Reduction of Triple-Charged Chromium Ions on Indium Cathode in Alkaline Sulfate Solutions

The problem of how the surface coverage of an indium electrode with chromium adatoms formed during the discharge of Cr(III) ions affects electrolytic evolution of hydrogen in alkaline solutions is studied using linear and cyclic voltametry. The method used permits stating that, in alkalinized media, the chromium deposition abruptly decelerates due to the formation of a hydroxide film on the electrode surface. Based on the results obtained, it is assumed that this film consists of mixed Cr(III)- and Cr(II)-containing hydroxo compounds (of the chromite type) and coadsorbed Cr_n clusters formed in the course of electrolysis.     

Sorption of chromium(VI) and chromium(III) on aluminium hydroxide

Factors that influence the sorption of Cr(VI) and Cr(III) on aluminium hydroxide were investigated. The sorption of chromates decreases as the pH of the suspension increases. The mechanism of CrO ₄ ^2– sorption was interpreted in terms of reactions between chromates and –OH and/or H₂O groups at the hydroxide/liquid interface. It has been shown that chromates are more tightly sorbed on aluminium hydroxide compared to other anions, e.g. chlorides. On the other hand, specifically absorbed anions, such as molybdates, compete strongly with chromates for the sorption sites. The sorption of chromium(III) increases with the pH of the suspension. Also, the sorption of chromium(III) is suppressed in the presence of citrate ions. The best conditions for the fixation of Cr(VI) and Cr(III) by aluminium hydroxide are presented.     

Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Reduction of hexavalent chromium at solid electrodes in acidic media: reaction mechanism and analytical applications

The electrochemical detection of hexavalent chromium species was investigated. It was found that Cr(VI) can undergo chemically irreversible reduction in acidic solutions at gold, glassy carbon and boron-doped diamond electrodes. The process was found to be diffusionally controlled at all three electrodes studied. The response obtained at a gold electrode towards the reduction of chromium(VI) produced an electrochemically reversible wave in contrast to those recorded at glassy carbon and boron-doped diamond electrodes. The analytical response of the hexavalent species was studied at gold electrodes in the presence of common environmental interferences: Ni²⁺, Cu²⁺, Fe³⁺, Cr³⁺ and Triton X-100 (surfactant), with an LoD of 4.3 μM obtained in the presence of 5 mM Cr(III).     

Determination of As(III) and total as in water by graphite furnace atomic absorption spectrometry after electrochemical preconcentration on a gold-plated porous glassy carbon electrode

Arsenic(III) was preconcentrated in a flow-through electrochemical cell on a gold coated porous carbon electrode. On stripping, arsenic was eluted with diluted nitric acid and determined off-line by GF AAS. The deposition and stripping steps were optimized. The limit of detection and limit of quantification were found to be 1.9 μg L⁻1 and 6.4 μg L⁻¹, respectively. The repeatability and reproducibility were found to be 5.3 % and 9.3 %, respectively. Total arsenic was determined after a microwave assisted chemical reduction of As(V) to As(III) making the procedure suitable for speciation analysis. The method was applied in analysis of water samples.     

Preconcentration and separation of Cr(III) and Cr(VI) using sawdust as a sorbent

A simple, inexpensive method based on solid-phase extraction (SPE) on sawdust from Cedrus deodera has been developed for speciation of Cr(III) and Cr(VI) in environmental water samples. Because different exchange capacities were observed for the two forms of chromium at different pH—Cr(III) was selectively retained at pH 3 to 4 whereas Cr(VI) was retained at pH 1—complete separation of the two forms of chromium is possible. Retained species were eluted with 2.5 mL 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH. Detection limits of 0.05 and 0.04 μg mL⁻¹ were achieved for Cr(III) and Cr(VI), respectively, with enrichment factors of 100 and 80. Recovery was quantitative using 250 mL sample volume for Cr(III) and 200 mL for Cr(VI). Different kinetic and thermodynamic properties that affect sorption of the chromium species on the sawdust were also determined. Metal ion concentration was measured as the Cr(VI)–diphenylcarbazide complex by UV–visible spectroscopy. The method was successfully applied for speciation of chromium in environmental and industrial water samples.     

