Study on photocatalytic oxidation for determination of chemical oxygen demand using a nano-TiO2-K2Cr2O7 system

A photocatalytic method for the determination of chemical oxygen demand (COD) using a nano-TiO₂-K₂Cr₂O₇ system is described. The measuring principle is based on direct determination of the change of Cr(III) concentration resulting from photocatalytic oxidation of organic compounds and simultaneous photocatalytic reduction of stoichiometrically involved K₂Cr₂O₇ in the solution. The operation conditions were optimized. The determinative COD value using this method was calculated from the absorbance of Cr(III). The operational characteristics of this method were demonstrated by use of a standard glucose solution as substrate. This method was also applied to the determination of the COD of wastewater samples. The results were in good agreement with those from the conventional (i.e., dichromate) COD methods.     

CO2在Na2Cr2O7溶液中的平衡溶解度模型

为研究重铬酸钠(Na₂Cr₂O₇)对CO₂溶解的影响, 本文在带有搅拌的气液相高压平衡釜内, 采用静态法测定了温度在313.2-333.2 K, 压力在0.1-1.9 MPa 范围内, 重铬酸钠浓度分别为0、0.361、0.650、0.901 mol·kg^-1 时, CO₂ 在Na₂Cr₂O₇溶液中的溶解度. 结果表明: (1) Na₂Cr₂O₇对CO₂ 的溶解有盐析效应; (2) CO₂ 在Na₂Cr₂O₇溶液中的溶解符合亨利定律, 并且CO₂溶解度是温度和Na₂Cr₂O₇浓度的函数, 且用改进的Setschenow方程和Peng-Robinson-Pitzer (PR-Pitzer)方程拟合了在此温度、压力及重铬酸钠浓度范围内的实验数据, 拟合效果较好, 并且其平均相对误差分别为4.24%和3.32%.     

Calorimetric determination of the thermodynamic properties of the alkali metal salts NaNO3, KNO3, Na2Cr2O7, K2Cr2O7 and their binary eutectic solutions

This work is part of more general project in thermal energy storage by means of the solid—liquid phase transformation. The phase diagrams were checked and slightly modified. A drop calorimeter was used to measure the enthalpies over a small temperature range near the melting points, so enabling the determination of the heats of fusion and the specific heats of the solid and liquid phases over these ranges.For NaNO₃, KNO₃, Na₂Cr₂O₇, K₂Cr₂O₇, NaNO₃KNO₃, NaNO₃Na₂Cr₂O₇, NaNO₃, K₂Cr₂O₇, KNO₃K₂Cr₂O₇ and Na₂Cr₂O₇K₂Cr₂O₇, the enthalpies in the solid and liquid states and the heats of fusion were found to be, respectively, in cal mole^?1; 9300, 12800, 3500 at 581 K; 10300, 12500, 2200 at 611 K; 23600, 29600, 6000 at 625 K; 25000, 34400, 9400 at 666 K; 5700, 8200, 2500 at 495 K; 10300, 13800, 3500 at 535 K; 8700, 11300, 2600 at 495 K; 11100, 13500, 2400 at 537 K; 20300, 23600, 3300 at 573 K. The results for the heats of fusion of the solutions are compared with values obtained by means of simple methods of estimation.     

Formation of Ferrite-Chromites BaFe10Cr2O19 and SrFe10Cr2O19 in the Solid-Phase Reaction of Fe2O3 and Cr2O3 with Barium or Strontium Carbonate

X-ray phase analysis and the thermomagnetic method were applied to study solid-phase reactions in mixtures of powders 5Fe₂O₃ + Cr₂O₃ + BaCO₃ and 5Fe₂O₃ + Cr₂O₃ + SrCO₃, yielding, respectively, barium and strontium ferrite-chromite solid solutions BaFe₁₀Cr₂O₁₉ and SrFe₁₀Cr₂O₁₉.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3, 2005, pp. 357–361.Original Russian Text Copyright © 2005 by Bashkirov, Kostyushko.     

