posted on 2023-01-11, 21:29authored byGyörgy Csekő, Qingyu Gao, Attila K. Horváth
The thiourea–iodate
reaction has been investigated simultaneously
by ultraviolet–visible spectroscopy and high-performance liquid
chromatography (HPLC). Absorbance–time traces measured at the
isosbestic point of the iodine–triiodide system have revealed
a special dual-clock behavior. During the first kinetic stage of the
title reaction, iodine suddenly appears only after a well-defined
time lag when thiourea is totally consumed due to the rapid thiourea–iodine
system giving rise to a substrate-depletive clock reaction. After
this delay, iodine in the system starts to build up suddenly to a
certain level, where the system remains for quite a while. During
this period, hydrolysis of formamidine disulfide as well as the formamidine
disulfide–iodine system along with the Dushman reaction and
subsequent reactions of the intermediates governs the parallel formation
and disappearance of iodine, resulting in a fairly constant absorbance.
The kinetic phase mentioned above is then followed by a more slowly
increasing sigmoidally shaped profile that is characteristic of autocatalysis-driven
clock reactions. HPLC studies have clearly shown that the thiourea
dioxide–iodate system is responsible mainly for the latter
characteristics. Of course, depending on the initial concentration
ratio of the reactants, the absorbance–time curve may level
off or reach a maximum followed by a declining phase. With an excess
of thiourea, iodine may completely disappear from the solution as
a result of the thiourea dioxide–iodine reaction. In the opposite
case, with an excess of iodate, the final absorbance reaches a finite
value, and at the same time, iodide ion will disappear completely
from the solution due to the well-known Dushman (iodide–iodate)
reaction. In addition, we have also shown that in the case of the
formamidine disulfide–iodine reaction, unexpectedly the triiodide
ion is more reactive toward formamidine disulfide than iodine. This
feature can readily be interpreted by the enhancement of the rate
of formation of the transition complex containing oppositely charged
reactants. A 25-step kinetic model is proposed with just 10 fitted
parameters to fit the 68 kinetic traces measured in the thiourea–iodate
system and the second, but slower, kinetic phase of the thiourea–iodine
reaction. The comprehensive kinetic model is constituted in such a
way as to remain coherent in quantitatively describing all of the
most important characteristics of the formamidine disulfide–iodine,
thiourea dioxide–iodine, and thiourea dioxide–iodate
systems.