Oxidation with alkaline permanganate using formic acid for the back-titration : Potentiometric determination of chromium

Determination of chromium by oxidation of chromite with permanganate does not give accurate results. KmnO₄ is reduced to MnO₂. Titration of KmnO₄ with Cr⁺³ solution in the presence of 0.8–1.5N NaOH and Ba⁺² ions yields manganate and gives good results. In the absence of Ba⁺² ions and in the presence of 0.5–2N NaOH reduction of KmnO₄ passes quantitatively to MnO₂.Cr⁺³ can be determined by adding the chromic solution to KmnO₄ while stirring in presence of 1N NaOH and Ba⁺² ions, or a. 2.5N NaOH in the absence ofBa⁺² ions. The excess KmnO₄ is then back-titrated with formic acid.     

Characterization and Catalytic Properties of Aerogel Chromium Oxide Supported by Alumina or Silica

Alumina or silica supported chromium oxide catalysts prepared by sol-gel technique have been investigated by X-ray diffraction, BET analysis, combined diffuse reflectance spectroscopy, EPR and reduction-extraction by ethane 1,2 diol. The results reveal the presence of Cr³⁺, Cr⁵⁺ and Cr⁶⁺ ions. Chromium species Cr⁶⁺ are isolated and atomically dispersed on the alumina support as free chromate CrO ₄ ^2– while on silica, dichromate or polychromate species like Cr₂O ₇ ^2– or Cr₃O ₁₀ ^2– predominate. Chromium oxide exhibits a better catalytic properties for paraxylene partial oxidation, when it is supported on alumina. The difference of oxidation states and degree of oligomerisation of chromium on the two supports affects the catalytic properties.     

A selective colorimetric and fluorescence chemosensing sensor for Cr<Superscript>3+</Superscript> based on a rhodamine base derivative

A rhodamine-conjugated coumarin (L) was used in designing a selective fluorescence chemosensor for the determination of trace amounts of Cr³⁺ ions in acetonitrile–water (MeCN/H₂O (90:10, %v/v) solutions. The intensity of the fluoresce emission of the chemosensor is intensified upon addition of Cr³⁺ ions in MeCN/H₂O (90:10, %v/v) solutions, due to the formation of a selective 1:1 complex between L and Cr³⁺ ions. The fluorescence enhancement versus Cr³⁺ concentration has been found to be linear from 1.0?×?10^?7 to 1.8?×?10^?5 M and a detection limit of 7.5?×?10^?8 M. The proposed fluorescent probe proved to be highly selective towards Cr³⁺ ions as compared to other common metal ions and could be successfully applied to the determination of Cr³⁺ concentrations in some water and wastewater samples.     

Application of H-Point standard addition method and multivariate calibration methods to the simultaneous kinetic-potentiometric determination of permanganate and dichromate

Simultaneous kinetic-potentiometric determination of binary mixture of permanganate (MnO₄ ^?) and dichromate (Cr₂O₇ ^2?) by H-point standard addition method (HPSAM), partial least squares (PLS) and principal component regression (PCR) is described. In this work, the difference between the rate of the oxidation reaction of Fe(II) to Fe(III) in the presence of MnO₄ ^? and Cr₂O₇ ^2? formed the basis of the method. The rate of the consumed fluoride ion for making the complex was detected by a fluoride ion selective electrode (FISE). The results show that the simultaneous determination of MnO₄ ^? and Cr₂O₇ ^2? could be conducted in their concentration ranges of 0.5?C10.0 and 0.1?C20.0 ??g ml^?1, respectively. The total relative standard error (RSE) for applying the PLS and PCR methods to 9 synthetic samples was 5.30 and 3.17, respectively in the concentration range of MnO₄ ^?, and 3.30 and 2.04, respectively, in the concentration range of Cr₂O₇ ^2?. In order for the selectivity of the method to be assessed, we evaluated the effects of certain foreign ions upon the reaction rate. The proposed methods (HPSAM, PLS and PCR) were evaluated using a set of synthetic sample mixtures and then applied to the simultaneous determination of MnO₄ ^? and Cr₂O₇ ^2? in different water samples.     

