/mol · s were determined at T = 298 K. In temperature dependent experiments the Arrhenius expressions k2(T) = (8.1 ± 0.4) · 1011 exp[-(2700 ± 400)K/T] l/mol · s and k3(T) = (1.9 ± 0.2) · 1013 exp[-(4300 ± 500) K/T] l/mol · s were derived. - The rate constant of the reaction of NO3 with dissolved sulfur (IV) has been studied over a range of pH-values and temperature. For the reactions with SO2(aq) (4). and HSO3− (5) rate coefficients of k4 = (2.3 ± 0.5) · 108 and k5 = (1.4 ± 0.1) · 109 l/mol · s, respectively, at 298 K were obtained. The corresponding temperature dependences are described by: k4(T) = (5.6 ± 0.7) · 1013 exp[-(3700 ± 1100)K/T] l/mol · s and k5(T) = (1.1 ± 0.2) · 1012 exp[-(2000 ± 400)K/T] l mol · s. For the reaction of NO3 with SO32- (6) a rate constant of k6 = (3.0 ± 0.4) · 108 l/mol · s was derived at T = 278 K.