The isothermal conversion of iodine to iodate with hydrogen peroxide is an intriguing step of the Bray-Leibhafsky (BL) oscillatory reaction. On a light note, this reaction is the process of catalytic decomposition of hydrogen peroxide into water and oxygen in the presence of hydrogen and iodate ions. Basically, it is composed of two periodically dominating processes the reduction of iodate to iodine and the backward oxidation of iodine to iodate. These oxidation and reduction reactions are inherently intricate and involve several intermediate species.
The most intriguing reaction is the oxidation of iodine to components with high oxidation numbers, which conducted as a separate BL subsystem in presence of mild mechanical mixing may take very long duration to end. Although it suggests large energy barrier, in periodic intervals the reaction dominates the whole BL reaction with still undetermined degree of conversion of iodine to iodate. This can however be speeded up by the absence of mixing and introduction of iodate or silver ions, bearing in mind that addition of new components may induce new reactions which may mask crucial properties of the mechanism. The fact that it is difficult to quantify iodate in the presence of hydrogen peroxide, which is present at the end of the reaction, makes the determination of iodine conversion to iodate a demanding task. More so, the energetically limiting step in this multi electron process may be identified by analyzing their thermodynamics.
Researchers led by Professor Dragomir Stanisavljev from the Faculty of Physical Chemistry at University of Belgrade in Serbia investigated the dominance of thermodynamically demanding oxidative processes over simple catalytic peroxide decomposition that take place during the reaction of iodine with hydrogen peroxide. Their main aim was to quantitatively measure the amount of iodate generated during iodine oxidation with hydrogen peroxide and examine its efficiency despite of the large thermodynamic barrier. Their research work is now published in the journal, Chemical Physics Letters.
The empirical procedure used involved, first, preparation of fresh solutions of reactants for the five experiments to be undertaken. Secondly, the concentrations of both hydrogen peroxide and sulphuric acid to be used were determined by iodometric titration and alkalimetric titration, respectively. The team also ensured that they verified spectrophotometrically the concentrations of iodine before every experiment. Lastly, the main procedures that involved running the iodine oxidation reaction and determination of iodate quantity using stopped-flow technique after reaction is completed, were carried out over five different days.
The research team observed that from the five different experiments, high degree of iodine conversion to iodate was achieved. More so, it was confirmed that after the induction period of slow spontaneous hydrogen peroxide catalytic decomposition, reaction dynamics were dominated by thermodynamically demanding oxidative processes. The research team also noted that the isothermal change of the reaction dynamics introduced some specific energy redistribution which was seen as a possible initiator of the oxidizing radicals.
The reaction of isothermal iodine oxidation with hydrogen peroxide simultaneously followed by potentiometric and spectrophotometric methods has been presented. The iodate amount generated in this reaction was determined using adjusted stopped-flow kinetic technique. The values obtained from the various experiments have been seen to be in good agreement and indicate that using the considered conditions, almost all iodine ends up as iodate despite of a large thermodynamic barrier. In conclusion, this is a positive leap towards a better understanding of the Bray-Leibhafsky reaction mechanism, which, despite almost 100 years of continuous research, is still unknown in many details. Better characterization of the involved energy redistribution may help understanding the mechanisms of other similar systems as well.
Kristina Z. Stevanovic´, Itana Nuša M. Bubanja, Dragomir R. Stanisavljev. Domination of thermodynamically demanding oxidative processes in reaction of iodine with hydrogen peroxide. Chemical Physics Letters volume 684 (2017) page 257–261
Go To Chemical Physics Letters