Showing papers on "Chromium published in 1985"

Supported Chromium Catalysts for Ethylene Polymerization

Abstract: Publisher Summary This chapter discusses supported chromium (Cr) catalysts for ethylene polymerization. The Phillips Cr/silica polymerization catalyst is prepared by impregnating a Cr compound onto wide pore silica and then calcining in oxygen to activate the catalyst. This leaves Cr in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (CrO3) has been impregnated most commonly, but even a trivalent Cr salt can be used because oxidation to Cr(VI) occurs during calcining. Dichromate is the main species in CrO3 solutions. Trivalent Cr salts, notably CrCl3, impregnated onto silica and calcined under vacuum, are found to polymerize ethylene. Three processes are used commercially to make linear polyethylene—namely, solution, slurry, and gas phase. All these processes are called “low-pressure processes” to distinguish them from the free radical or high-pressure process that makes highly branched polyethylene. The slurry process is very convenient for batch polymerization studies. The diluent permits precise control of the temperature and serves to dissolve ethylene and other reactants that must contact the catalyst during polymerization.

Calculated electronic structure of chromium surfaces and chromium monolayers on iron.

Abstract: A self-consistent calculation of the magnetic and electronic properties of the chromium (100) and (110) surfaces and of a chromium monolayer on the (100) and (110) iron surfaces is presented. It is found that (i) the (100) chromium surface is ferromagnetic with a greatly enhanced spin polarization (3.00 electrons); (ii) a substantial enhancement of the spin imbalance exists several (g5) layers into the bulk; (iii) the (110) chromium surface is antiferromagnetic with a large (2.31) spin imbalance; (iv) the (100) chromium monolayer on ferromagnetic iron is ferromagnetic, with a huge spin imbalance (3.63), and aligned antiferromagnetically with respect to the bulk iron; (v) the (110) chromium monolayer on ferromagnetic iron is also ferromagnetic, with a spin imbalance of 2.25 and antiferromagnetically aligned to the iron. The spin imbalance of chromium on iron (100) is possibly the largest of any transition-metal system.

False-positive, follicular and irritant patch test reactions to metal salts

Abstract: 853 hard metal workers were patch tested with nickel sulphate 5%, potassium dichromate 0.5% and cobalt chloride 1%, each in petrolatum. Non-allergic reactions appeared in 6.5% of the nickel tests, 13% of the chromium tests and 18.3% of the cobalt tests. Most of the individuals with positive, poral or pustular reactions were retested with serial dilutions of metal salts in pet. and in water. The accuracy of a positive initial nickel reaction was 83%, a chromium reaction 40% and a cobalt reaction 62%. The nonallergic reactions were partly reproducible and correlated with both the type of patch test material and with individual factors. Weak and moderately strong positive patch test reactions to metal salts may be irritant and should be checked with serial dilution tests or at least be retested. A reduction of the cobalt chloride concentration from 1% to 0.5% in the standard test material is discussed.

Analysis of Passive Films on Stainless Steel by Cyclic Voltammetry and Auger Spectroscopy

Abstract: Anodic passivation of Type 316L stainless steel in a borate-boric acid buffer solution was studied using cyclic voltammetry and Auger spectroscopy. Based on a comparison with the results obtained on the metals (iron, chromium, and nickel), the reduction peaks appearing in the voltammograms for the steel were attributed to the reductive dissolution of a ferric oxide and to valence transitions associated with chromium and nickel in the oxide. It is shown that cyclic voltammetry in the buffer solution provides a qualitative and semiquantitative analysis of the passive film growth. Okamoto's model of formation of two types of films, determined by the anodization potential, is found to be applicable; the mechanism of oxide growth is related to selective enrichment of iron or chromium, which is based on solubility relationships predicted by thermodynamic considerations. 12 references, 8 figures.

