Kinetics and Mechanism of the Oxidation of Naphthol Green B by Peroxydisulphate Ion in Aqueous Acidic Medium
The kinetics of the oxidation of naphthol green B (NGB3−) by peroxydisulphate ion has been carried out in aqueous acidic medium at of 700 nm, °C, and mol dm−3 (NaCl). The reaction shows a first-order dependence on oxidant and reductant concentration, respectively. The stoichiometry of the NGB— reaction is 1 : 2. Change in hydrogen ions concentration of the reaction medium has no effect on the rate of the reaction. Added cations and anions decreased the rate of the reaction. The results of spectroscopic and kinetic investigation indicate that no intermediate complex is probably formed in the course of this reaction.
Naphthol green B is used in histology to stain collagen. The dye is a lake, in which the mordant metal is ferric iron. However, the iron appears to play no part in its staining ability . Naphthol green B is very soluble in water and has an absorption maximum of 714 nm. As an anion, it acts as an acid dye . Kinetic spectrophotometric method for the determination of cerium (IV) with naphthol green B has been studied . The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of naphthol green B at 710 nm. It was found that the reaction rate is very slow when the temperature is below 60°C, but the rate of the catalyzed reaction increases rapidly when the reaction temperatures are above 70°C.
Peroxydisulphate ion is one of the powerful oxidizing agents known [4, 5]. A large number of investigations have been carried out in the oxidation of metal ions [6, 7], metal complex [8, 9], and various organic compounds  by . Most electron transfer reactions of occur by the outer-sphere mechanism [11–13]. However, reactions with some positively charged metal complexes such as Fe, Fe, and Os occur by both the outer-sphere and the inner-sphere mechanisms [5, 9]. Except when the reducing agent can be protonated, the redox reaction of was found to be independent of hydrogen ion concentration . Despite its reactions with other compounds, the redox reaction of this oxidant with naphthol green B is scanty.
It is our hope that this investigation will help to gain more insight into the redox reaction of naphthol green B with peroxydisulphate ion.
2. Materials and Methods
The chemicals used were of analytical grade and were used without further purification. Standard solution of was prepared by dissolving 0.088 g in 100 cm3 volumetric flask using distilled water. Sodium peroxydisulphate solution was prepared by dissolving known quantities in distilled water. All other reagents used were of analytical grade.
The stoichiometry of the reaction was determined spectrophotometrically using the mole ratio method by keeping the concentration of the dye constant at 4.0 × 10−5 mol dm−3, mol dm−3, , nm, , and mol dm−3. The stoichiometry was evaluated from the plot of absorbance versus after the reaction had reached completion by the observation of a steady zero absorbance value over a period of two days.
A Corning Colorimeter Model 252 spectrophotometer was used to follow the decrease in absorbance of the dye at 700 nm, , mol dm−3 (NaCl), and mol dm−3 (HCl). The kinetic runs were conducted under pseudo-first-order conditions with  in at least 120-fold excess over . The pseudo-first-order plots of log () versus time were made (where and are the absorbance at time and at the end of the reaction, resp.). From the slope of the plots, the pseudo-first-order rate constant () was determined .
The effect of acid on the rate of reaction was studied by varying  in the range mol dm−3, while  and  were kept constant at and mol dm−3 (NaCl). The range was narrow because acid was stable at such range.
The effect of ionic strength on the rate of the reaction was investigated in the range of 0.05–0.7 mol dm−3 (NaCl) while the concentrations of other reagents were kept constant at .
3. Results and Discussion
The result of stoichiometric studies reveals that one mole of reacted quantitatively with two moles of . Hence the overall equation for the reaction is
The presence of was confirmed qualitatively by the addition of BaCl2 followed by HCl which formed an insoluble white precipitate in excess HCl. Kinetic studies indicated first-order dependence of rate of reaction on  and . Under the experimental conditions employed was found to be dm3 mol−1 s−1.
The rate equation for the reaction is
Effect of changes in ionic strength of the reaction medium indicated that the rate constant decreases with increase in ionic strength. This obeyed a negative Bronsted-Debye salt effect . Plot of log against (Figure 1) gave a slope of −0.71 ().
The reaction was found to be independent of  in the range of mol dm−3 investigated, indicating that the reaction occurs through an acid-independent pathway (Table 1). Similar results have been reported for the redox reaction of [17–19]. Added cation and anion inhibited the rate of the reaction (Tables 2 and 3). This inhibited effect by the ions suggested that the reaction might be operating via the outersphere mechanism .
Spectroscopic evidence and the Michaelis-Menten plot of 1/ versus 1/ (Figure 2) suggest that intermediates may be unimportant in the rate determining step. Free radical test did not yield gel formation or if present it might be transient.
On the basis of the above findings, the mechanism below is proposed for this reaction.
4. Reaction Scheme
The redox reaction of naphthol green “B” and peroxydisulphate ions in aqueous acidic medium showed a stoichiometry of 1 : 2; a first order was observed for and ions, respectively. The rate constant decreases with increase in ionic strength and the reaction was found to be independent of . Spectroscopic evidence and the Michaelis-Menten plot of 1/ versus 1/ suggest that intermediates may be unimportant in the rate determining step. Polymerization test suggests the absence of free radicals in the reaction medium. Based on the above results, it is proposed that the reaction most probably operates through the outer-sphere mechanism.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
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