Thionolin als analytisches Reagens

Summary According to the measurements performed the redox potential of thionoline changes at the temperature of 40° C in the p_H-range of 0,66 to 8,11 from 0,542 V to 0,087 V (SCE). Thionoline can be used for the selective detection of BrO₃ ^– in the presence of ClO₃ ^–, IO₃ ^– and the free halogens (sensitivity p_D=5,4), as well as for the selective detection of Sn²⁺ in the presence of Cr²⁺, Ti³⁺, V²⁺ and Fe²⁺ (p_D=4,6). Sn²⁺, Ti³⁺ and Cr²⁺ can be titrated potentiometrically with thionoline solution in the presence of Co²⁺, Ni²⁺, Mn²⁺, Al³⁺ and Zn²⁺. V²⁺ and Fe²⁺ interfere with this reaction. Sb³⁺ interferes with the determination of Sn²⁺ and Cr²⁺.Chlorine and bromine water in ethanolic solution convert the thionoline easily and quantitatively in tetrahalogenated products. The halogenation can be watched potentiometrically.     

Ca10(CrVO4)6(CrVIO4), a disordered mixed‐valence chromium compound exhibiting inversion twinning

The structure analysis of so‐called 9CaO·4CrO₃·Cr₂O₃ proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr^VIO₄ tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr^VO₄ tetrahedra are in general positions and are ordered. The charge is balanced by Ca²⁺ cations, one of which is located on a threefold axis. The Ca²⁺ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO–CrO_x system and its formation reduces the refractoriness of calcium‐rich compositions in an oxidizing atmosphere.     

Photocatalytic selective oxidation of CO with O2 in the presence of H2 over highly dispersed chromium oxide on silica under visible or solar light irradiation

A highly dispersed Cr⁶⁺-oxide species on silica (Cr/SiO₂) was found to act as an efficient photocatalyst for the selective oxidation of CO into CO₂ with O₂ in the presence of H₂ under visible (λ>420 nm) or solar light irradiation at 293 K. UV-Vis, photoluminescence and FT-IR investigations revealed that the selective reactivity of the photoexcited tetrahedral Cr⁶⁺-oxide species ([Cr⁵⁺−O⁻]^*) with CO, as well as the high reactivity of the photoreduced Cr⁶⁺-oxide species (Cr⁴⁺-oxide species) with O₂ both play significant roles in this reaction.     

Studies on the hydrolytic polymerization of chromium (III) ion: IX. Revision of graphical method for the reaction in the presence of maleic acid by computer

The hydrolytic polymerization of Cr²⁺ in the presence of maleic acid was studied by equilibrium pH method at 60°C and constant ionic strength. Both maleic acid and Cr³⁺ are of three different concentrations: 0.006, 0.008, 0.01 mol·L^?1. The state of Cr³⁺ in aqueous solution were determined by graphical method and pqr analysis. The following species were found (CrA)^+, Cr-(OH)A and Cr₂(OH)₂A₂. The results by graphical method were revised by computer calculation with data of about fifty experimental points. Hydrolysis constants of all species were obtained with good fitting. It is obvious that the results obtained by program LEMIT are more accurate than those by graphical method. Mechanism of Cr³⁺ polymerization in the presence of maleic acid is also discussed.     

Die Additionsmethode in der Photometrie

Standard curves for the photometric determination of Cu²⁺, Ni²⁺, Co²⁺, Fe³⁺ and Cr³⁺, using the colour of their aqueous solutions, are not suitable if the composition of the solutions is unknown. It is more useful, to determine the respective absorption coefficient of the ions in the present solution by adding an known amount of the element. Fe³⁺ is reduced to Fe²⁺, then the absorption of Fe²⁺ is measured at 920 nm. Cr³⁺ must be oxidized to CrO ₄ ^2? and then be reduced again. The standard deviation of the procedures described exceeds 1% only in a few cases. The possibility to determine in the same manner two or three compounds in presence of each other is mentioned.     