The effect of phase composition of Ti-20 at % Cr alloy on its anodic activation in the chloride and bromide solutions

The effect of phase composition of Ti-20 at % Cr alloy on its anodic behavior in the solutions containing aggressive Cl⁻ and Br⁻ anions and, for comparison, non-aggressive SO₄²⁻ anion is studied. Different phase compositions of the alloy were obtained by using different modes of heat treatment. The features of anodic behavior of alloys with various phase compositions are related to the electrochemical properties of titanium and chromium, their tendency to passivation and breakdown of passivity in the presence of various anions.     

Synthesis, crystal structures and magnetic properties of cyanide-bridged macrocyclic complexes

Three cyanide-bridged dodecanuclear macrocyclic wheel-like complexes [Cr(bpmb)(CN)₂]₆[Mn(5-Brsalpn)]₆·12H₂O (1), [Co(bpmb)(CN)₂]₆[Mn(5-Brsalpn)]₆·12H₂O (2) and [Co(bpmb)(CN)₂]₆[Mn(5-Clsalpn)]₆·24H₂O·8CH₃CN (3) [bpmb²⁻= 1,2-bis(pyridine-2-carboxamido)-4-methylbenzenate dianion; 5-Brsalpn²⁻ = N,N′-propylenebis(5-bromosalicylideneaminato) dianion; 5-Clsalpn²⁻ = N,N′-propylenebis(5-chlorosalicylideneaminato) dianion] have been synthesized and their crystal structures and magnetic properties have been investigated. The three compounds are structurally isomorphous and consist of alternating Mn(III)-Schiff base cations and [M(bpmb)(CN)₂]⁻ anions, generating cyanide-bridged nanosized dodecanuclear macrocyclic structures with an approximate diameter of 2 nm. The study of the magnetic properties of complex 1 reveals an antiferromagnetic interaction between the Cr(III) and Mn(III) ions through the cyanide bridges. A best-fit to the magnetic susceptibility of the complex leads to a magnetic coupling constant of J _CrMn = −2.65(6) cm⁻¹ on the basis of a one-dimensional alternating chain model with the Hamiltonian $ H = - J_{CrMn} \sum\limits_{i = 0}^N {S_i \cdot S_{i + 1} } $ H = - J_{CrMn} \sum\limits_{i = 0}^N {S_i \cdot S_{i + 1} } .     

Effect of current density on poisoning rate of Co-containing fuel cell cathodes by chromium

Variation of electrochemical performance of a La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode due to chromium gas-phase deposition has been studied at 800°C. The highest degradation rate is observed under open circuit conditions and is related to formation of a SrCrO₄ layer on the surface of an LSCF cathode. This results in an increase in both polarization and ohmic electrode resistance. The degradation rate is 3.7 and 0.5 Ohm cm²/1000 h, accordingly. When external polarization is applied, the degradation rate decreases considerably. It is found that the amount of chromium in the cathode bulk changes nonlinearly at an increase in current density. At 0.2 A/cm², the overall amount of chromium is twice as large as under open circuit conditions, but the degradation rate is three times lower. Herewith, a considerable amount of chromium was found both on the cathode surface and in its bulk. The results of model experiments show that exposure to external electric current leads to migration of chromium cations in the bulk of the porous cathode. It is shown that the growth of a SrCrO₄ layer on the surface of an LSCF cathode and penetration of chromium into the cathode bulk becomes slower at the current density of 0.5 A/cm². Under similar conditions (temperature, current density, and time), the amount of chromium in an LSCF cathode is 2.5–7.3 times larger as compared to that in La_0.65Sr_0.3MnO₃. However, the rate of degradation of electrochemical performance of an LSCF cathode is lower, which points to its higher tolerance towards the presence of chromium. This is due to higher oxygenionic conductivity of an LSCF cathode.     

Discussion of thermodynamic data obtained by adsorption gas chromatography of hydrocarbons on a new graphitized carbon black with a high specific surface area

Summary A new graphitized carbon black (Carbograph 5) with a specific surface area (560 m² g⁻¹) greater than those of commerically available graphitized carbons was studied by gas chromatography to determine the enthalpy, entropy, and free energy of adsorption of a series of alkanes (C₂−C₆). The adsorption properties were also investigated by considering changes in the isosteric heats and entropies of adsorption when a nonpolar stationary phase (squalane) was added to the adsorbent. The data obtained are discussed and compared with literature values for other graphitized carbon blacks.     

