Inhibiting Effect of Different Carboxylic Acid on Uncatalysed and Co₂O₃ Catalysed Autoxidation of SO₂ in Alkaline Medium- Brief Comparision

The study of air pollution has been the interest area to the scientific community, since the end of the World War II. The earliest perceived problems were those related to the incomplete combustion of coal, soot and ash abounded in the major industrial cities of the world [1-5]. The oxidation of sulfur dioxide has been one of the most frequently studied reactions in aqueous atmospheric droplets. Three reaction pathways are considered to be dominantly responsible for oxidation of SO₂ in atmospheric water droplets. These are the oxidation of dissolved SO₂ by H₂O₂, O₃ and O₂ in the presence of transition metal ions as catalysts [6-10]. Recent studies show that the sulfur (IV) oxidation in atmospheric water droplets can be affected by other reactions. Organic compounds may dissolve into water droplets and react with sulfoxy radicals and transition metal ions, and thus alter the rate of catalytic S(IV) oxidation [11-15]. In most of the studies the role of organics has been reported in the metal ion catalysed autoxidation of sulfur (IV) in aqueous medium [16-17]. Very few studies are available on the role of organics on the metal oxide catalysed autoxidation of sulfur (IV) in aqueous medium. This led us to investigate the kinetics of sulfur (IV) autoxidation catalyzed by Co₂O₃ in the pH range 7.8-9.4. and the effect of various carboxylic acids have been studied in alkaline media to delineate the nature of the mechanism.

The experimental procedure was exactly the same as described earlier [18-20].

The qualitative tests showed sulfate to be the only oxidation product [21-22].

The major aim of the present study was to examine the effect of organic inhibitors on the autoxidation of S(IV) in alkaline medium. For this purpose, oxalic acid, acetic acid, succinic acid, and malonic acid were chosen as the organic inhibitors. On varying the (carboxylic acid) from 1×10^-7 to 4×10^-3mol L^-1, the rate of the reaction become decelerated. The nature of the (S(IV))-dependence in presence of carboxylic acid did not change and remained first order. By plotting a graph between 1/K_inhv/s (carboxylic acid) gives a linear line with non-zero intercept [23-25]. The value of intercept=1/k_cat and slope=B/k_cat from these values the value of inhibition parameter B can be calculated, inhibition parameter B=slope/intercept (Table 1).

Table 1: Calculated value of B (Inhibition parameter) in absence and presence of Co2O3.

As reported by Gupta et al. [1] a radical mechanism operates in those reactions in which the inhibition parameter lies in the range 10³-10⁴. In this study the value of inhibitor parameter is found to be 10⁴-10⁵, which lies in above the range [26,27]. These results strongly support the radical mechanism. In contrast the results conclusively show that (oxalic acid, acetic acid, succinic acid and malonic acid) act as inhibitors for S(IV) autoxidation and the order in uncatalyzed reaction is -

oxalic acid<acetic acid<malonic acid< succinic acid

In case of Co (III) catalysed reaction the order is -

acetic acid<malonic acid< succinic acid<oxalic acid

Hence it is concluded that succinic acid is better inhibitor out of the four organics in uncatalysed reaction, whereas oxalic acid is better inhibitor among the four organics in Co (III) catalysed reaction.

The authors pay their sincere gratitude to Principal, Govt. P.G. College, Jhalawar- Rajasthan (India) for providing necessary research facilities to accomplish this study.

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