Kinetics of the Oxidation of Thiourea by Tris(I,10–Phenanthroline)Iron(III) Complex Ion in Aqueous Perchloric Acid Media
Abstract
The redox reaction between tris(I,10–phenanthroline)iron(III) complex ion, [Fe(phen)3]3+, and thiourea (H2NCSNH2) in aqueous perchloric acid media has been investigated at temperature of 26.0 ± 1.0 oC over the hydrogen ion concentration range 0.50–1.0 mol dm-3 using a UNICO UV–2102PC Spectrophotometer at 510 nm, the visible absorption maximum (λmax) of the product iron specie, [Fe(phen)3]2+ (tris(1,10–phenanthroline)iron(II) complex ion). The ionic strength was maintained at 1.0 mol dm-3 using sodium perchlorate salts, except for the determination of the effect of ionic strength on the rate of the reaction were it was varied between 0.50 and 1.0 mol dm-3. One mole of thiourea was consumed per mole of [Fe(phen)3]3+ with the formation of [Fe(phen)3]2+ and formamidine disulphide (H2N)HN=C–S–S–C=NH(NH2) as products. Under the conditions in which the reaction was carried out, the reaction is of the first order in each of the reactants in aqueous perchloric acidic solutions, and second orders overall. The kinetics of the oxidation obeys the empirical rate law:  with the rate constant, k2, found to be 19.01 ± 0.21 dm3 mol-1 s-1. Taking into account the kinetic data and general observations, a five–step outer–sphere mechanism which lack intermediate complex was proposed for the redox reaction.
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