The extraction of Cr(III) from aqueous thiocyanate solutions and its separation from Co(II) and Ni(II)

For the system liquid anion-exchanger—Cr(III)−NCS⁻, an investigation has been made of the dependence of the percentage extraction of Cr(III) on parameters such as standing time of the Cr(III)−NCS⁻ solution, temperature, pH and type of exchanger. Quantitative extraction of e.g. 4·10⁻⁴ M Cr(III) by 0.1M Aliquat in CCl₄ is easily achieved at room temperature, using 4.75M KNCS−0.05N HCl as aqueous phase. At high Cr(III) concentrations, the complex anion present in the organic phase is Cr(NCS) ₆ ³⁻ ; when working with dilute metal ion solutions, the species extracted is Cr(NCS)₄ (H₂O) ₂ ⁻ . Separations of mixtures containing 10⁻²−10⁻⁴ M Co(II), Ni(II) and Cr(III) have successfully been accomplished.     

Electroreduction of chromic acid during the chromium deposition: An RRDE study

The rate of the trivalent chromium ion formation during the chromic acid reduction in the presence of sulfate ions is studied in a wide potential range by the RRDE method. At potentials of incomplete reduction of chromate ions, the reaction is attendant by a side process (probably, the hydrogen ion reduction). With a phase film covering the disk cathode, the ring current does not correspond to the number of Cr(III) ions formed at the disk, rather it characterizes the oxidation of products of a phase film comprising oxide-hydroxide compounds of Cr(III). The hypothesis about the formation of a phase cathodic film, presumably preceding the metallic chromium deposition, is confirmed experimentally. The oxide-hydroxide species of Cr(III), which are main film-forming substances, undergo phase transitions in a zone that depends on the sulfate ion concentration. The phase transitions whose products may reach the ring electrode occur in the near-electrode layer. The lower the sulfate ion concentration, the larger the extension of the layer.     

Formation of Cr(III) Hydroxides from Chrome Alum Solutions: 1. Precipitation of Active Chromium Hydroxide

The formation of active chromium hydroxide, Cr(OH)₃·3H₂O, was studied through potentiometric titrations and turbidimetric measurements. UV-Vis and IR spectroscopies were also employed to characterize the synthesized solid. The rapid addition of NaOH solution to aqueous chrome alum (KCr(SO₄)₂·12H₂O) solutions caused the immediate precipitation of the active material. Only monomeric Cr(III) species seemed to be participating in the precipitation process; neither chromium polymers nor complexes with anions (SO²⁻₄, Cl⁻, NO⁻₃, ClO⁻₄) influenced the fast formation of Cr(OH)₃·3H₂O. Titration studies allowed the determination of several hydrolysis and precipitation constants for Cr(III). Nevertheless, they cannot be used for the estimate of Cr(OH)⁰₃formation constant.     

Electron transfer in the course of chromium ion reduction/oxidation in the potassium chloride melt

A hypothesis about formation of a polynuclear chromium complex is made on the basis of a voltammetric study of the processes of trivalent and divalent chromium reduction within complex chloride ions in a potassium chloride melt. The effect of the reaction of Cr₂Cl₇³⁻ formation on deviation of theoretical dependences from the experimentally obtained dependences under the terms of linear voltammetry is shown. Models are suggested for the complex chromium ions, quantum-chemical calculations of the bond lengths in them are performed and steric energies of ions are determined. Diffusion coefficients of Cr(III) and Cr(II) are calculated, diffusion activation energies are determined.     

