Oxidation of L-ascorbic acid by peroxodisulphate in acidic medium Section A-Research paper Eur. Chem. Bull., 2014, 3(9), 873-877 DOI: 10.17628/ECB.2014.3.873 873 SULFATE FREE RADICALS –A KINETIC APPROACH FOR OXIDATION OF L-ASCORBIC ACID IN AQUEOUS ACID MEDIUM N. K. Soni, Riya Sailani, [a] * Anita Meena, [a] Vinita K. Gupta, [a] C. L. Khandelwal [a] and P. D. Sharma [a] Keywords: sulfate free radicals; ascorbic acid; peroxodisulfate; radical kinetics; acid aqueous medium. The kinetics of oxidation of L-ascorbic acid in absence and presence of silver(I) as catalyst has been studied in aqueous acid medium. The stoichiometry corresponds to the reaction as represented by eqn: S2O8 2- + C6H8O6 → 2SO4 2- + C6H6O6 + 2H + . The order with respect to ascorbic acid is unity in uncatalyzed reaction where as it is zero order in silver (I) catalyzed reaction. The reaction mechanism has been suggested in both the conditions delineating the role of sulfate radical ion. Corresponding Authors Tel:+91-9928194174 E-Mail: l.p_riya@yahoo.co.in [a] Department of Chemistry, University of Rajasthan, Jaipur- 302055 (India)A Introduction Numerous studies on uncatalyzed and silver(I) catalyzed oxidations of various types of substrates by peroxodisulfate have been reported. 1-11 The rate of silver(I) catalyzed reactions was generally found to be independent of substrate(s) concentration in oxidation reactions of persulfate. However, certain questions still remain to be replied. The thermal decomposition of peroxodisulfate is first order whereas the reaction is second order in the presence of certain substrate(s). Therefore, such observations are not only interesting but help in understanding chemistry of the oxidant owing to the formation of sulfate free radicals. Similar is the situation in silver(I) catalyzed oxidation of the substrate which is also a second order reaction and the rate is still controlled by the interaction of catalyst and oxidant. However, no firm evidence is provided to discriminate between catalyst redox cycles viz. Ag I /Ag II or Ag I /Ag III . Ascorbic acid (vitamin C) has played an essential role as a water soluble vitamin in diet. Survey is developed in a person due to its deficiency vitamin C is also known to help in cardiovascular and cancer diseases. 12 Interestingly plants and some animals make their own vitamin C but it is not in case of human bodies and is, therefore, sought from other sources. 13 It is added as a supplementary source in pharmaceutical preparations. Also, it is a known anti- oxidant in large number of biological processes. The kinetic based determinations in analytical chemistry have been exploited owing to its strong reducing properties. 14 The spectrofluorometric methods are based on vanadium(V) catalysed and ascorbic acid activated oxidation of rhodamine 15,16 or Saframine T 17 by bromate. Such were the observations that tempted us to undertake the title study to gain detailed insight of the reaction events viz. to viz. the role of sulfate free radicals in both silver (I) catalyzed and uncatalyzed oxidations of l-ascorbic acid by peroxodisulfate. Experimental Material and method All the reagents employed in the kinetics study were either of AnalaR or guaranteed reagent grade and were employed as supplied. Silver nitrate (BDH) solution was prepared by dissolving requisite amount of the salt in doubly distilled water. The solution of silver nitrate, however, was kept in bottles painted black from the outside at refrigerated temperature (~ 5 ˚C) to check photochemical decomposition. Nevertheless, afresh solution of the catalyst was employed. Twice distilled water was employed throughout the study; the second distillation was from an alkaline potassium permanganate solution in an all glass apparatus. Other reagents were employed as received. Kinetic procedure The reaction was conducted in Corning glass vessels painted black from the outside. These flasks were immersed in a water-bath thermostated at ± 0.1 °C unless stated otherwise. All other reaction ingredients of requisite concentrations except peroxodisulfate were taken in these flasks, the reaction was initiated by adding temperature pre- equilibrated peroxodisulfate solution and the time of initiation of the reaction was recorded when the pipette was half-emptied. An aliquot sample (5 cm 3 ) of the reaction mixture was withdrawn at different time intervals and then discharged into an ice-cold dil. H2SO4 (~1.0 mol dm -3 ), the remaining ascorbic acid was then estimated by titrating