Indian Journal of Chemistry Vol. 46A, October 2007, pp. 1579-1584 Bromide mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium D S Munavalli, K A Thabaj, S A Chimatadar & S T Nandibewoor* P.G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India Email: stnandibewoor@yahoo.com Received 18 May 2007; revised 9 August 2007 The micro amount (10 -3 mol dm -3 ) of bromide mediated oxidation of antimony(III) by cerium(IV) in an aqueous sulphuric acid medium has been studied spectrophotometrically at 25°C and I = 3.10 mol dm -3 . In the presence of bromide, one mole of antimony(III) requires two moles of cerium(IV). The reaction is first order with respect to [cerium(IV)] and [bromide], whereas the order with respect to [antimony(III)] is less than unity. Increase in [sulphuric acid] accelerates the reaction rate. The order with respect to [H + ion] is less than unity. Added products, cerium(III) and antimony(V) do not have any significant effect on the rate of reaction. The active species of oxidant is identified as H 3 Ce(SO 4 ) 4 - , whereas that of reductant as SbBr 2 + . The possible reaction mechanism is proposed and the activation parameters determined and discussed. IPC Code: Int. Cl. 8 C07B33/00 Halides play an important role in our daily life. Antimony(III) is stable in high acid concentration and its oxidation by different oxidants is reported in acid media 1 . The reduction potential 2 of the couple Sb V/ Sb III in dilute acid is –0.581 V. In presence of Br - , antimony(III) forms several bromide complexes, such as SbBr 2+ , SbBr 2 + , SbBr 3 , SbBr 4 - , SbBr 5 2- and SbBr 6 3- , similar to that of chloride 3 , but their role is not understood clearly. Cerium(IV) is a well known oxidant in acid media 4 having reduction potential 2 of the couple Ce IV /Ce III (1.70 V) and is stable only in high acid concentration. In sulphuric acid and sulphate media, cerium(IV) forms several sulphate complexes 5 , but their role has not received much attention so far. The slow reaction between cerium(IV) and antimony(III) is facilitated by minute amounts 6 of I - (10 -7 mol dm -3 ), and Cl - (10 -2 mol dm -3 ), and also by Br – (10 -3 mol dm -3 ) (present work) in aqueous sulphuric acid. We report herein the Br - mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium. Materials and Methods The preparation of solutions and the method of following kinetics were as reported in our earlier study 6 . During the kinetic studies, it was observed that under the present experimental conditions in the absence of Br - , the oxidation of antimony(III) by cerium(IV) occurs very slowly, though in a measurable quantities. Hence, during the calculation of the pseudo-first order rate constants (k c ), the uncatalysed rate (k u ) has also to be taken in to account. Thus, k t = k c + k u and, k c = k t – k u The rate constants, k c (average of at least four independent kinetic runs) obtained in this way were reproducible within ±5% (Table 1). Regression analysis of experimental data was performed with Microsoft Office Excel-2003 programme. Results and Discussion Different sets of concentrations of reactants in 1.0 mol dm -3 sulphuric acid, at constant ionic strength, I = 3.10 mol dm -3 and at constant catalyst concentrations were kept over 10 h in a closed container under nitrogen atmosphere at 25°C and were analyzed. The analysis indicated that in presence of Br - , two moles of cerium(IV) reacted with one mole of antimony(III), as given in Eq. (1). 2Ce (IV) + Sb (III) 2Ce (III) + Sb (V) …(1) The orders with respect to cerium(IV) and antimony(III) concentrations were determined as in earlier work 6 and found to be unity and less than unity (0.70), respectively. Br -