Electrochemistry of iodine and iodide in chloroaluminate melts

The electrochemical behavior of iodine and iodide has been studied in AlCl₃+NaCl mixtures with compositions ranging from NaCl saturated melts to AlCl₃+NaCl (63+37 mol %) at platinum and tungsten electrodes. Iodide is oxidized in two steps to iodine and I(I); a reduction wave to iodide and an oxidation wave to I(I) are obtained in iodine solutions. The equilibrium constant for the reaction, I^?+I(I)=I₂, is 6×10⁸ l mol^?1 in molten chloroaluminate melts at 175°C.     

In the presence of very fast comproportionation, sampled current voltammetry and rotating disk electrode voltammetry yield equal two versus one-electron limiting current ratios. Reconciliation through analysis of concentration profiles

A species having three sequential redox states is able to react with its higher oxidation (or lower reduction) state producing two equivalents of its middle redox state. A possible electrochemical signature of that comproportionation reaction is that the faradaic current from the two-electron process (I_1,1) might not be twice the current of the corresponding one-electron process (I_1,2). In this paper, using redox-active species with well-separated one- and two-electron processes, such as N,N′-di-n-heptylviologen, N-methyl-4-benzoylpyridinium perchlorate, TCNQ, TTF, N,N′-dimethylphenazine and TMPD, it is reported that within a wide range of the experimental parameters, two seemingly different electrochemical methods, namely sampled current voltammetry (SCV), a diffusion-controlled method, and rotating disk electrode (RDE) voltammetry, a convection-dominated method, give equal mass-transfer limited current ratios (I_1,2/I_1,1). These phenomena have been traced to the fact that close to the electrode distance-normalized concentration profiles generated from both SCV and RDE voltammetry are superimposable. Digital simulations have confirmed these conclusions, and have led to the elucidation of the relative roles of the comproportionation reaction rate constant, k_f, and the diffusion-layer thickness, δ, in determining the value of the (I_1,2/I_1.1)_{SCV or RDE} ratio: when the diffusion-layer is thicker, the comproportionation reaction time is longer and limiting (I_1,2/I_1,1) ratios are reached with lower k_f values. (The larger δ corresponds to longer sampling times in SCV and slower electrode rotation rates in RDE voltammetry.). Ultimately, the limiting values of the (I_1,2/I_1,1)_{SCV or RDE} ratios are controlled by the relative values of the diffusion coefficients of all three species involved in the comproportionation reaction. According to our results, the (I_1,2/I_1,1)_{SCV or RDE} ratio can afford kinetic information on the comproportionation reaction, and comprises a diagnostic criterion for the relative diffusion coefficients of a redox-active species and its one-electron oxidized (or reduced) form.     

Determination of the standard Gibbs energies of transfer of cations across the nitrobenzene|water interface utilizing the reduction of iodine in an immobilized nitrobenzene droplet

When a nitrobenzene (NB) droplet containing iodine is attached to a graphite electrode and immersed into a chloride containing aqueous (AQ) solution, the electrochemical reduction of iodine is accompanied by a transfer of chloride ions from NB to water. These chloride ions enter the NB phase in a preceding partition between the AQ and the NB phases, supported by formation of I₂Cl⁻ ions in NB and accompanied by the transfer of stoichiometric amounts of cations. The overall electrode reaction is of CE_rev type, where C refers to the preceding chemical step forming I₂Cl⁻, and E_rev refers to the reversible reduction of iodine at the graphite|NB interface and the simultaneous transfer of chloride from NB to water. If the chloride concentration in NB is insufficient to compensate by leaving the NB the amount of electrochemically produced iodide, a second voltammetric signal occurs at more negative potentials due to the transfer of iodide from NB to water. The kinetics and thermodynamics of the preceding chemical step C, determine the voltammetric behaviour of the system in such way that the ratio of peak currents of the first and second signals depends linearly on the Gibbs energy of transfer of the co-partitioned cations. The method was validated for cations of known Gibbs energies of transfer, and it was applied to cations of amino acids.     

