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 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/Kinhv/s (carboxylic acid) gives a linear line with non-zero intercept [23-25]. The value of intercept=1/kcat and slope=B/kcat 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.  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 -
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.
Gupta KS, Singh R, Saxena D, Manoj SV, Sharma M (1999) Role of manganese dioxide in the autoxidation of sulfur (IV) in oxic and anoxic suspensions. Ind J Chem 38A: 1129-1138.
Sharma AK, Sharma R, Prasad DSN (2017) Acid rain chemistry, catalysis and inhibition of SO2 in environment. P Lambert Academic Publishing, Germany.
Sharma AK, Prasad DSN, Sharma R (2018) Catalysis and inhibition of SO2 in atmospheric environment- a perspective of acid rain chemistry. Analytical techniques in Chemical and biological sciences Discovery publishing house Pvt Ltd, New Delhi, India.
Sharma AK, Acharya S, Sharma R, Saxena M (2012) Recovery and reuse of SO2 from thermal power plant emission. In: Mukesh K (Ed.), Air Pollution-Monitoring, Modelling, Health and Control. In Tech Open Access Publisher, University campus, Croatia.
Sharma H, Sharma AK, Kumar M, Prasad DSN (2019) The influence of succinic acid on the kinetics of the atmospheric oxidation of dissolved SO2 Catalysed by Co2O3. International Journal on Emerging Technologies 10(3): 82-86.
Wilkosz I, Mainka A (2008) Mn (II)-catalysed S(IV) oxidation and its inhibition by acetic acid in acidic aqueous solutions. J Atmos Chem 60: 1-17.
Ziajka P, Pasiuk-Bronikowska W (2003) Autoxidation of sulfur dioxide in the presence of alcohols under conditions related to tropospheric aqueous phase. Atmos Environ 37(28): 3913-3922.
Sharma AK, Sharma R, Prasad DSN, Parashar P, Gupta AK (2015) Formic acid inhibited Ag (I) catalysed autoxidation of S(IV) in acidic medium. J Chem Chem Sci 5(12): 760-771.
Sharma AK, Sharma R, Prasad DSN, Parashar P, Gupta AK (2016) Ag (I) catalyzed autoxidation of S(IV) and its inhibition by isopropyl alcohol in acidic medium. Chem Sci Rev Lett 17(5): 14-23.
Sharma AK, Sharma R, Prasad DSN (2015) Kinetics and mechanism of uncatalysed and Ag (I) catalysed autoxidation of S(IV) and its inhibition by isoamyl alcohol in acidic aqueous solutions. Int J Mod Sci Eng Technol 2(12): 31-40.
Sharma AK, Sharma R, Prasad DSN, Parashar P (2016) The inhibitive action of aniline on the autoxidation of sodium sulfite in acidic medium. J Anal Phar Res 17(5): 14-23.
Sharma AK, Sharma R, Prasad DSN (2017) The effect of atmospheric aromatic amides on the Ag(I) catalyzed S(IV) autoxidation in aqueous solution. The Experiment 40(1): 2354-2363.
Sharma AK, Sharma R, Prasad DSN, Parashar P (2017) Ag(I) catalyzed oxidation of S(IV) in aqueous solution differing effect of benzoate ions in acidic medium. Curr Phy Chem 7(2): 338-347.
Sharma AK, Prasad DSN (2017) Influence of pH and organics on autoxidation of S(IV) catalyzed by Ag(I). Recent Adv Petrochem Sci 3(1): 1-2.
Professor, Chief Doctor, Director of Department of Pediatric Surgery, Associate Director of Department of Surgery, Doctoral Supervisor Tongji hospital, Tongji medical college, Huazhong University of Science and Technology