Indian Journal of Chemistry Vol. 46A, July 2007, pp. 1090-1094 Oxidation of hexacyanoferrate(II) by peroxodisulphate in aqueous perchloric acid⎯A kinetic and mechanistic study S A Chimatadar, S V Madawale, K A Thabaj & S T Nandibewoor* PG Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India Email: stnandibewoor@yahoo.co Received 26 June 2006; revised 16 May 2007 The kinetics of the reaction between hexacyanoferrate(II) and peroxodisulphate has been studied spectroscopically in perchloric acid. The reaction is first order each in peroxodisulphate and hexacyanoferrate(II) concentrations. Increase in perchloric acid concentration increases the reaction rate. The order with respect to perchloric acid concentration is less than unity. The active species of hexacyanoferrate(II) and peroxodisulphate are HFe(CN) 6 3- and S 2 O 8 2- respectively. A suitable mechanism is proposed for the studied reaction. The thermodynamic quantities with respect to the first step and the activation parameters with respect to the slow step of the mechanism have been calculated. Numerical simulation with this mechanism agrees well with the experimental results. IPC Code: Int. Cl. 8 C07B33/00 The hexacyanoferrate(II) ion is a one-electron reducing agent with a relatively stable octahedral coordination sphere 1 , with the ground state being 2 1 A 1g . The reduction potential 3 of the couple [Fe(CN) 6 ] 3- /[Fe(CN) 6 ] 4- in the pH range 1-13 is 0.41 V. The hexacyanoferrate(II) ion is oxidised to Prussian Blue by oxygen in acidic, but not significantly in neutral or alkaline solutions. The oxidation of hexacyanoferrate(II) by different oxidants has been reported earlier 4,5 . Several oxidations by peroxodisulphate do not pro- ceed at convenient rate at ambient temperature despite a relatively high redox potential (E = 2.01 V). In spite of this, some oxidation-reduction reactions have been investigated using peroxodisulphate as an oxidant 6,7 . The peroxodisulphate-hexacyanoferrate(II) reaction has been studied in aqueous solutions by a number of workers 8,9 and the results have been interpreted by postulating the formation of ion pairs. When the reactants are present as potassium salts, the rate determining step may involve the participation of the KFe(CN) 6 3- and KS 2 O 8 - ion pairs as reactive species. This hypothesis was used by Chlebeck and Lister 10 , but has been criticized by Kershaw and Prue 11 because one can always improve the fit of data by introducing more adjustable parameters to take account of additional reaction pathways and associated species. However, unless independently obtained values of the association constants of the latter are available, the exercise is of doubtful value. Similar ideas have been pointed out by other authors in different reactions. Hexacyanoferrate(II) is known to form different protonated species in aqueous acid medium 12 and one needs to know the active form of it in the reaction. The mechanism may be quite complicated due to the formation of different intermediates. The earlier workers 8,9 have not mentioned the role of protonated species of hexacyanoferrate(II) in their work. Herein, we have investigated the title reaction in order to understand the behaviour of protonated species of hexacyanoferrate(II) and peroxodisulphate in acid media. A suitable mechanism has also been proposed in terms of active species. Materials and Methods The solutions were prepared in doubly distilled water, in an all-glass unit in the presence of potassium permanganate. Reagent grade chemicals were used. Solutions of the oxidant, peroxodisulphate and reductant, hexacyanoferrate(II), were prepared by dissolving potassium peroxodisulphate (SD Fine Chem) and potassium hexacyanoferrate(II) (BDH, AR) in water and standardized by known methods 13 . Freshly prepared solutions were used in all the kinetic studies. Hexacyanoferrate(III) and sodium sulphate solutions were prepared in water. Perchloric acid (Glaxo, AR) was used to provide the required acidity. The ionic strength of the solution was kept constant with sodium perchlorate.