Solubility in the system Na2Cr2O7 + 2NH4Cl ⇆ (NH4)2Cr2O7 + 2NaCl-H2O

The solubility in the water-salt quaternary reciprocal system Na₂Cr₂O₇ + 2NH₄Cl ⇆ (NH₄)₂Cr₂O₇ + 2NaCl-H₂O has been investigated for the first time at 25, 50, and 75°C. Using a formal analytical model, the boundaries of the phase fields have been determined, and the univariant lines and invariant points have been calculated. The experimental data have been used to calculate the temperature and concentration parameters of the circular isohydric process of potassium dichromate preparation with the participation of ammonium salts as intermediates.     

The mercury chromates Hg6Cr2O9 and Hg6Cr2O10—Preparation and crystal structures, and thermal behaviour of Hg6Cr2O9

The basic mercury(I) chromate(VI), Hg₆Cr₂O₉ (=2Hg₂CrO₄·Hg₂O), has been obtained under hydrothermal conditions (200 °C, 5 days) in the form of orange needles as a by-product from reacting elemental mercury and K₂Cr₂O₇. Hydrothermal treatment of microcrystalline Hg₆Cr₂O₉ in demineralised water at 200 °C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg₆Cr₂O₁₀ (=2Hg₂CrO₄·2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg₆Cr₂O₉: space group P2₁2₁2₁, Z=4, a=7.3573(12), b=8.0336(13), , 3492 structure factors, 109 parameters, R[F²>2σ(F²)]=0.0371, wR(F² all)=0.0517; Hg₆Cr₂O₁₀: space group Pca2₁, Z=4, a=11.4745(15), b=9.4359(12), , 3249 structure factors, 114 parameters, R[F²>2σ(F²)]=0.0398, wR(F² all)=0.0625. Both crystal structures are made up of an intricate mercury-oxygen network, subdivided into single building blocks [O-Hg-Hg-O] for the mercurous compound, and [O-Hg-Hg-O] and [O-Hg-O] for the mixed-valent compound. Hg₆Cr₂O₉ contains three different Hg₂²⁺ dumbbells, whereas Hg₆Cr₂O₁₀ contains two different Hg₂²⁺ dumbbells and two Hg²⁺ cations. The Hg^I-Hg^I distances are characteristic and range between 2.5031(15) and 2.5286(9) Å. All Hg₂²⁺ groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the Hg²⁺ cations is nearly linear with two tightly bonded O atoms at distances around 2.07 Å. For both structures, the chromate(VI) anions reside in the vacancies of the Hg-O network and deviate only slightly from the ideal tetrahedral geometry with average Cr-O distances of ca. 1.66 Å. Upon heating at temperatures above 385 °C, Hg₆Cr₂O₉ decomposes in a four-step mechanism with Cr₂O₃ as the end-product at temperatures above 620 °C.     

The lowest triplet state of K2Cr2O7. Optical spectra and optically detected electron spin resonance at 1.2 K

High-resolution emission and excitation spectra are reported for a K₂Cr₂O₇ crystal at 1.2 K, and the effect of microwave saturation on thes     

Caracte´risation ete´tude structurale du bichromate mixte AgKCr2O7

The crystal structure of AgKCr₂O₇ has been determined by single crystal X-ray diffraction. The structure is orthorhombic, space groupPnam, with cell constantsa = 15.175(7),b = 7.414(4), andc = 6.014(2)A? forZ = 4. The structure was refined to a finalR of 0.072 for 934 reflections. The configuration of the dichromate ion, pseudo-C_2v, is similar to that found in most other dichromate structures. The potassium ion is surrounded by eight oxygen atoms and the silver ion has a square planar coordination.     