Electrochemical studies of iodine in an aluminium chloride-butylpyridinium chloride ionic liquid part II. Neutral and basic solvent composition

The electrochemical behavior of iodine in an ambient temperature molten salt system, aluminum chloride-N-(1-butyl)pyridinium chloride (BuPyCl), have been studied in basic (excess BuPyCl) and neutral (1.0:1.0 AlCl₃: BuPyCI mole ratio) melt compositions. Acid-base interactions of iodine in different oxidation states with the ionic solvent are observed. High stability of triiodide ion in neutral butylpyridinium tetrachloroaluminate indicates relatively weak intermolecular interactions in this solvent. In basic solutions polyhalogen equilibria involving iodine in different oxidation states and chloride ions are established. In iodine and tetraethylammonium triiodide solutions a mixture of ICI₂^?, I₂Cl^?, I₃^? and I^? ions forms. The formation constants of I₂Cl^? and I₃^? and the equilibrium constant for I₂Cl^? disproportionation are estimated.     

127I Mössbauer spectra for phenyliodonium ylides

¹²⁷I Mössbauer spectra for the phenyliodonium ylides were measured at 20 K. The valence electron populations (N_s, N_x-z) and the charge number (Z_I) for iodine atom are estimated from the Mössbauer parameters. The obtained populations were very close to those of diphenyliodonium chloride having two I-C primary bonds. To examine the possibility of some double bond character, the electron populations for the case of N_z = 1.90 are estimated. In this case, the Z_I values become larger as 1.2-1.3, and these values were unreasonably large because the values are close to those of PhICl₂, PhI(OAc)₂ having electron withdrawing ligands. Thereby, ¹²⁷I Mössbauer parameters suggest little double bond character for phenyliodonium ylides.     

Redox Denaturation of Proteins: Electrochemical Treatment of Egg Plasma

Although acid/base reactions are used widely to denature proteins during food processing, the application of reduction/oxidation (redox) reactions are relatively rare in this field. Herein, we demonstrate a “redox denaturation” of proteins, using egg plasma as a model. Electrochemical treatment of egg plasma in the presence of iodide ion (I^?) selectively induced intramolecular disulfide bond formations, resulting in a different type of denaturation than that achieved via heat treatment. The reaction mechanism was examined through electrochemical analysis using cyclic voltammetry. Although the involvement of hypervalent iodine (“I⁺”) cannot be ruled out, molecular iodine (I₂) generated at the surface of anode is the most likely oxidizing agent that formed the disulfide bonds. We believe that the redox denaturation of proteins described herein would be a promising approach in food processing and can find practical applications in this field.     

Disposable screen-printed ring disk carbon electrode coupled with wall-jet electrogenerated iodine for flow injection analysis of arsenic(III)

We report here a wall-jet electrogenerated iodine approach for sensitive detection of arsenite (As^III) by using a disposable screen-printed ring disk carbon electrode. Iodide (I⁻) is first oxidized to iodine (I₂) at the disk electrode; the electrogenerated I₂ can be effectively reduced back to I⁻ in the presence of As^III. The inhibited reduction current of I₂ to I⁻ can thus be monitored at the ring electrode and used for As^III analysis. Various factors influencing the flow injection analysis (FIA) of As^III were thoroughly investigated in this study. Under the optimized conditions, a linear calibration plot up to 10 μM with a detection limit (S/N = 3) of 70 nM was obtained by using 50 μM KI as the mobile phase in FIA. Practical utility of the proposed method was demonstrated to detect As^III in “Blackfoot” disease endemic village groundwater from southwestern coast area of Taiwan (Pei-Men).     

