Reactions anodiques et cathodiques des composes de Cr(VI) en milieu de nitrates alcalins fondus

The electrochemical behavior of chromate and dichromate, ions in molten NaNO₃-KNO₃ at 250°C has been studied by single-sweep voltammetry and cyclic voltammetry. The anodic dimerization of CrO₄^2- ions has been shown to proceed in three steps, the reaction: CrO₄^2-→CrO_4ad^?+e^? being rate-determining. The cathodic reduction of Cr₂O₇^2- ions includes the coupled acid-base reactions; it forms solid chromium oxide Cr₂O₃ and the soluble chromate and chromite ions.     

Free radical reactions of bromoform (BF) in air free and air saturated aqueous solutions. Effect of pH

G(Br̄) and G(H₂O₂) have been reported in Ar, N₂O and air saturated bromoform, containing 0.1 M t-butanol at various pHs. The results demonstrate an increase in G(Br̄) with increasing bromoform concentration and increasing pH. e^-_aq and Ḣ atoms react with bromoform to give Br̄, whereas ȮH radicals produce the t-butanol radical which interacts with bromoform and generates Br̄. This reaction is catalysed at acidic and alkaline pH. The results suggest that the reactivity of the primary radicals and t-butanol radical appear to be in the order e^-_aq > ḢȮH > ĊH₂(CH₃)₂COH. Ḃr radicals formed as a result of reactions of dibromomethyl radical with bromoform, further react with bromoform to give Br̄.G(Br̄) values at high pH are attributed to the formation of a reactive intermediate BrOḢ^- (Ḃr + OH^-), which decays to Br̄ and ȮH at higher pH. G(Br̄) values are considerably higher in the presence of oxygen. This is because of the formation of peroxy radicals CHBr₂Ȯ and CBr₃Ȯ which decompose to Br̄ and CO, and Br̄ and CO₂, respectively. High G(Br̄) values are found in the presence of isopropanol, ethanol and methanol suggesting a chain reaction. Rate constants for the reaction of ċHBr₂ radical with isopropanol, ethanol and methanol have been measured as 553, 136 and 56 mol^-1dm³s^-1 respectively. G(H₂O₂) ∼ 0.6 in Ar and N₂O saturated solution, and G(H₂O₂) ∼ 1.0 in air saturated solutions.     

Synthesis and dichromate anion extraction ability of p-tert-butylcalix[4]arene diamide derivatives with different binding sites

The article describes the synthesis and evaluation of the dichromate anion (Cr₂O₇²⁻/HCr₂O₇⁻) extraction properties of p-tert-butylcalix[4]arene diamide derivatives (5-7) containing different binding sites. Among these compounds, 6 and 7 have been synthesized via aminolysis in a toluene-methanol solvent system with 3-aminomethylpyridine and 3,6-dioxa-1,8-diamino octane, respectively. On the other hand, compound 5 has been synthesized via an acid chloride method due to its inefficiency under aminolysis. The extraction properties of these diamides toward dichromate anions are studied by liquid-liquid extraction. The results show that p-tert-butylcalix[4]arene diamide derivative 7 exhibited a much higher affinity toward dichromate anions than that of 6 due to its special structure, while 5 was an ineffective ligand for these anions.     

Radiation chemistry of xylenol orange in aqueous solutions at different acidities

The radiolytic decolourization and peroxide formation have been studied in aqueous solutions of xylenol orange (XO) at different acidities. The G(-XO) increases from 0.78 at pH 11, to 3.70 at pH 3. The peroxide yield also increases from 1.19 at pH 11, to 3.34 in 0.025 mol dm^-3 H₂SO₄. In alkaline solutions only the OH^. decolourizes XO whereas in acidic solutions both the H and OH^. decolourize XO though G(-XO) due to H-atoms is less. The ionization of the phenolic group in XO influences the e^-_aq reaction with it. In alkaline solution, the oxidized and reduced XO formed by OH^. and e^-_aq reactions, respectively, react together regenerating original XO. Near 0.025 mol dm^-3 H₂SO₄, there is an abstraction of H-atom from XO by HO₂ whereas at other acidities, H₂O₂ is formed by disproportionation of peroxides. Reaction schemes have been given to explain the various radiolytic yields.     