The lowest triplet state of K2Cr2O7. Electron spin resonance in an external magnetic field at 1.2 K

ESR experiments on the luminescent triplet state of triclinic K₂Cr₂O₇ are reported. For the A site D = 24.300, E = 2.324 GHz, for t     

Cryoscopic studies in molten salts. Dissociation state of some alkali isopolymolybdates and some related molybdenum(VI) compounds in molten K2Cr2O7and KNO3

The dissociation state of the solutes M₂MoO₄, M₂Mo₃O₁₀, M₂Mo₄O₁₃, M₂Mo₅O₁₆ (MRb or Cs), Na₂CrO₄·MoO₃, K₂CrO₄·2 MoO₃, Cr₂Mo₃O₁₂ and V₂MoO₈ was studied cryoscopically in molten K₂ Cr₂O₇ and KNO₃ solvents. The freezing point depression, ΔT, of the solvents was obtained by measuring the cooling curves of the binary salt mixtures over unlimited range of solute concentration. The number of foreign ions obtained ν, showed that the solutes were either simply dissociated in the melt into the probable stable species (MoO₄)^2?, (Mo₃O₁₀)^2?, (Mo₄O₁₃)^2? and (Mo₅O₁₆)^2? or, in some cases after reactions and rearrangements, into (CrMo₂O₁₀)^2? heteropolyions. The solute V₂MoO₈, on the other hand, was found to dissolve without any apparent dissociation. An agreement between the experimental and calculated values of activity, a, based on the Temkin and Random Mixing models and that of Van't Hoff's equation support the proposed simple dissocia- tion scheme for K₂Cr₂O₇Cs₂MoO₄ system.     

Structure and Ionic Conductivity of Bi6Cr2O15, a New Structure Type Containing (Bi12O14)n Columns and CrO4 Tetrahedra

Powder samples of the Cr⁶⁺-containing compound Bi₆Cr₂O₁₅ were prepared by solid state reaction of Bi₂O₃ and Cr₂O₃ in air at 650°C. The structure was solved and refined using high-resolution neutron powder diffraction data in space group Ccc2, with anisotropic thermal displacement parameters a=12.30184(5), b=19.87492(7), and c=5.88162(2) Å, V=1438.0 ų, and 126 variables to R_F=1.8%. Bi₆Cr₂O₁₅ exhibits a new structure type that contains (Bi₁₂O₁₄)⁸ⁿ⁺_n columns, of the kind previously found only for phases isotypic with Bi₁₃Mo₄VO₃₄. Each column is surrounded by eight CrO²⁻₄ tetrahedra. The ionic conductivity of Bi₆Cr₂O₁₅ was determined by impedance measurements to be 3.5×10⁻⁵ (Ω cm)⁻¹ at 600°C.     

β‐K2Cr2O7

The monoclinic modification of dipotassium dichromate, β‐K₂Cr₂O₇, has been synthesized in the K₂Cr₂O₇–H₂O system. The structure consists of K⁺ cations and Cr₂O₇^2? dimers. In contrast with triclinic α‐K₂Cr₂O₇ [Kuz'min, Ilyukhin, Kharitonov & Belov (1969). Krist.Tech. 4 , 441–461], the Cr₂O₇^2? groups in β‐K₂Cr₂O₇ have twofold crystallographic symmetry and are parallel to each other.     

Polymorphism in the Sc2Si2O7-Y2Si2O7 system

This paper examines the structural changes with temperature and composition in the Sc₂Si₂O₇-Y₂Si₂O₇ system; members of this system are expected to form in the intergranular region of Si₃N₄ and SiC structural ceramics when sintered with the aid of Y₂O₃ and Sc₂O₃ mixtures. A set of different compositions have been synthesized using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1750 °C during different times. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β-RE₂Si₂O₇ polymorph, with γ-RE₂Si₂O₇ and δ-RE₂Si₂O₇ showing very reduced stability fields. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of the components. Although, the XRD data show a complete solid solubility of β-Sc₂Si₂O₇ in β-Y₂Si₂O₇ at 1300 °C, the ²⁹Si MAS-NMR spectra indicate a local structural change at x ca. 1.15 (Sc_2−xY_xSi₂O₇) related to the configuration of the Si tetrahedron, which does not affect the long-range order of the β-RE₂Si₂O₇ structure. Finally, it is interesting to note that, although Sc₂Si₂O₇ shows a unique stable polymorph (β), Sc³⁺ is able to replace Y³⁺ in γ-Y₂Si₂O₇ in the compositional range 1.86?x?2 (where x is Sc_2−xY_xSi₂O₇) as well as in δ-Y₂Si₂O₇ for compositions much closer to the pure Y₂Si₂O₇.     

