Showing papers on "Chromium published in 1993"

High-Tc Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium(III)-Chromium(II) Cyanides with Tc at 240 and 190 Kelvin

Abstract: Molecular-based magnets with high magnetic-ordering temperatures, T(c), can be obtained by mild chemistry methods by focusing on the bimetallic and mixed-valence transition metal micro-cyanide of the Prussian blue family. A simple orbital model was used to predict the electronic structure of the metal ions required to achieve a high ordering temperature. The synthesis and magnetic properties of two compounds, [Cr(5)(CN)(12)].10H(2)O and Cs(0.75) [Cr(2.125)(CN)(6)].5H(2)O, which exhibited magnetic-ordering temperatures of 240 and 190 kelvin, respectively, are reported, together with the strategy for further work.

The toxicology of chromium with respect to its chemical speciation: A review

Abstract: The properties of trivalent and hexavalent chromium are reviewed with respect to acute and chronic oral toxicity, dermal toxicity, systemic toxicity, toxicokinetics, cytotoxicity, genotoxicity and carcinogenicity. The hexavalent chromium compounds appear to be 10-100 times more toxic than the trivalent chromium compounds when both are administered by the oral route. Dermal irritancy and allergy are more frequently caused by contact with soluble hexavalent chromium compounds. The cytotoxicity of soluble and insoluble hexavalent chromium compounds to fibroblasts is 100-1000 times greater than that demonstrated by trivalent chromium compounds. In short-term tests, the hexavalent chromium compounds demonstrated genotoxic effects four times more frequently than did the trivalent chromium compounds. Carcinogenicity appears to be associated with the inhalation of the less soluble/insoluble hexavalent chromium compounds. The toxicology of chromium does not reside with the elemental form. It varies greatly among a wide variety of very different chromium compounds. Oxidation state and solubility are particularly important factors in considering the toxicity of chromium with respect to its chemical speciation.

Mechanisms of Chromium Carcinogenicity and Toxicity

Abstract: Chromium, like many transition metal elements, is essential to life at low concentrations yet toxic to many systems at higher concentrations. In addition to the overt symptoms of acute chromium tox...

Characterization of enzymatic reduction of hexavalent chromium by Escherichia coli ATCC 33456.

Abstract: Chromium reduction by Escherichia coli ATCC 33456 quantitatively transferred hexavalent chromium, Cr(VI), to trivalent chromium, Cr(III). The reduced chromium was predominantly present in the external medium. Supernatant fluids of cell extract, obtained by centrifugation at 12,000 and 150,000 x g, showed almost the same Cr(VI) reduction activity, indicating that Cr(VI) reduction by E. coli ATCC 33456 was a largely soluble reductase activity. In studies with respiratory inhibitors, no inhibitory effects on aerobic and anaerobic Cr(VI) reduction were demonstrated by addition of cyanide, azide, and rotenone into both intact cell cultures and supernatant fluids of E. coli ATCC 33456. Although cytochromes b and d were identified in the membrane fraction of cell extracts, Cr(VI) was not reduced by the membrane fraction alone. The cytochrome difference spectra analysis also indicated that these cytochromes of the respiratory chain require the presence of the soluble Cr(VI) reductase to mediate electron transport to Cr(VI). Stimulation of Cr(VI) reduction by an uncoupler, 2,4-dinitrophenol, indicated that the respiratory-chain-linked electron transport to Cr(VI) was limited by the rate of dissipation of the proton motive force.

A quantitative diffuse reflectance spectroscopy study of supported chromium catalysts

Abstract: The diffuse reflectance spectra of chromium supported on alumina, on silica, and on silica-alumina have been investigated after calcination, CO reduction, and recalcination. A method for quantifying the Cr 6+ , Cr 3+ , and Cr 2+ concentrations was developed. After calcination at 550 o C, the chromate:dichromate ratio is ∞, 0.56, and 2. 18 on respectively alumina, silica, and silica-alumina. Reduction results in the formation of three new species: octahedral Cr 3+ , octahedral Cr 2+ , and pseudotetrahedral Cr 2+

Metallo-carbohedrenes : chromium, iron, and molybdenum analogues

Abstract: The formation of especially stable metal-carbon clusters, which have been designated metallo-carbohedrenes by Castleman and co-workers, has previously been reported for the early transition metals (Ti, Zr, Hf, V) Clusters having the formula M[sub 8]C[sub 12] were recognized as especially stable species which were formed in plasmas containing the metal and a carbon-containing precursor A similar laser induced plasma technique is employed to synthesize chromium, molybdenum, and iron analogues of these so-called [open quotes]met-cars[close quotes] clusters and to study their photodissociation behavior These observations suggest that met-cars cluster formation is a more general phenomenon than previously recognized While chromium and molybdenum form the [open quotes]super-magic[close quotes] 8/12 stochiometry efficiently, met-cars formation competes with other special stoichiometries in the iron clusters Implications for the bonding properties in met-cars systems are discussed 22 refs, 5 figs

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Abstract: Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

High-performance flow flame atomic absorption spectrometry for automated on-line separation and determination of chromium(III)/chromium(VI) and preconcentration of chromium(VI)

Abstract: High-performance flow atomic spectrometry permits the fully automated separation of Cr(III)/Cr(VI) species and subsequent flame AAS determination within only 1 min. An HPLC integrator at the output of the flame AA spectrometer renders possible the simultaneous signal processing of both oxidation states. Samples of drinking water, waste water, and extracts of soils were investigated

Chiral (.eta.6-arene)chromium complexes in organic synthesis: stereoselective synthesis of chiral (arene)chromium complexes possessing amine and hydroxy groups and their application to asymmetric reactions

Abstract: Optically active (1,2-disubstituted arene)chromium complexes with an amine and a hydroxy group in the two ortho benzylic positions were stereoselectively synthesized from a commercially available α(R)-phenylethylamine. These chiral chromium complexes can be used a chiral ligands in a catalytic asymmetric ethylation of benzaldehyde with diethylzinc. Also, moderate enantioselectivity was observed in the 1,4-conjugated addition reaction of chalcone

Nitrogen adsorption with a divalent cation exchanged lithium x-zeolite

Abstract: The present invention is directed to a process for separating nitrogen from gas mixtures containing nitrogen and less strongly adsorbed components such as oxygen, hydrogen, argon or helium by use of an at least binary exchanged X-zeolite having lithium and a divalent cation selected from the group consisting of barium, cobalt, copper, chromium, iron, magnesium, manganese, nickel, zinc and mixtures thereof in a ratio of preferably 5% to 50% of the divalent cation and 50% to 95% lithium.