Kinetics and mechanism of the chromic oxidation of 3-O-methyl-d-glucopyranose

The oxidation of 3-O-methyl-d-glucopyranose (Glc3Me) by Cr^VI in acid medium yields Cr^III, formic acid and 2-O-methyl-d-arabinose as final products when a 50-times or higher excess of Glc3Me over Cr^VI is used. The redox reaction takes place through the combination of Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. Intermediacy of free radicals and Cr^II in the reaction was demonstrated by the observation of induced polymerization of acrylamide and detection of CrO₂²⁺ formed by reaction of Cr^II with O₂. Intermediate oxo-Cr^V–Glc3Me species were detected by EPR spectroscopy. In 0.3–0.5 mol/L HClO₄, intermediate Cr^V rapidly decompose to the reaction products, while, at pH 5.5–7.5, where the redox processes are very slow, five-coordinate Cr^V bis-chelates of the pyranose and furanose forms of Glc3Me remain more than 15 h in solution. The C1–C2 bond cleavage of Glc3Me upon reaction with Cr^VI distinguishes this derivative from glucose, which is oxidized to gluconic acid.     

Solubility in the Na2Cr2O7-(NH4)2Cr2O7-K2Cr2O7-H2O system

Solubility in the Na₂Cr₂O₇-(NH₄)₂Cr₂O₇-K₂Cr₂O₇-H₂O four-component water-salt system at 25, 50, and 75°C was studied for the first time. Phase field boundaries for individual salts and potassium and ammonium dichromate solid solutions, monovariant lines, and invariant points were determined. Experimental data were used to optimize the looped isohydric process of potassium dichromate preparation involving additional salts.     

A thermodynamic study of the Cu-Cr-O system by the EMF method

The equilibrium oxygen pressures of the three-phase regions [Cu, Cr₂O₃, Cu₂Cr₂O₄], [Cu, Cu₂O, Cu₂O₂O₄] and [CuO, Cu₂O, Cu₂Cr₂O₄] were measured as a function of temperature by the solid oxide electrolyte electromotive force method. The measured Gibbs energy of the reaction Cu₂O+ Cr₂O₂ = Cu₂Cr₂O₄ (ΔG°) was found to be −46608 + 7.8328 T J mol⁻¹ (1075–1275 K). The evaluated Gibbs energy of formation of Cu₂Cr₂O₄ (ΔG°(inf,Cu₂Cr₂O₄)) was found to be −1332900 + 332.761 T J mol⁻¹ (900–1350 K).     

Radiolysis of tetrachloromethane-water two-phase systems

The radiolysis of two-phase systems CCl₄-water proceeds in kinetical regime up to dose 12 kGy. Groos radiation yields of chloride ions are the same as the radiolysis of saturated solutions in this period. A two-phase rule of additivity is valid and the partial yields for both phases were calculated; G_CCl4(Cl^- = 5.61 ± 0.10 and G_H2O(Cl^-) = 8.29 ± 0.51 molecules/100 eV, respectively.The radiolysis proceeds in diffusional regime at the absorbed dose of more than 50 kGy. The gross radiation yield of chloride ions is determined by hydrolysis of molecular chlorine which is produced with G(Cl₂) = 0.68 ± 0.14 molecules/100 eV. An additional part of chloride ions is produced by radiolysis of substrates which diffuse into both phases with value G_dif(Cl^-) = 2.38 ± 0.31 molecules/eV. This value is approximately three times less than the gross radiation yields in kinetical regime of radiolysis of two-phase systems in saturated solutions of these substrates.     