Phase relations of the Cr2O3TiO2 system

The phase relations of the system Cr₂O₃TiO₂ were determined at temperatures between 1400 and 1765°C in air. Discrete homologous series of Cr₂Ti_n?2O_2n?1, with n = 6, 7, 8, were found to be stable as single phases in the range of certain temperatures, while a continuous solid solution existed in the composition of n > 8 below 1425°C. This presence and its stable region of a new compound of Cr₂TiO₅ corresponding to n = 3 are revealed in the present paper. Cr₂Ti₂O₇, the so-called E phase, existed in wide homogeneity range, corresponding to the composition of approximately 3 < n < 5. High-temperature phases (called n and n′ phases in the present work) existed above 1425°C and seemed to be closely related to each other from the viewpoint of the structure except that some X-ray diffraction lines of n phase were strongly diffused. Both rutile and chromia had limited solid solubilities. In the present paper, phase relations between Cr₂O₃ and TiO₂ are summarized in a phase diagram.     

Polymerization of aniline derivatives by K2Cr2O7 and production of Cr2O3 nanoparticles

Oxidative polymerization of aniline, anthranilic acid, and aniline‐co‐anthranilic acid by potassium dichromate Cr(VI) as an oxidant in acidic medium was investigated. In this study, the polymerization process of aniline, o‐anthranilic acid as well as aniline/o‐anthranlic acid using K₂Cr₂O₇ produced, coordinated Cr(III)/polyaniline (PANI), Cr(III)/polyanthranilic acid (PAA) and Cr(III)/poly aniline‐co‐anthranilic acid (PANAA). The mechanism of polymerization reaction in the presence of dichromate was hypothesized. The precursor chromium doped polymers were characterized by TGA, FT‐IR, UV‐visible, XRD analyses. Cr₂O₃ nanoparticles size were determined using TEM analysis. The calcinations process of synthesized chromium doped PANI, PAA and PANAA yields Cr₂O₃ nanoparticles 26%, 31%, and 34% wt. respectively. Rhombohedral phase of Cr₂O₃ particles in the range from 33 to 61 nm was produced from chromium/polyanthranilic acid (PAA) and chromium/poly(aniline‐co‐anthranilic acid) PANAA. UV‐ visible analysis showed that optical band gaps (E_g) of doped poly aniline and its derivatives are in the range from1.55 to 1.80 using Tacu's law. The band gap values reveal that the doped chromium emeraldine base can be used as semiconductor materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Some studies on the solid solutions in the systems Sr2Nb2O7Eu2Nb2O7 and Sr2Ta2O7Eu2Ta2O7

Preparation of new solid solutions containing divalent europium have been tried in the systems Eu₂Nb₂O₇Sr₂Nb₂O₇ and Eu₂Ta₂O₇Sr₂Ta₂O₇. These solid solutions described as Eu_2xSr_2(1?x)M₂O₇ (M = Nb and Ta) exist in a pure orthorhombic phase in a limited region of x from 0 to about 0.5. The compounds with compositions close to Eu₂M₂O₇ exist but techniques have not been found to prepare them in pure form.     

Solubility in the NaCl-NH4Cl-KCl-H2O system

This is the first study of solubility in the NaCl-NH₄Cl-KCl-H₂O four-component water-salt system at 25, 50, and 75°C. Phase fields of individual salts and potassium and ammonium chloride solid solutions were demarcated. Experimental data were used to develop a mathematical model of the K⁺, Na⁺, NH₄⁺/Cl⁻, Cr₂O₇²⁻-H₂O five-component reciprocal system, which includes the title four-component system.     

