Abstract
The peroxo compounds H2O2 and K2S2O8 represent one of the major constituents in many oscillatory chemical systems. In this contribution we demonstrate that beside H2O2 and S2 the sodium perborate (NaBO3·H2O) can act as alternative oxidizing agent in oscillatory reactions. So far the H2O2 has been successfully substituted with NaBO3 in two oscillators: in the BO–S2–Cu(II) flow system potential and pH oscillations, in the strongly alkaline Cu(II)-catalyzed BO–SCN− batch reaction, which are rather different in their chemistry and dynamics, potential oscillations were observed. In spite of the significant differences in the oxidizing nature of H2O2 and NaBO3 we assume that the oscillatory cycle in the BO–substrate and in the H2O2–substrate systems is similar in many aspects, therefore the numbers of this new subgroup of the oscillators may be considered to be borate-mediated H2O2 oscillators. Mechanisms are suggested and simulations are shown to describe the oscillatory behaviors observed in the perborate chemistry based oscillators by using the assumption that the oxidation reactions of the intermediates (HO)3B(OOH)− and (HO)2B(OOH) anions, which are dominant species in alkaline and neutral pH solutions of perborate, are much faster than that of H2O2.
Highlights
The known oscillatory chemical reactions in which peroxo compounds are involved as oxidant include the classical Bray (H2O2–IO−3 ) and Briggs-Rauscher [H2O2–IO−3 –MA—Mn(II)] reactions, some one-substrate pH-oscillators [H2O2–S2−; H2O2–S2O32−–Cu(II); H2O2–S2O42−; H2O2–Fe(CN)64−], a few two-substrate pH-oscillators [H2O2–SO32−–Fe(CN)64−; H2O2–SO32−– S2O32−; H2O2–HSO−3 –HCO−3 ], the Cu(II) catalyzed H2O2–SCN− and S2O82−–S2O32− systems, and the Ag(I)-catalyzed S2O82−–S2− reaction
Using the procedures described in the Materials and Methods section we were successful with observing periodic responses in both perborate—reductant—Cu(II) systems but the oscillatory concentrations differed significantly from those when H2O2 was used as oxidant
The question, what we address here is the following: do the perborate species contribute substantially to the development of the oscillations or the dynamics is mainly governed by the reactions of hydrogen peroxide? According to the literature between pH = 8 and 12 the kinetic effect of the existing different perborate species can be significant (Burgess and Hubbard, 2013)
Summary
The known oscillatory chemical reactions in which peroxo compounds are involved as oxidant include the classical Bray (H2O2–IO−3 ) and Briggs-Rauscher [H2O2–IO−3 –MA—Mn(II)] reactions, some one-substrate pH-oscillators [H2O2–S2−; H2O2–S2O32−–Cu(II); H2O2–S2O42−; H2O2–Fe(CN)64−], a few two-substrate pH-oscillators [H2O2–SO32−–Fe(CN)64−; H2O2–SO32−– S2O32−; H2O2–HSO−3 –HCO−3 ], the Cu(II) catalyzed H2O2–SCN− and S2O82−–S2O32− systems, and the Ag(I)-catalyzed S2O82−–S2− reaction. Among the listed systems two representative oscillators, the Cu(II)-catalyzed H2O2–S2O32− (Orbán and Epstein, 1987) and the H2O2–SCN− reactions (Orbán, 1986) were selected to test for observing oscillations when the H2O2 is replaced with NaBO3. In the H2O2–S2O32−–Cu(II) system addition of acid (H2SO4), in the H2O2–SCN−– Cu(II) reaction addition of base (NaOH) as auxiliary reagent was necessary to tune the systems to the oscillatory domain. Both oscillators are highly reproducible, they show wide variety of dynamical phenomena and the main reaction steps in their chemical mechanism are well-established. The H2O2–SCN−–Cu(II) reaction displays even richer dynamics. Under flow conditions it shows three types of bistability
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