Free Ḃr atom and free radical reactions in the radiolysis of 1,2-dibromoethane (DBE) and other bromohydrocarbons in air-free aqueous solutions

G(Br^-)-values have been evaluated in Ar- and N₂O-saturated 1,2-dibromoethane (DBE), bromobutane, propylbromide and bromopentane (BP) under a variety of experimental conditions in aqueous solutions. G(Br^-) increases with increasing halogenated hydrocarbon concentration and with increasing pH. Free Ḃr atoms formed as a result of e^-_aq and ȮH radical reactions with bromohydrocarbons are considered to initiate a chain reaction. Another chain reaction has been invoked to explain the high G(Br^-) at high pH. Ḃr radicals may form a reactive intermediate with OH^- (BrOH^⨪), which decays to Br^- and ȮH. The ȮH radicals have been found to interact with bromohydrocarbons and eventually yields Br^- ions. The formation of Br^- ions is accelerated both at acidic and alkaline pH in systems containing t-butanol. Dose rate studies in Ar- and N₂O-saturated 1,2-DBE and BP show an increase in G(Br^-)-values at low dose rate and confirm the occurrence of a chain reaction initiated by Ḃr atoms. From these studies the rate constant for the reaction of Ḃr with 1,2-DBE has been calculated to be ∼ 1 × 10⁶M^-1s^-1.     

Single-drop micro-extraction and diffuse reflectance Fourier transform infrared spectroscopic determination of chromium in biological fluids

The present paper deals with a new micro-extraction procedure for selective separation of Cr(VI) in the form of a metaloxy anionic species namely dichromate (Cr₂O₇²⁻) with N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) in to dichloromethane and its subsequent and rapid diffuse reflectance Fourier transform infrared spectroscopic (DRS-FTIR) determination employing potassium bromide matrix. The diffuse reflectance Fourier transform infrared spectroscopy gives both qualitative and quantitative information about the dichromate. The determination of chromium is based on the analytical peak selection, among the various vibrational peaks, at 902 cm⁻¹. The micro-extraction was based on the liquid-liquid solvent extraction (LLSE) principle. The dichromate binds with the nitrogen and oxygen atoms of N¹-hydroxy-N¹,N²-diphenylbenzamidine (HOA) and forms 1:2, Cr(VI):HOA complex in 0.1 mol L⁻¹ HCl medium. The formation of above complex, in the acidic medium, is confirmed by the appearance of chocolate-brown color in the micro-extract. The speciation studies of Cr(III) and Cr(VI) is done by conversion of Cr(III) into Cr(VI) utilizing H₂O₂ as an oxidizing agent. The chemistry of pure dichromate and that of its HOA complex is discussed. The limit of detection (LoD) and the limit of quantification (LoQ) of the method are found to be 0.01 μg g⁻¹ Cr₂O₇²⁻ and 0.05 μg g⁻¹ Cr₂O₇²⁻, respectively. The standard deviation value and the relative standard value at a level of 10 μg Cr₂O₇²⁻/0.1 g KBr for n = 10 is found to be 0.26 μg Cr₂O₇²⁻ and 2.6%, respectively. The relative standard deviation (n = 8 and 6) for the determination of dichromate (Cr₂O₇²⁻) in real human biological fluid samples is observed to be in the range 3.1-7.8%.     

Structural characterization and catalytic properties of bis(1,1,3,3-tetramethylguanidinium) dichromate

The structure of bis(1,1,3,3-tetramethylguanidinium) dichromate was determined from powder X-ray diffraction data. The compound crystallizes in the monoclinic system (space group P2₁/n) with a = 10.79714 (15) Å, b = 11.75844 (16) Å, c = 8.15097 (11) Å, β = 109.5248 (6)°. The structure consists of dichromate anions (Cr₂O₇²⁻) stabilized by tetramethylguanidinium cations ([H₂NC(N(CH₃)₂)₂]⁺ or [TMGH]⁺). Phase transitions of [TMGH]₂Cr₂O₇ were determined by differential scanning calorimetry, thermal gravimetric analysis and in situ Raman spectroscopy, where the decomposition of the matrix into CrO_x was found at 171-172 °C. Further heat treatment to above 400 °C resulted in formation of the thermodynamically stable Cr₂O₃, most likely with the [TMGH]⁺ cation as reductant. The catalytic activity of [TMGH]₂Cr₂O₇ supported on TiO₂ anatase in the selective catalytic reduction (SCR) of nitrogen oxide was also investigated, however only moderate activity was observed in the temperature range 100-400 °C compared to the activity of e.g., vanadia supported on titania.     