A tandem mass spectrometry study of the gas phase RSC6H5Cr+L (R = H,C6H5; L = arene) and C6H5SSC6H5Cr+ ions

Ion-molecule reactions of chromium containing ions with arylsulfides have been studied in the gas phase and their products have been characterized by tandem mass spectrometry. C₆H₅SH and (C₆H₅)₂S react as typical aromatic compounds and give rise to Cr⁺C₆H₅SR] and RC₆H₅Cr⁺QH₅SR′ [R = H, CH₃, CH(CH₃)₂; R′ = H, C₆H₅] ions. Metastable ion mass spectra of the latter species show that the metal is more strongly bound to diphenylsulfide than to alkylbenzenes. C₆H₅SSC₆H₅ reacts with chromium-containing ions to form only Cr⁺(C₆H₅SSC₆H₅). The decomposition characteristics of this ion and, in particular, the presence of a recovery signal in the neutralization-reionization mass spectrum are in keeping with the formation of a 1,2-dithia[2]cyclophane complex ion, which rearranges into a structurel(s) that contains Cr?S bond(s). No evidence was found for metal atom insertion into S?S, C?S, or S?H bonds.     

Determination of stoichiometry of Li1+yCoO2 materials by flame atomic absorption spectrometry and automated potentiometric titration

A rapid, accurate and precise analytical method to determine the stoichiometry of pure, excess lithium and magnesium-doped Li_1+yCoO₂ materials is described. The method is based on the determination of lithium, cobalt and magnesium by flame atomic absorption spectrometry after dissolution of samples in dilute hydrochloric acid. Five-point calibration curves using aqueous standard solutions have been constructed for all the analytes. Relative standard deviation (R.S.D.) of five repeated measurements are better than 0.3% for all metals when the absorbance signal of analytes is set near the middle of the regression lines.The total oxygen content is indirectly determined by potentiometric titration of average oxidation state of cobalt. The Co³⁺ present in the oxide powders is reduced to Co²⁺ at room temperature with a known excess of 0.1 M Fe²⁺ solution in 6 M sulphuric and phosphoric acid mixture. The samples completely dissolved in a few minutes developing a pink coloured and clear solution. The excess of Fe²⁺ ions is back titrated with potassium dichromate standard solution using automated potentiometric end-point detection. The difference between the total cobalt content (found by AAS) and Co³⁺ (found by potentiometric titration) gives the amount of Co²⁺ present in the materials. The S.D. of the determined Co³⁺ value has been estimated to be below 0.03 mg for samples of 10-20 mg. By the proposed method the LiCoO₂ is well characterised and can be applied as a standard reference material for use in lithium batteries technology.     

Two-step one-electron transfer reaction of chromium(III) complex containing levodopa and uridine with N-bromosuccinimide

[Cr^III(LD)(Urd)(H₂O)₄](NO₃)₂?·?3H₂O (LD?=?Levodopa; Urd?=?uridine) was prepared and characterized. The product of the oxidation reaction was examined using HPLC. Kinetics of the oxidation of [Cr^III(LD)(Urd)(H₂O)₄]²⁺ with N-bromosuccinimide (NBS) in an aqueous solution was studied spectrophotometrically, with 1.0–5.0?×?10^?4?mol?dm^?3 complex, 0.5–5.0?×?10^?2?mol?dm^?3 NBS, 0.2–0.3?mol?dm^?3 ionic strength (I), and 30–50°C. The reaction is first order with respect to [Cr^III] and [NBS], decreases as pH increases in the range 5.46–6.54 and increases with the addition of sodium dodecyl sulfate (SDS, 0.0–1.0?×?10^?3?mol?dm^?3). Activation parameters including enthalpy, ΔH*, and entropy, ΔS*, were calculated. The experimental rate law is consistent with a mechanism in which the protonated species is more reactive than its conjugate base. It is assumed that the two-step one-electron transfer takes place via an inner-sphere mechanism. A mechanism for this reaction is proposed and supported by an excellent isokinetic relationship between ΔH* and ΔS* for some Cr^III complexes. Formation of [Cr^III(LD)(Urd)(H₂O)₄]²⁺ in vivo probably occurs with patients who administer the anti-Parkinson drug (Levodopa), since Cr^III is a natural food element. This work provides an opportunity to identify the nature of such interactions in vivo similar to that in vitro.     

