pH Oscillations and Mechanistic Analysis in the Hydrogen Peroxide–Sulfite–Thiourea Reaction System

Abstract

A new pH oscillator has been constructed by combining the pH clock reaction H2O2-SO3(2-)-H(+) with thiourea (Tu, (NH2)2CS) as a proton-consuming species. The system exhibited oligo-oscillatory behavior in a closed system, and large amplitude oscillations in a continuous-flow stirred tank reactor (CSTR) were observed in a narrow range of input concentrations, flow rate, and temperature. For the purpose of constructing the kinetic model, a reversed-phase ion-pair high-performance liquid chromatography (HPLC) and mass spectrometer (MS) were used to track and determine intermediate species during the oxidation of thiourea by hydrogen peroxide. Experimental results illustrated that the four species: thiourea monoxide (TuO), formamidine disulfide (Tu2(2+)), thiourea dioxide (TuO2), and thiourea trioxide (TuO3) were formed during the oxidation process. A ten-step mechanistic model was proposed, where TuO was another key species participating in two proton feedback loops in addition to bisulfite. Numerical simulations based on this model agreed well with the experimental results.

Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl+ Versus O Atom Transfer

“…Formation of those species would lead to an absorbance decrease rather than the rise shown in Figure . The HPLC chromatogram of the product mixture (Figure S4) clearly reveals the formation of thiourea monoxide (TUO, (NH 2 ) 2 CSO) by comparing the retention time with that reported in literature; moreover, the UV spectrum of this species obtained by HPLC with diode-array detection corresponds to the spectrum of the product mixture (Figure S5). Figure S5a also shows that TUO has an absorbance maximum at 260 nm and a significant but smaller absorbance at 292 nm.…”

Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium

“…Thiourea dioxide ((NH 2 ) 2 CSO 2 , TDO) has been extensively used in chemistry and chemical technology as an effective reducing agent despite the fact that its transformation in redox reactions is not yet fully understood. − Even nowadays new applications of this versatile compound have been reported that span a wide range of different areas in chemistry and chemical technology. Without providing an exhaustive survey the following important fields should be emphasized to support this statement: reduction of graphene and graphite oxides, preparation of nanometer-sized metal powders, organocatalysis, , polymerization reactions, and even in demonstrating nonlinear dynamical phenomena in chemical kinetics. , Majority of these new fields usually exploits rich chemistry of TDO such as its unusual structure, rearrangement in aqueous solutions, as well as its complex decomposition in different solvents. Focusing on the application of TDO in aqueous solution it is well-known that strong reducing feature of TDO in alkaline condition stems from a controlled release of sulfoxylic acid or sulfoxylate ion by splitting the unusually long C–S bond of the target molecule .…”

“…Without providing an exhaustive survey the following important fields should be emphasized to support this statement: reduction of graphene 4 and graphite oxides, 5 preparation of nanometer-sized metal powders, 6 organocatalysis, 7,8 polymerization reactions, 9 and even in demonstrating nonlinear dynamical phenomena in chemical kinetics. 10,11 Majority of these new fields usually exploits rich chemistry of TDO such as its unusual structure, rearrangement in aqueous solutions, as well as its complex decomposition in different solvents. Focusing on the application of TDO in aqueous solution it is well-known that strong reducing feature of TDO in alkaline condition stems from a controlled release of sulfoxylic acid or sulfoxylate ion by splitting the unusually long C−S bond of the target molecule.…”

Autonomous Chemical Modulation and Unidirectional Coupling in Two Oscillatory Chemical Systems

“…The master system was the bromate–sulfite pH oscillator − and the slave system was the hydrogen peroxide–sulfite pH oscillator (Figure ). , Both chemical systems have been intensively studied in the literature, and several nonlinear behaviors have been discovered and observed in these systems (e.g., bistability, bifurcation). − In experiments, we used the following reagent-grade chemicals, NaBrO 3 (Sigma-Aldrich), Na 2 SO 3 (Sigma-Aldrich), H 2 SO 4 (Sigma-Aldrich), NaHCO 3 (Sigma-Aldrich), and H 2 O 2 (Sigma-Aldrich). The aqueous solutions of all chemicals were always prepared freshly before the experiments to avoid any decay of chemicals and the oxidation of sulfite.…”