Inhibition of lead chromate clastogenesis by ascorbate: relationship to particle dissolution and uptake

Abstract: Chromium metal salts are considered to be human carcinogens, especially the salts of low solubility. Lead chromate, a highly insoluble chromium metal salt, has been shown to be tumorigenic, genotoxic and clastogenic. In this study, the roles of particle-cell contact, particle dissolution and particle uptake in the clastogenic activity of lead chromate were investigated. Using Pb51CrO4 it was found that lead chromate particles (1.2 microns mean diameter, -28 mV surface charge) were slightly soluble in water; solubility increased 2-fold when particles were incubated in culture medium, but was not increased further by the addition of serum. The extracellular concentration of chromium increased 7-fold when lead chromate was incubated in the presence of Chinese hamster ovary (CHO) cells compared with culture medium alone. The intracellular concentration of ionic chromium increased in a dose-dependent manner following exposure of CHO cells to clastogenic doses of lead chromate reaching estimated levels as high as 1.2 mM per cell. Treatment of cells with lead chromate particles in the presence of a nontoxic dose of vitamin C blocked uptake of ionic chromium and eliminated the clastogenic activity of the particles. Transmission electron microscopy showed that lead chromate particles were internalized by CHO cells in phagocytic vacuoles in as little as 1 h; internalization was unaffected by co-treatment with vitamin C. It was demonstrated that particle-cell contact was required for lead chromate-induced clastogenesis. These data show that although phagocytic particle uptake occurs, particle-cell contact and extracellular dissolution are responsible for the clastogenic activity of lead chromate. These data also demonstrate that the genotoxicity of particulate hexavalent chromates can be blocked by vitamin C.

Synergistic Effects of Inorganic and Organic Cations on Inhibitive Performance of Propargyl Alcohol on Steel Dissolution in Boiling Hydrochloric Acid Solution

Abstract: The role of different inorganic cations of several elements and organic materials in combination with different concentrations of propargyl alcohol (PA) was investigated in an effort to control the dissolution of mild steel in boiling (102 ± 1°C) hydrochloric acid (HCl) solution. Inorganic cations of chromium (Cr6+), arsenic (As3+), tin (Sn2+), copper (Cu2+), nickel (Ni2+), and mercury (Hg2+) were studied. Organic materials studied included phenol, formaldehyde, O-aminobenzoic acid, sodium hypophosphide, and hydroquinone. Cr6+, As3+, and Ni2+ provided higher synergism, followed by Cu2+, Sn2+, and Hg2+. Phenol and formaldehyde provided higher beneficial effect. Except for phenol and formaldehyde, the other organics stimulated corrosion when added at higher concentrations. Organic additives were more effective than inorganic cations. Cu2+, Cr6+, and Ni2+ accelerated corrosion when used without PA but strongly reduced dissolution in its presence. As3+ reduced the corrosion rate alone and in combinat...

Chromium ribbon-like silicate clay A-olefin catalysts

Abstract: A process for preparing pillared chromium ribbon-like silicate clay α-olefin catalysts is disclosed which comprises contacting a ribbon-like clay selected from the group consisting of sepiolites with a chromium solution. Preparation of said chromium solution comprises the steps of: a) dissolving a chromium salt and a base in water by heating solution at a temperature in the range of about 20° C. to about 100° C. thereby producing a first chromium solution having a pH in the range of about 1.5 to 2.5; b) diluting said first chromium solution with water so as to produce a second chromium solution wherein first chromium solution is diluted such that the ratio of added water to first chromium solution is in the range of about 5 ml H 2 O to about 10 mls H 2 O to per ml first solution, and thereafter heating said second chromium solution at a temperature in the range of about 60° C. to about 95° C. for a time period in the range of about 60 to about 180 minutes; thereafter said pillard clays are activated to produce a chromium clay catalyst system. Pillared clays are then heat activated to produce chromium clay catalyst systems.

Aqueous trivalent chromium photoproduction in natural waters

Abstract: Field studies indicate Cr(III) is photochemically produced at two freshwater lakes in southeastern North Carolina. Results of controlled photolysis experiments with marine and estuarine samples suggest this process may be common to many natural water systems. This photogeneration mechanism sheds new insight into the existence of the thermodynamically unstable Cr(III) ion in oxygenated surface waters. Sixty-eight water samples from seven water bodies in the southeastern United States (fresh, estuarine, and marine) displayed a range of total aqueous inorganic chromium from <0.01 nM to 1.05 nM, with variable amounts of the trivalent and hexavalent species. These are the first Cr(III) and Cr(VI) values reported for all these aqueous systems and some of the first ever for nonmarine environments. 24 refs., 5 figs., 1 tab.

A kinetic study on the reactivity of chromium(IV)

Abstract: The inhibition effect of Mn2+ on the rates of the Cr(VI) oxidations of 13 different organic reductants (RH) in aqueous perchloric acid has been studied. The reductants were formaldehyde, formic aci...