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Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium
journal contribution
posted on 2017-03-24, 14:18 authored by Li Xu, László Valkai, Alena A. Kuznetsova, Sergei V. Makarov, Attila K. HorváthThe thiourea dioxide
(TDO)–iodine reaction
was investigated spectrophotometrically
monitoring the consumption of total amount of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength
in buffered slightly acidic medium. The nitrogen- and carbon-containing
products were found to be ammonium ion and dissolved carbon dioxide,
respectively, while from sulfur part sulfate ion was exclusively detected,
when fresh TDO solution was used. The stoichiometry of the reaction
was established as 2I2 + TDO + 4H2O →
SO42– + 2NH4+ +
4I– + CO2 + 4H+ indicating
a strict 2:1 stoichiometric ratio. However, using aged TDO solution
this stoichiometric ratio is shifted to lower values suggesting the
formation of elementary sulfur augmented by the 2TDO + I2 + 4H2O → S + SO42– + 4NH4+ + 2I– + 2CO2 hypothetical
limiting stoichiometry. We also confirmed experimentally that in aqueous
solution TDO slowly rearranges into an unindentified species. This
species then produces elementary sulfur at a later stage of the aging
process via subsequent reactions accounting for a loss of reducing
power. The direct reaction between TDO and iodine was found to be
relatively rapid and completed within seconds in absence of initially
added iodide ion. Formation of the latter ion, however, strongly inhibits
the oxidation process; hence, the system is autoinhibitory with respect
to iodide ion. Furthermore, increase of pH markedly accelerates the
reaction as well. These observations suggest that a short-lived steady-state
intermediate (iodinated TDO) is produced in a rapid pre-equilibrium,
where iodide and hydrogen ions are also involved. A nine-step kinetic
model, to be able to describe the most important characteristics of
the experimental curves with four fitted parameters, is proposed and
discussed.