Kinetics and Mechanism of Redox Reaction of Neutral Red with Nitrite Ion in Aqueous Acidic Medium

The kinetics of redox reaction of neutral red, NR⁺, with nitrite ion, NO₂^-, was studied in aqueous hydrochloric acid medium under pseudo-first order conditions at 25 ± 1°C, [H⁺] = 2.0 × 10^-3 mol dm^-3, I = 0.1 mol dm^-3 (NaCl) and λ_max = 525 nm. The reaction was first order with respect to [NR⁺], [NO₂^-] and [H⁺]. The reaction displayed a negative Bronsted-Debye salt effect. There was no evidence of the formation of an intermediate complex of significant stability and free radicals are probably not present in the reaction. The observations above, coupled with the result of Michaelis-Menten plot suggests an outer sphere mechanism for the reaction. The reaction obeys the rate law: -d [NR⁺]/dt = (a + b [H⁺])[NR⁺][NO₂^-]. A plausible mechanism has been proposed for the reaction.