Zum Chemischen Transport von Cr2O3 mit Cl2 und mit HgCl2 ? Experimente und Modellrechungen

On the Chemical Transport of Cr₂O₃ with Cl₂ and with HgCl₂ — Experiments and Model Calculations The migration of Cr₂O₃ in a temperature gradient (1 000°C → 900°C) in the presence of low concentrations of chlorine and water (from the wall of silica ampoules) is a result from the endothermic reactions (1) Cr₂O_3,s + H₂O_g + 3 Cl_2,g = 2 CrO₂Cl_2,g + 2 HCl_g (2) Cr₂O_3,s + 1/2 O_2,g + 2 Cl_2,g = 2 CrO₂Cl_2,g With higher concentrations of chlorine, the transport reaction is (3) Cr₂O_3,s + 5/2 Cl_2,g = 3/2 CrO₂Cl_2,g + 1/2 CrCl_4,g The gas phase of the transport system Cr₂O₃/Cl₂ can be reduced step by step by adding small amounts of chromium, so that CrCl₃ and finally also CrCl₂ become more important. Further, at a lower ratio n°(Cl)/n°(Cr) three transport reactions have to be taken into consideration; with the participation of CrOCl_2,g (5). (4) Cr₂O_3,s + 9/2 CrCl_4,g = 3/2 CrO₂Cl_2,g + 5 CrCl_3,g (5) Cr₂O_3,s + 3 CrCl_4,g = 3 CrOCl_2,g + 2 CrCl_3,g (6) Cr₂O_3,s + H_2,g + 4 HCl_g = 2 CrCl_2,g + 3 H₂O_g The reactions (1), (2) and (6) become possible through the cooperation of two transport agents at a time. The migration of Cr₂O₃ with HgCl₂ can also be described with reactions (1) – (3). The decomposition of HgCl₂ Produces the small chlorine pressure for the transport reaction. The oxidation potential of the transport agent HgCl₂ is too low for the oxidation of Cr^III to Cr^VI.     

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.     

Synthèse et propriétés magnétiques de solutions solides entre l'oxyde de chrome CrO2 et l'oxyhydroxyde de cobalt CoOOH

Solid solutions of general formula (1 ? x)CrO₂, xCoOOH have been prepared by hydrothermal synthesis under very high pressure conditions (80 kb). Cr⁶⁺O₃ and CoCr⁶⁺O₄ were used as starting materials. Homogeneous ferromagnetic phases were obtained when 0 ? x ? 0.5. X ray powder patterns clearly demonstrate the isotype with InOOH, an orthorhombic distorted rutile type structure. The results of the magnetic measures performed on samples with different compositions indicate that part of the Cr⁴⁺ cation have been reduced to Cr³⁺ and that the general formula of the solid solutions should be written Cr⁴⁺_1?(x+y)Cr³⁺_yCo²⁺_xO₂H_x+y.     

A kinetic study of the oxidation of 3- and 4-pyridinemethanol,and 3- and 4-pyridinecarboxaldehyde by chromium(VI) in acidic aqueous media