Photofragmentation of alpha-oxocarboxylic acids in aqueous solution. An EPR study. Formation of semidione radicals by decarboxylative substitution of alpha-oxocarboxylic acids by acyl radicals--II. Oxaloacetic acid (oxosuccinic acid).

Abstract— –On in situ photolysis (Λ= 250–400 nm) of aqueous oxaloacetic acid solutions, between pH 5 and 10, the radicals ^-O₂CCH₂C(O^->=C(O⁺)CH₂CO₂^- and ^-O₂CCH₂C(OH)CO₂ are identified. With acetone present, CH₂CO₂, CH₃C(OH)CO^-₂, CH₃C(O^-)=C(O)CH₂CO₂ and -O₂CCHCOCO₂ are also observed. CO₂ and CO are identified as reaction products. The experimental results are explained in terms of α-cleavage of the electronically excited keto-isomer dianion of oxaloacetic acid to yield O₂CCH₂CO and CO₂^-. ^-O₂CCH₂CO adds to the keto-isomer of oxaloacetic acid and to pyruvic acid, which is formed from oxaloacetic acid by thermal decarboxylation, to yield ^-O₂CCH₂C(O^-)= C(O^-)CH₂C0^- and CH₃C(O^-)=C(O)CH₂CO^-₂, respectively, via a decarboxylase substitution reaction. CH₂CO₂ is derived from ^-O₂CCH₂CO by decarbonylation. CO₂^- is scavenged by oxaloacetic acid and pyruvic acid to yield O₂CCH₂C(OH)CO₂ and CH₃C(OH)CO₂^-, respectively.     

The first chromate containing both Cr<Subscript>2</Subscript>O<Subscript>7</Subscript> and Cr<Subscript>3</Subscript>O<Subscript>10</Subscript> groups,bis(1,3-bis(4-piperidinium)propane) dichromate trichromate monohydrate: Synthesis and crystal structure

A new organic-inorganic hybrid material (C₁₃H₂₈N₂)₂[Cr₂O₇][Cr₃O₁₀]·H₂O (1) was synthesized by slow solvent evaporation at room temperature, and its crystal structure was determined by single crystal X-ray diffraction. This compound crystallizes in the monoclinic space group P2₁/c. The asymmetric unit contains two crystallographically independent 1,3-bis(4-piperidinium)propane, (H₂bppp)²⁺ cations (A and B), one trichromate Cr₃O ₁₀ ^2? anion, one dichromate Cr₂O ₇ ^2? anion, and one water molecule. All these entities are interconnected into a complicated two-dimensional hydrogen bonded network via N–H?O and O–H?O hydrogen bonds. Furthermore, this structure is stabilized by a large number of C–H?O interactions, thus establishing a three-dimensional network structure. This compound appears to be the first example of chromates containing both Cr₂O₇ and Cr₃O₁₀ groups.     

Crystal structure and thermal analysis of the tetramethylammonium dichromate and trichromate

The structures of the tetramethylammonium dichromate, [(CH₃)₄N]₂Cr₂O₇ and trichromate, [(CH₃)₄N]₂Cr₃O₁₀, were determined from single-crystal X-ray diffraction data. These compounds crystallize in the orthorhombic system (space group Pnma, with Z=4 and a=17.192(1) Å, b=8.55(1) Å, c=10.637(1) Å), for the dichromate and in the monoclinic system (space group P2₁/n, with Z=4 and a=11.366(2) Å, b=8.493(2) Å, c=20.187(4) Å, β=103.98(3)° for the trichromate. The structures consist of discrete dichromate anions (Cr₂O₇)^2– or trichromate anions (Cr₃O₁₀)^2–, respectively, stabilized by quaternary ammonium [(CH₃)₄N]⁺. Phase transitions in [(CH₃)₄N]₂Cr₂O₇ have been evidenced by differential scanning calorimetry as well as a new allotropic variety of [(CH₃)₄N]₂Cr₂O₇ which was characterized by X-ray powder diffraction. It crystallizes in an orthorhombic system with the unit cell parameters a=24.49(1) Å, b=8.85(1) Å, c=8.705(8) Å.     