DDTC-Na-based colorimetric chemosensor for the sensing of cyanide in water

Sodium diethyldithiocarbamate (DDTC-Na) was demonstrated to be a new colorimetric cyanide chemosensor by utilizing an indirect trick. First, some copper ions were added to the colorless aqueous solution of DDTC-Na. Then, the resultant brown solution was studied upon the addition of different anions, including Cl⁻, I⁻, IO₃⁻, SO₄²⁻, NO₂⁻, Br⁻, H₂PO₄⁻, F⁻, SCN⁻, HSO₄⁻, ClO₄⁻ and CN⁻. It was observed by naked eyes that the brown solution changed to colorless immediately after the addition of the trace cyanide, but there were no changes towards other anions, making DDTC-Na a good selective cyanide chemosensor in pure water. Supported by the National Natural Science Foundation of China (Grant Nos. 20674059 & 20402011)     

Formation of chromium composite electrochemical coatings from sulfate oxalate solutions based on Cr(III)

The mechanism responsible for the inclusion of Al₂O₃ and SiC nanoparticles, the mixture of Nb₂N and Ta₂N (1: 1), MoS₂, Cr₂O₃, and SiO₂ with diverse electric conductivity, hydrophilicity, and resistance to solution components in chromium deposits from the sulfate–oxalate suspension solutions based on Cr(III) was studied. The main factors that determine the formation of chromium composite electrochemical coatings, their composition, and surface morphology were determined. The film on the surface of the growing deposit of the intermediates of the reduction of chromium ions plays the key role in the formation of composite coatings from Cr(III) and Cr(VI) sulfate–oxalate suspension solutions. The film can play the role of a structural mechanical barrier that hinders the incorporation of particles in the deposit, or it can fix the particles on the electrode surface by creating hydroxo bridges with chemisorbed hydroxide compounds on the particle surface.     

Metallosurfactants of Chromium(III) Coordination Complexes. Synthesis, Characterization and Determination of CMC Values

A number of mixed ligand chromium(III)–surfactant coordination complexes, of the type cis-[Cr(en)₂(A)X]²⁺ and cis-α-[Cr(trien)(A)X]²⁺ (A = Dodecyl or Cetylamine; X = F⁻, Cl⁻, Br⁻) were synthesized from the corresponding dihalogeno complexes by ligand substitution. These compounds form foam in aqueous solution when shaken. The critical micelle concentration (CMC) values of these surfactant metal complexes in aqueous solution were obtained from conductance measurements. Specific conductivity data (at 303, 308 and 313 K) served for evaluation of the temperature-dependent critical micelle concentration (cmc) and the thermodynamics of micellization (Δ G_m⁰, Δ H_m⁰ and Δ S_m⁰).     

Kinetic studies on acid-catalysed aquation of some [Cr(ox)<Subscript>2</Subscript>(amino acid)]<Superscript>2−</Superscript> complexes

Three chromium(III) complexes of general formula [Cr(ox)₂(Aa)]²⁻ (Aa is an α-amino acid, namely alanine, valine or cysteine) were obtained and characterized in solution. In acidic solutions, [Cr(ox)₂(Aa)]²⁻ undergo acid-catalysed aquation to cis-[Cr(ox)₂(H₂O)₂]⁻ and the appropriate amino acid. The process goes through a metastable intermediate with monodentate amino acid coordinated via the carboxylate oxygen atom. The kinetics of the chelate ring opening were studied under isolation conditions. The determined pseudo-first-order rate constants were linearly dependent on [H⁺]. A mechanism is proposed, in which the reactive form of substrate is in the form of the conjugate acid.     

Composition, Structure, and Corrosion–Electrochemical Properties of Chromium Coatings Deposited from Chromium(III) Electrolytes Containing Formic Acid and Its Derivatives

To study electrocatalytic properties of chromium, its deposits are obtained from Cr(III) electrolytes containing formic acid and its four derivatives (formaldehyde, methyl alcohol, formamide, dimethylformamide) and comprehensively examined. Amorphous structure of the deposits is established with an x-ray diffraction analysis. The valence state of chromium and major extrinsic elements is studied by x-ray photoelectron spectroscopy. The carbon reduction degree and the organization of carbon particles in deep layers of the deposits (chainlike or graphite-like structures) is shown to depend on the nature of organic compounds added into the chromium-plating electrolyte. The corrosion–electrochemical behavior of the obtained deposits in 0.5 M H₂SO₄ is compared to that of polycrystalline chromium and deposits plated from sulfate–oxalate Cr(III) electrolytes.