Spectroscopic and electrochemical study of conducting films of tetrathiotetracene iodides

Films of tetrathiotetracene (TTT) iodides were prepared by electrochemical doping of deposited TTT films and were subjected to manyfold cycling in 0.2 mol L^–1 aqueous KI solution in the range of potentials from +0.04 to +0.34 V relative to Ag/AgCl. The thus formed films were electrochromic: their color changed reversibly as the potential changed. The results of spectral and electrochemical studies show that radical-cation salts of (TTT)I_x type are accounted for by the presence of several forms of such salts that differ in the content of iodine and reversibly transform into each other with potential changes. The formal compositions of the salts formed under high and low potentials can be approximately described as (TTT)I_0.9 and (TTT)I_0.5, correspondingly.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2400–2403, December, 1995.The authors thank G. M. Mikhailov for measuring XPS and G. A. Krasochka for assistance in electrochemical experiments.This work is supported by the Russian Foundation for Basic Research, Project No. 93-03-4531.     

Transformation of aromatic bromides into aromatic nitriles via formations of grignard reagents and their DMF adducts

Various aromatic bromides were efficiently transformed into the corresponding aromatic nitriles in good yields via the formations of Grignard reagents and subsequently N,N-dimethyl formamide (DMF) adducts, followed by treatment with molecular iodine (I₂) in aq NH₃ at room temperature. The present reaction is an easy and practical method for the preparation of aromatic nitriles from aromatic bromides with less toxic reagents, such as Mg, DMF, I₂, and aq NH₃.     

Electronic spectral studies and solvent effects on the reaction of iodine with 1,4,8,11-tetraazacyclotetradecane

The reaction between iodine and 1,4,8,11-tetraazacyclotetradecane (TACTD) is studied photometrically in various solvents like CCl₄, CHCl₃, CH₂Cl₂ and 1,2-dichloroethane. The results reveal that in each solvent the (TACTD):I₂ ratio is 1:2 and the iodine complex is formulated as (TACTD)I⁺·I₃⁻. The obtained values of the formation constant (K), extinction coefficient (ε) and oscillator strength (ƒ) for the iodine complex are shown to be strongly dependent on the polarity of the solvent. A linear correlation is obtained between either (ƒ) or (ε) and the dielectric constant (D) of the solvent. The important role of the solvent is mainly suggested to be due to the interaction of the ionic iodine complex with the solvent.     

Iodine Clathrated: A Solid-State Analogue of the Iodine–Starch Complex

Co-crystallizing iodine with a simple dicationic salt (1,8-diammoniumoctane chloride) results in the clathration of the iodine (I₂) molecules inside trigonal and hexagonal helical channels of the crystal lattice with 72 wt % overall I₂ loading. The I₂ inside the bigger trigonal channel forms a I−I⋅⋅⋅I−I⋅⋅⋅I−I halogen-bonded infinite helical chain, while the I₂ in the smaller hexagonal channel is disordered. In both channels the I₂ interaction with the channel wall happens through I−I⋅⋅⋅Cl⁻ halogen bonds. The helical channels in the crystal lattice are constructed via the strong charge-assisted H₂N⁺H⋅⋅⋅Cl⁻ hydrogen bonds between the dications and the chloride anions. The structure shows a marked similarity with the well-known starch–I₂ system, and thus may provide insight for the yet unresolved structure of the I₂ in the helical starch channel.     

Investigation of the reaction of 1-methylimidazoline-2-thione with molecular iodine

The reactions of 1-methylimidazoline-2-thione and its chemical oxidation product, viz., 2,2′-dithiobis(1-methylimidazolin-2-yl) diiodide, with molecular iodine were studied by UV-Vis spectroscopy. The crystal and molecular structure of the compound of composition [(C₃H₂N₂)(HCH₃)S]₂(I₃)₂I₂ was established by X-ray diffraction. In the crystal structure, the dications and the triiodide anions form mixed stacks, between which iodine molecules are located. The dications, anions, and iodine molecules are linked by numerous intermolecular interactions to form three-dimensional supramolecular associates.     