Crystal structure of 1-cyclohexylpiperazine-1,4-diium dichromate(VI)

Single crystals of 1-cyclohexylpiperazine-1,4-diium dichromate(VI), (C₁₀H₂₂N₂)[Cr₂O₇], were obtained by slow evaporation at room temperature from an aqueous solution of potassium dichromate, hydrochloric acid and 1-cyclohexylpiperazine. (C₁₀H₂₂N₂)[Cr₂O₇] is triclinic (P\(\bar 1\)) with a = 10.351(2) Å, b = 12.766(3) Å, c = 6.111(1) Å, α = 91.50(2)°, β = 104.26(3)°, γ = 94.91(2)°, V = 778.8(3) ų, and Z = 2. The structure determination performed from single crystal X-ray diffraction data leads to R₁/wR₂ reliability factors of 0.032/0.078. The asymmetric unit of the title salt C₁₀H₂₂N₂²⁺·Cr₂O₇^2?, consists of one 1-cyclohexylpiperazine-1,4-diium dication and one dichromate dianion. These entities are linked together by N–H···O hydrogen bonds to form {(C₁₀H₂₂N₂)[Cr₂O₇]}_n infinite chains lying parallel to the (100) plane and running along the c axis. The intermolecular N–H···O hydrogen bonds link these chains into a two-dimensional network structure consolidated through C–H···O weak interactions.     

Inorganic interferences in the 2,4-xylenol spectro-photometric method for nitrate and their elimination

A study of inorganic interferences with the 2,4-xylenol spectrophotometric method for nitrate and their elimination is reported. Fifty-three substances do not interfere with the original method. Nitrite interferes somewhat by producing a faint yellow color. Certain reducing agents (Fe²⁺, S^2-, S₂O₃^2-, and SCN^-) cause low results by reducing the nitrate in the strong sulfuric acid solution, while some oxidizing agents (Mn⁷⁺, Cr⁶⁺, V⁵⁺, and ClO₃^-) cause low results by inactivating or destroying the 2,4-xylenol. Persulfate and small amounts of H₂O₂ produce a slight deepening of the color; larger amounts of H₂O₂; cause low results, as do Cl^-, Br^-, I^-, and metals. The recommended maximum permissible limits (mg per 10-ml aliquot) for the original method are NO₂^--N, Fe²⁺, S^2-, SCN^-, V⁵⁺, ClO₃^-, Cl^-, I^-, 0.2; Mn⁷⁺, Cr⁶⁺, S₂O₈^2-, 5; H₂O₂, 0.02; S₂O₃^2-, Br^-, 0.1; metals, none. Procedures for the elimination of most of the interferences are described. Nitrite is destroyed with sulfamic acid. The interferences of reductants (Fe²⁺, S^2-, S₂O₃^2-, and SCN^-) and oxidants (Mn⁷⁺ and Cr⁶⁺) are eliminated with hydrogen peroxide, the excess of which (and S₂O₈^2-) is destroyed by boiling in the presence of Fe³⁺. The interference of Cl^-, Br^-, and I^- is eliminated by precipitation with silver sulfate. An alternative to the sulfamic acid procedure is to oxidize nitrite to nitrate with peroxide and deduct NO₂^--N from the total NO₃^--N. After elimination of interferences, a 10-ml aliquot of sample solution is treated with 17.0 ml of sulfuric acid and 2,4-xylenol, the 6-nitro-2,4-xylenol is steam-distilled into an ammonia—water—isopropanol mixture, and the yellow color is measured.     

