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, NO 2- , 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 + ], [NO 2- ] 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:

Neutral red is a eurhodin dye which finds a variety of applications in the biological system [8,9]. It is also used for the dyeing of silk, paper and cotton as well as for the production of inks [10]. Redox reactions of neutral red were investigated in aqueous media [10,11].
In spite of these and numerous other uses, redox chemistry of this dye with NO 2 has not been reported. The present study is undertaken to obtain kinetic data with a view of gaining insight into the mechanistic pathway of this reaction. Therefore, in this paper, we report on the kinetics and mechanisms of redox reaction of neutral red with nitrite ion.

Materials and Methods.
All the reagents used were Analar grade. Neutral red (Gurr) and Sodium nitrite (BDH) were used without further purification. Sodium chloride (M&B) was used to maintain the ionic strength of the reaction medium. Hydrocholoric acid (Analar grade) was standardized titrimetrically using sodium carbonate. All other reagents were used as supplied.
The stoichiometry of the reaction was determined by spectrometric titration using the mole ratio method. The concentration of [NR + ] was kept constant while that of ] was varied in the reaction mixtures after which excess [NR + ] was determined by measuring the absorbance of the solutions at 525 nm until a constant value was obtained. The stoichiometry was evaluated from the plot of absorbance against mole ratio [12].
The kinetic studies were carried out under pseudo-first order conditions with the [NO 2 -] in excess over the [NR⁺] at 525 nm, [H⁺] = 2.0 × 10 -3 mol dm -3 , I = 0.1 mol dm -3 and T = 25 ± 1°C. The Pseudo-first order rate plots of log(A t -A ∞ ) versus time were made (where A ∞ and A t are the absorbance at the end of the reaction and at time, t) and from the slopes of the plots, the pseudo-first order rate constants (k 1 ) were determined. The second order rate constants (k 2 ) were obtained using Equation 1. (1)

Results and Discussion
Stoichiometric Studies. From the stoichiometric studies, the mole ratio of the reaction was found to be 1:1 (Fig. 1), and is represented below (Eq. 2).
Kinetics. The result of kinetic studies were obtained by plotting log(A t -A ∞ ) against time t (Fig. 2). The slope was calculated and k 1 (pseudo-first order rate constant), was determined by multiplying the slope by 2.303. The order of the reaction with respect to [NO 2 -] was determined by plotting a graph of log k 1 against log[NO 2 -]. The gradient of the linear graph was obtained as 0.99 (Fig. 3). The values of second order rate constants k 2 determined as the ratio of k 1 and [NO 2 -] were fairly constant ( Table 1). The rate equation for the reaction can be written as; (3) Similar order was reported in the redox reactions of nitrite [1,4,5,7].
where a = 1.41 dm 3 mol -1 s -1 and b = 0.70 dm 6 mol -2 s -1 Acid dependence of this nature shows that there is a rapid pre-equilibrium between the protonated and non-protonated forms in which both forms are reactive [4]. The overall rate equation in the acid range investigated is given as: Similar acid dependence has been established [6,7].

Effect of Ionic Strength and Dielectric Constant.
The effect of varying the ionic strength on the rate of reaction as the concentration of other reactants were kept constant was investigated in the range of (0.06 -0.12) mol dm -3 . The rate constants were observed to decrease with increase in ionic strength suggesting negative Bronsted -Debye salt effect ( Figure 5) [13]. This indicates that the species at the rate determining step are of different charges. This result was supported by the observed decrease in the reaction rate as the dielectric constant of the medium was decreased ( Table 2). 4 2and NO 3 -) ( Table 3) had no effect on the rate of the reaction while added cations (Mg 2+ and Ca 2+ ) (Table 3 and Figure 6) increased the reaction rate. This is an indication that the reaction might be operating via the outer sphere mechanism [1,7]. Polymerization Studies. Addition of acrylamide to the partially reacted solution to serve as radical scavenger in the presence of large excess of methanol did not produce a gelatinous precipitate. This indicates that the involvement of free radicals in the reaction is unlikely.

International Letters of Chemistry, Physics and Astronomy Vol. 67
Reaction Mechanism. On the basis of the results above from our investigation, the following reaction scheme is proposed for this reaction:

Conclusion
The redox reaction of neutral red with nitrite ion in aqueous acidic medium showed a stoichiometry of 1:1. The reaction is second order overall. The reaction showed both acid dependent and independent pathways. It also displayed a negative Bronsted-Debye salt effect. Both kinetic and spectroscopic investigations showed no evidence of intermediate complex formation. Based on the foregoing, the outer sphere mechanism is proposed as the plausible mechanistic pathway for this reaction.