Abiotic reaction of iodate with sphagnum peat and other natural organic matter

Previous studies have shown that iodine (including ¹²⁹I) can be strongly retained in organic-rich surface soils and sediment and that a large fraction of soluble iodine may be associated with dissolved humic material. Iodate (IO₃⁻) reacts with natural organic matter (NOM) producing either hypoiodous acid (HIO) or I₂ as an intermediate. This intermediate is subsequently incorporated into the organic matter. Based on reactions of model compounds, we infer that iodine reacts with peat by aromatic substitution of hydrogen on phenolic constituents of the peat. Alternatively, the intermediate, HIO or I₂, may be reduced to iodide (I⁻). The pH (and temperature) dependence of the IO₃⁻ reaction (reduction) has been explored with sphagnum peat, alkali lignin, and several model compounds. The incorporation of iodine into NOM has been verified by pyrolysis gas chromatography/mass spectrometry (GC/MS). Model compound studies indicate that reduction of IO₃⁻ to HIO may result from reaction with hydroquinone (or semiquinone) moieties of the peat.     

Principes de l'iodimétrie absorptiométrique

A general method for the determination of oxidising or reducing agents has been investigated It is based on the measurement of the absorption of the spectrum of the 1₃^- complex Oxidizing agents may be determined by thu oxidation of iodide into I₃^-, reducing agents by reduction of I₃^-For a right application of this method one must have (a) all the iodine present in the state of 1₃^-, (b) the iodine produced by autoxidation of the iodide ion in negligible quantity Theconstants of the chief equilibria in which 13- is involved (equilibrium with I₂,I_2Br-,I₂Cl^-, IOH) and the kinetics of the autoxidation of the iodide ion in the presence and the absence of the catalyst Cu⁺² have been reinvestigated The acquned information enabled us to determine the general conditions of Applicability of this method and the realization of special analyses, The limit of detection by this method is 2.10^-7 gram equivalent of oxidizing or reducing agent per liter when absorptions are measured through a 5 cm layer.     

Design of BiOBr0.25I0.75for synergy photoreduction Cr（Ⅵ） and capture Cr（Ⅲ） over wide pH range

Conversion of hexavalent chromium (Cr（Ⅵ）) to trivalent chromium (Cr（Ⅲ）) is an effective way to reduce its environmental risk,especially via photoreduction process.However,over a wide range of p H values,it is still a great challenge to achieve a high removal rate,and the disposal of produced Cr（Ⅲ） should be concerned.In this work,we implemented a high removal rate at 98%for Cr（Ⅵ） and total chromium(Cr（T）) over a wide p H range （4–10） through the synergistic effect of adsorption,photoreduction an...     

Catalytic Detection of Iodide at Cationic Kaolinite Modified Gold Electrode in Presence of Thiosulfate

A gold electrode modified by a thin film of cationic kaolinite was used for the electrochemical detection of iodide in aqueous solution in the presence of thiosulfate. At gold electrode, iodide showed two electrochemical systems in the potential range explored (0.10 V to 0.85 V). The pH‐independent system was assigned to the redox couple I₂/I^? and the pH‐dependent one assigned to the redox couple HIO/ . For increased amount of thiosulfate the oxidation peak intensity of the first system increases sharply followed by the gradual decrease of the reduction peak, due to the chemical reaction between thiosulfate and oxidized iodide. The calibration curve in the presence of excess thiosulfate resulted in an increase of the sensitivity by a factor of 7. To improve this sensitivity, the bare gold electrode was coated by a thin film of an anionic exchanger kaolinite, obtained by grafting the ionic liquid (1‐(2‐hydroxyethyl)‐4‐(tert‐butyl) pyridinium chloride). Accumulation‐detection method yielded a spectacular increase of the oxidation peak current of iodide in the presence of thiosulfate ions. At optimized experimental conditions, a sensitivity of 2.45 μA.μM^?1 and a detection limit of 65 nM were obtained. The method was successfully applied for total iodine determination in povidone?iodine formulation.     