A novel dichromate-sensitive fluorescent nano-chemosensor using new functionalized SBA-15

A novel fluorescence nano-chemosensor for Cr₂O₇²⁻ anion has been developed by assembly of fluorescent aluminum complex of 8-hydroxyquinoline (AlQ_x) within the channels of modified SBA-15. SBA-SPS-AlQ_x shows a fluorescence emission at 486 nm. The observed remarkable fluorescence of SBA-SPS-AlQ_x quenches in presence of Cr₂O₇²⁻ anion. The results showed that this fluorescent nano-material can be a useful chemo-sensor for determination of dichromate anions in aqueous solutions. The linear detecting range of fluorescent nano-chemosensor for Cr₂O₇²⁻ anion was 0.16–2.9 μmol L⁻¹. The lowest limit of detection (LDL) was also found to be 0.2 ng mL⁻¹ in aqueous solutions. SBA-SPS-AlQ_x showed selectively and sensitively fluorescent quenching response toward Cr₂O₇²⁻ ion in comparison with I₃⁻, NO₃⁻, CN⁻, CO₃²⁻, Br⁻, Cl⁻, F⁻, H₂PO₄⁻ and SO₄²⁻ ions, which was because of the higher stability of its inorganic complex with dichromate ion.     

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr⁶⁺ ions (solution of potassium dichromate, K₂Cr₂O₇) in a water cathode was studied during a DC discharge in air. The concentration range of Cr⁶⁺ was (5.7–19) ×10^?5 mol/l and discharge current range was 20–80 mA. Cr⁶⁺ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr⁶⁺ kinetics. The main processes of Cr⁶⁺ reduction and Cr³⁺ oxidation were revealed. HO ₂ ^· radicals and hydrogen peroxide were shown to be responsible for Cr⁶⁺ reduction whereas ^·OH radicals and O₂ molecules provide the reverse process of Cr³⁺ oxidation to Cr⁶⁺. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.     

Identification,composition, thermodynamic and structural properties of a pyroaurite-like iron(II)-iron (III) hydroxy-oxalate green rust

The aerial oxidation of aqueous suspensions of ferrous hydroxide precipitated from ferrous oxalate and caustic soda can lead to an iron (II)-iron (III) hydroxy-oxalate of the pyroaurite group, a GR(C₂O₄^2-) Green Rust. As other GR compounds, it is unstable with respect to the action of oxygen and oxidises later on. Its chemical composition was determined to be [Fe^II₆ Fe^III₂(OH)₁₆]²⁺[C₂O₄^2- · nH₂O], with n more likely equal to 3 on the basis of structural considerations. The composition does not vary and the Fe (II) / Fe (III) ratio in the compound is measured by means of transmission Mössbauer spectroscopy at 78 K and 20 K in the range from 2.8 to 3.2 for various samples at various stages of the reaction. GR(C₂O₄^2-) is paramagnetic at both temperatures and is unambiguously distinguished from ferrous hydroxide, the initial reactant, and magnetite, the main final product, which are magnetically ordered at 20 K. The spectrum of the GR compound is composed of three quadrupole doublets, one due to the Fe(III) cations characterised by a small quadrupole splitting ΔE_Q of 0.40 mm s⁻¹, and two due to the Fe(II) cations, characterised by larger ΔE_Q values of about 2.55 and 2.85 mm s⁻¹. Finally, from the observed equilibrium conditions between ferrous hydroxide and GR(C₂O₄^2-), the standard free enthalpy of formation of GR(C₂O₄^2-) was computed to be : ΔG°_f[Fe^II₆ Fe^III₂ (OH)₁₆]²⁺[C₂O₄^2- · 3H₂O] = −5383 ± 3 kJ mol⁻¹.     

The gamma radiolysis of aqueous solutions of ferriciminodiacetate and ferricglycinate

Radiolytic G-values were determined for [−Fe(III)IDA], Fe(II), (−IDA) and CHO-COOH in deoxygenated aqueous solutions of Fe(III)IDA and Fe(III)gly, and in the presence of scavengers, t-butanol, 2-propanol, methanol, sodium formate and O₂. The metal ion was reduced by e^-_aq, H or secondary radicals HO₂, CO⁻₂, CH₂-OH and (CH₃)₂C-OH, while the OH radical did so indirectly through α-hydrogen abstraction from the ligand followed by intramolecular electron transfer. The rate constant k[OH + Fe(III)IDA], determined by the competition kinetics method at pH 2.0, was 1.7 × 10⁸ M^-1 s^-1.