Kinetics and Mechanism of the Redox Reaction of Naphthol Green B with Hydrazine Dihydrochloride in Aqueous Acidic Medium

The kinetics of the redox reaction between naphthol green B and hydrazine dihydrochloride has been studied in aqueous hydrochloric acid medium at an ionic strength, I = 0.50 mol dm -3 (NaCl), [H] = 1.0×10 moldm (HCl) and T = 21±1°C. The redox reaction displayed a stoichiometry of 1:1 and obeys the rate law: -d[NGB3-]/dt=k2[NGB3-][N2H4.2HCl]. Change in hydrogen ion concentration of the reaction medium has no effect on the rate of the reaction. Added cations and anions inhibited the rate of the reaction. The redox reaction showed negative salt effect, with the rate decreasing with increase in ionic strength of the reaction medium. Results of the Michaelis–Menten’s plot show that an intermediate complex was not formed during the course of the reaction. The outersphere mechanism is proposed for this reaction.


Introduction
Naphthol green B (NGB 3-) is used in the production of drugs, cosmetics and for staining purposes. It possesses excellent redox characteristic. A new amperometric glucose biosensor with naphthol green B as a mediatior has been reported [1]. It was found that naphthol green B is a good mediator, promoting electron transfer from glucose oxidase to graphite electrode.
Hydrazine dihydrochloride, which is a powerful reducing agent, has similarity to the thiourea by possessing nitrogen in its structure can also be possible inhibitor and similar compounds [2]. Hydrazine and its derivatives have been used in industry, agriculture and other fields, including photographic development, rocketry, explosives, and insecticides and blowing agents for plastics. Kinetic study of the oxidation of hydrazine dihydrochloride by aqueous iodine has been reported [3]. It was found that the reaction is first order in both reactants. Reduction of aqueous silver nitrate by hydrazine dihydrochloride in weakly alkaline solution results in a polydisperse colloid that is stable for many months without addition of any stabilizing compounds [4].
This work is carried out to further understand the reaction of naphthol green B and that of hydrazine dihydrochloride.

Materials and Methods
A1.0×10 -3 mol dm -3 stock solution of naphthol green B (Analar grade) was prepared by dissolving 0.02196g in 25cm 3 flask using distilled water. Hydrazine dihydrochloride solution was prepared by dissolving known quantities in distilled water. 1.0 mol dm -3 solution of hydrochloric acid (BDH) was prepared (36%, specific gravity 1.18) and was standardized titrimetrically. Sodium chloride was used to maintain constant ionic strength of the reaction.
The reaction stoichiometry was determined spectrophotometrically using the mole ratio method [5] . The pseudo-first order plots of log(A t -A∞) versus time were made and the slope of the plots gave the pseudo-first order rate constant, k 1 . The second order rate constants, k 2 were determined from k 1 as k 1 /[N 2 H 4 .2HCl] [6].
The effect of added ions on the reaction rate was observed by addition of (1-50)×10 -3 mol dm -3 of the ions (Ca 2+ , Mg 2+ , SO 4 2and CH 3 COO -), while the concentrations of naphthol green B, the oxidant, hydrogen ion, temperature and ionic strength of reaction medium were kept constant.
Test for the presence of stable, detected intermediate formed during the course of the reaction was carried out spectrophotometrically. The electronic spectra of partially reacted reaction mixture (after one minute of mixing) were recorded at various time intervals depending on the speed of the reaction. A similar run was made for the dye alone in each case. This was carried out in order to determine whether there is significant shift in λ max or enhacement of peak resulted as the reaction progressed.

Results and Discussion
The stoichiometric studies showed that one mole of naphthol green B was consumed by one mole of N 2 H 4 .2HCl. The overall stoichiometry equation is shown in equation (1).
Organic product of the reaction of NGB 3with N 2 H 4 .2HCl gave a yellow precipitate with 2, 4-dinitrophenylhydrazine, confirming the presence of carbonyl group. Ketone was further distinguished by the addition of acidified potassium dichromate, which does not change the colour of the reacting mixture.
The linearity of the pseudo-first order plots ( Figure 1) suggests a first order dependence of reaction rate on [NGB 3-] and [N 2 H 4 .2HCl] under the experimental conditions employed in this investigation. Similar first order dependence has been reported for the oxidation of hydrazine dihydrochloride by aqueous iodine [3]. The reaction is therefore second order overall at constant hydrogen ion concentration. This may be represented as: The rate of reaction showed lack of hydrogen ion concentration dependence in the range (0.1 -20) ×10 -4 mol dm -3 . This independence is in accord with the fact that neither the oxidant nor the reductant undergoes significant protonation changes under the reaction conditions. Similar result has been reported [7]. The redox reaction showed negative salt effect, with the rate decreasing with increase in ionic strength from 0.4-1.0 mol dm -3 (NaCl). This observation of negative Bronsted-Debye salt indicates that species that formed the activated complex are of opposite charges. Plot of logk 2 against I 1/2 gave a slope of -1.26 (R 2 =0.98). (Figure 2) Lack of spectrophotometric evidence for the formation of intermediate complex suggests an outersphere mechanism. Michaelis-Mentein's plot of 1/k 1 versus 1/[N 2 H 4 .2HCl] (Figure 3) had an intercept, suggesting an innersphere. However, ions inhibition as shown in Table 2 and 3 is a characteristic of the outersphere mechanism [8]. This evidence suggests that this reaction is probably operating by the outersphere mechanism. On the basis of the above the following reaction scheme is proposed for this reaction.

Conclusion
The redox reaction of naphthol green B and hydrazine dihydrochloride in acidic medium showed a stoichiometry of 1:1, a first order was observed for NGB 3-and [N 2 H 4 .2HCl]. The rate of reaction had no effects with increase in acid concentration. Increase in ionic strength decreases the rate of the reaction. Added ions inhibited the rate of the reaction. An intermediate complex was not implicated during the course of the reaction. Based on the above results, it is proposed that the reaction is most probably operates through the outersphere mechanism.