Cerium/Ascorbic Acid/Iodine Active Species for Redox Flow Energy Storage Battery

In this study, we developed a novel cerium/ascorbic acid/iodine active species to design a redox flow battery (RFB), in which the cerium nitrate hexahydrate [Ce(NO₃)₃·6H₂O] was used as a positive Ce³⁺/Ce⁴⁺ ion pair, and the potassium iodate (KIO₃) containing ascorbic acid was used as a negative I₂/I⁻ ion pair. In order to improve the electrochemical activity and to avoid cross-contamination of the redox pair ions, the electroless plating and sol–gel method were applied to modify the carbon paper electrode and the Nafion 117 membrane. The electrocatalytic and electrochemical properties of the composite electrode using methanesulfonic acid as a supporting electrolyte were assessed using the cyclic voltammetry (CV) test. The results showed that the Ce (III)/Ce (IV) active species presented a symmetric oxidation/reduction current ratio (1.09) on the C–TiO₂–PdO composite electrode. Adding a constant amount of ascorbic acid to the iodine solution led to a good reversible oxidation/reduction reaction. Therefore, a novel Ce/ascorbic acid/I RFB was developed with C–TiO₂–PdO composite electrodes and modified Nafion 117–SiO₂–SO₃H membrane using the staggered-type flow channel, of which the energy efficiency (EE%) can reach about 72%. The Ce/ascorbic acid/I active species can greatly reduce the electrolyte cost compared to the all-vanadium redox flow battery system, and it therefore has greater development potential.     

Spectroscopic study of interaction of 1H-1,2,4-triazoline-3-thione with molecular iodine

The formation by 1H-1,2,4-triazoline-3-thiones in dilute chloroform solution of n-σ*-complex with molecular iodine of the composition C₂H₂N₃S·I₂ was studied by electronic spectroscopy in the UV and visible regions (log β = 2.14±0.05). By the XRD method the crystal and molecular structure of the product of chemical interaction of the thione with molecular iodine in ethanol, 1,2-di(1H-1,2,4-triazol-3-yl) disulfide, was determined.     

Theoretical study on the adsorption mechanism of iodine molecule on platinum surface in dye-sensitized solar cells

By means of density functional theory calculations, the adsorption process of I₂ at Pt (111) surface in dye-sensitized solar cells (DSSCs) has been investigated. The obtained adsorption energies and stable structures depending on the adsorption sites of the Pt surface are in good agreement with experimental values. Our results show that the dissociative chemisorption and the non-dissociative chemisorption are competitive for the adsorption of I₂ on the Pt surface, and the dissociative pathway is more favored in energy. This study is expected to enrich the understanding on the origin of the excellent heterogeneous catalytic performance of Pt for triiodide reduction and the complex iodine chemistry in DSSCs. Understanding of this adsorption mechanism is helpful for rational screening for redox couple and the Pt-free alternative counter electrode materials.     

A New Dual‐Signalling Electrochemical Sensing Strategy Based on Competitive Host–Guest Interaction of a β‐Cyclodextrin/Poly(N‐acetylaniline)/Graphene‐Modified Electrode: Sensitive Electrochemical Determination of Organic Pollutants

Based on the competitive host–guest interaction between a β‐cyclodextrin/poly(N‐acetylaniline)/electrogenerated‐graphene (β‐CD/PNAANI/EG) film and probe or target molecules, a new dual‐signalling electrochemical sensing method has been developed for the sensitive and selective determination of organic pollutants. As a model system, rhodamine B (RhB) and 1‐aminopyrene (1‐AP) were adopted as the probe and target molecules, respectively. Due to the host–guest interaction, RhB molecules can enter into the hydrophobic inner cavity of β‐CD, and the β‐CD/PNAANI/EG‐modified glassy carbon electrode displays a remarkable oxidation peak due to RhB. In the presence of 1‐AP, competitive association to β‐CD occurs and the RhB molecules are displaced by 1‐AP. This results in a decreased oxidation peak current of RhB and the appearance of an oxidation peak current for 1‐AP, and the changes of these signals correlate linearly with the concentration of 1‐AP. When the value ΔI_1‐AP+∣ΔI_RhB∣ (ΔI_1‐AP and ΔI_RhB are the change values of the oxidation peak currents of 1‐AP and RhB, respectively) is used as the response signal to quantitatively determine the concentration of 1‐AP, the detection limit is much lower than that given by using ΔI_1‐AP or ΔI_RhB as the response signal. This dual‐signalling sensor can provide more sensitive target recognition and will have important applications in the sensitive and selective electrochemical determination of electroactive organic pollutants.