MOF-1和Tb@MOF-1荧光探针的设计、合成及对痕量Cr2O72-、S2O82-离子的识别

采用5-（（4-吡啶基）甲氧基）-异烟酸（H₂PLIA）、1,3,5-三（1-咪唑基）-苯（TIB）合成了金属有机骨架[Cd （PLIA）（TIB）]_n （MOF-1）。MOF-1是具有理想一维孔道的二维结构化合物,其一维孔道由柔性三角形PLIA^2-配体和刚性三角形TIB配体间隔形成。利用MOF-1易掺杂的优势,采用后修饰合成策略制备了Tb@MOF-1。对MOF-1和Tb@MOF-1进行了基本表征及荧光探针性能研究。2种探针材料具有相同的结构。MOF-1和Tb@MOF-1分别对水溶液中的Cr₂O₇^2-和S₂O₈^2-离子具有较强荧光识别能力,均有响应时间快,稳定性、选择性、灵敏度高的特点。研究了MOF-1和Tb@MOF-1对Cr₂O₇^2-和S₂O₈^2-的荧光识别机理,其不同可能与Tb³⁺离子掺杂有关。     

MOF-1和Tb@MOF-1荧光探针的设计、 合成及对痕量Cr2O72-、 S2O82-离子的识别

采用5-((4-吡啶基)甲氧基)-异烟酸(H₂PLIA)、1,3,5-三(1-咪唑基)-苯(TIB)合成了金属有机骨架[Cd(PLIA)(TIB)]_n (MOF-1),MOF-1是具有理想一维孔道的二维结构化合物,其一维孔道由柔性三角形PLIA^2-配体和刚性三角形TIB配体间隔形成。利用MOF-1 易掺杂的优势,采用后修饰合成策略制备了Tb@MOF-1。对MOF-1 和Tb@MOF-1 进行了基本表征及荧光探针性能研究。2种探针材料具有相同的结构。MOF-1和Tb@MOF-1分别对水溶液中的Cr₂O₇^2-和S₂O₈^2-离子具有较强荧光识别能力,均有响应时间快,稳定性、选择性、灵敏度高的特点。研究了MOF-1和Tb@MOF-1对Cr₂O₇^2-和S₂O₈^2-的荧光识别机理,其不同可能与Tb³⁺离子掺杂有关。     

Preparation of Li4Ti5O12 Microspheres with a Pure Cr2O3 Coating Layer and its Effect for Lithium Storage

The pure Cr₂O₃ coated Li₄Ti₅O₁₂ microspheres were prepared by a facile and cheap solutionbased method with basic chromium(III) nitrate solution (pH=11.9). And their Li-storage properties were investigated as anode materials for lithium rechargeable batteries. The pure Cr₂O₃ works as an adhesive interface to strengthen the connections between Li₄Ti₅O₁₂ particles, providing more electric conduction channels, and reduce the inter-particle resistance. Moreover, LixCr₂O₃, formed by the lithiation of Cr₂O₃, can further stabilize Li₇Ti₅O₁₂ with high electric conductivity on the surface of particles. While in the acid chromium solution (pH=3.2) modification, besides Cr₂O₃, Li₂CrO₄ and TiO₂ phases were also found in the final product. Li₂CrO₄ is toxic and the presence of TiO₂ is not welcome to improve the electrochemical performance of Li₄Ti₅O₁₂ microspheres. The reversible capacity of 1% Cr₂O₃-coated sample with the basic chromium solution modification was 180 mAh/g at 0.1 C, and 134 mAh/g at 10 C. Moreover, it was even as high as 127 mAh/g at 5 C after 600 cycles. At-20℃, its reversible specific capacity was still as high as 118 mAh/g.