Oxidation of 3-pyridinemethanol (3-pyol), 4-pyridinemethanol (4-pyol), 3-pyridinecarboxaldehyde (3-pyal) and 4-pyridinecarboxaldehyde (4-pyal) by Cr^VI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H_aq ⁺ concentrations; [Cr^VI] = 8 × 10^–4 M, [reductant] = 0.025–0.20 M, [HClO₄] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A linear dependence of the pseudo-first-order rate constant (k _obs) on [reductant] and a parabolic function of k _obs versus [H⁺] lead to the rate law: –d[Cr^VI]/dt = (a + b[H⁺]²)[reductant][Cr^VI], where a and b describe the reaction paths via HCrO₄ ^– and H₃CrO₄ ⁺ species respectively, and are composite values including rate constants and equilibrium constants. The apparent activation parameters were determined from second-order rate constants at 1.0 and 2.0 M HClO₄, at three temperatures within the 293–323 K range. The presence of chromium species with intermediate oxidation states – Cr^V, Cr^IV and Cr^II, was deduced based on e.s.r. measurements and the kinetic effects of Mn^II or O₂ (Ar), respectively. The alcohols were oxidized to the aldehydes, and carboxylic acids and the aldehydes to the carboxylic acids. Chromium(III) was in the form of the [Cr(H₂O)₆]³⁺ complex.     

The spectrophotometric and titrimetric determination of gold with ferroin as reagent

A spectrophotometric and a photometric titration method in a two-phase system for the determination of gold with ferroin is reported. Both methods are rapid and reproducible with an accuracy of ±1%. In the spectrophotometric determination Co²⁺, Cu²⁺, Ni²⁺ , Fe³⁺, Zn²⁺, Mn²⁺ and Cr³⁺ do not interfere with the determination of gold and Pt⁴⁺, Pd²⁺, Hg²⁺, Ir⁴⁺ and Os⁴⁺ can be tolerated up to a ratio of 1:1. The titrations can also be carried out in the presence of a number of diverse ions, e.g. Ni²⁺, Cu²⁺, Co²⁺, Fe³⁺, Zn²⁺, Cr³⁺, Mn²⁺, without interference; the platinum metals and Hg²⁺ cause interference but, by the use of the spectrophotometric procedure, this can be reduced.     

Effect of oxygen non-stoichiometry on the structural and magnetotransport properties of LaMn0.85Cr0.15O3+δ

Thermogravimetry, X-ray diffraction (XRD), d.c. magnetization, high-temperature susceptibility and electrical resistivity measurements were performed for LaMn_0.85Cr_0.15O_3+δ perovskites with accurate control of the oxygen content (0?δ?0.11). For 0?δ<0.09, three orthorhombic structures (Pnma) are found: for 0?δ<0.045, the O′ phase (b/√2<c<a), for 0.045?δ<0.06, the O″ (b/√2<a<c) and for 0.06?δ<0.09, the O^? (a<b/√2<c). For 0.09?δ?0.11, a rhombohedral symmetry coexists with O^? in a biphasic field. Magnetic measurements revealed the ferromagnetic interactions (FM) character of the Mn³⁺-O-Cr³⁺ interaction, but also the intricate magnetic phase diagram due to the presence of multiple interactions (Mn^3+,4+-O-Mn^3+,4+, Cr³⁺-O-Mn³⁺, etc.). The comparison of the results for LaMn_0.85Cr_0.15O_3+δ with those of LaMn_0.9Cr_0.1O_3+δ allows discuss the role of Cr³⁺ on the structural, magnetic and magnetotransport properties of the LaMn_1−xCr_xO_3+δ perovskites.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

Comparative study regarding the formation of La1-xSrxCrO3 perovskite using unconventional synthesis methods

The synthesis of strontium-doped lanthanum chromite, La_1−xSr_xCrO₃ (x=0.1 and 0.3), used as an interconnect material for solid oxide fuel cells (SOFC), was investigated using two unconventional synthesis methods: (1) organic precursors’ method based on the thermal conversion of complex combination resulted in the oxidation reaction of 1,2-ethanediol by La³⁺, Sr²⁺ and Cr³⁺ nitrates; (2) combustion synthesis based on the exothermic redox reaction of La³⁺, Sr²⁺ and Cr³⁺ nitrates with urea and glycine as fuels. We also used a mixture of urea and glycine as fuel. The samples were characterized by means of thermal analysis and X-ray diffraction.