Studies of reactions of atomic and diatomic chromium, manganese, iron, cobalt, nickel, copper, and zinc with molecular water at 15 K

Abstract: Formation de molecules HMOH (M=Cr, Mn, Fe, CO, Ni, Cu et Zn) qui photolysees conduisent a HMn 2 OH, HMnOMnH, HFeOFeH, Fe(OH) 2 HFe 2 OH et H x Cr(OH) 2 . Le cuivre et le zinc donnent CuOH et ZnOH

Modification of chromium(VI)-induced DNA damage by glutathione and cytochromes P-450 in chicken embryo hepatocytes

Abstract: The role of glutathione and cytochrome P-450 in the production of DNA damage by chromium(VI) was examined in chicken embryo hepatocytes by the alkaline elution technique. Cellular levels of glutathione and cytochrome P-450 were altered by treating the hepatocytes with N-acetyl-L-cysteine, buthionine sulfoximine, isopentanol, or beta-naphthoflavone. A dramatic increase in chromium(VI)-induced DNA strand breaks was observed after increasing glutathione levels in the cells. Chromium(VI)-induced DNA strand breaks were even more numerous when the level of cytochrome P-450 was also increased. Upon depletion of glutathione levels and induction of cytochrome P-450 or cytochrome P-448, little or no DNA strand breaks or DNA interstrand cross-links were observed after chromium(VI) treatment. Chromium(VI)-induced DNA-protein cross-links generally decreased after either increases or decreases in cellular levels of glutathione or cytochrome P-450 or P-448. These results suggest that glutathione enhances chromium(VI)-induced DNA damage through metabolic activation of chromium(VI). The possible production of reactive chromium species upon metabolism by glutathione and cytochrome P-450 or P-448 and their involvement in DNA damage is discussed.

Trace determination of chromium in various water types by adsorption differential pulse voltammetry

Abstract: A new sensitive voltammetric method is presented for the determination of trace amounts of total chromium [Cr(III) and Cr(VI)] in natural waters. The method is based on the preconcentration of the Cr(III)-DTPA complex by adsorption at the HMDE at the potential of −1.0 V. The adsorbed complex is then reduced producing a response with a peak potential of −1.22 V and the peak height of the Cr(III) reduction is measured. The catalytic action of nitrate and bromate ions on the Cr(III)-DTPA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a. c. voltammetry and the potential range of adsorption was determined. Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 20–2,000 ng/l have been established. The determination limit is 20 ng/l and the RSD is 5% for chromium concentrations ⩾200 ng/l.

Binding of chromium to chromatin and DNA from liver and kidney of rats treated with sodium dichromate and chromium(III) chloride in vivo.

Abstract: The in vivo binding of chromium to whole chromatin, polynucleosomes, DNA, and cytoplasmic RNA-protein fraction from liver and kidney was examined after treatment of rats with sodium dichromate and chromium(III) chloride. Significant amounts of chromium were bound to DNA and the nonhistone proteins of chromatin and to cytoplasmic RNA-protein fraction. The binding of chromium to the nuclear and cytoplasmic nucleic acid fractions varied considerably, depending on the tissue and the oxidation state of the chromium administered. The level of chromium bound to whole chromatin was greater in the liver than in the kidney after treatment with either chromium compound. Chromium entered the liver and kidney tissues at a slower rate after chromium(III) treatment than after chromium(VI) treatment. At early times after chromium(VI) treatment, more chromium was bound to the liver and kidney chromatin and DNA than after chromium(III) treatment. A much smaller proportion of the chromium bound to chromatin was associated with the DNA after treatment with chromium(III) than after treatment with chromium(VI). However, 40 hr after injection, there was no significant difference in the level of chromium on the DNA from both the liver and kidney of chromium(VI)- and chromium(III)-treated animals. No DNA damage was detected in either liver or kidney nuclei after chromium(III) treatment, using the technique of alkaline elution. A possible correlation between chromium binding to chromatin and DNA damage is discussed.

Chromium bond detection in isolated erythrocytes: a new principle of biological monitoring of exposure to hexavalent chromium.

Abstract: Internal stress to chromium is only relevant in occupational medicine if it is due to the handling of hexavalent chromium. Cr(VI) ions, after uptake by inhalation or percutaneously are carried in the blood plasma and penetrate—depending on the concentration—into the erythrocytes. Due to the intracellular reduction to Cr(III) and the concurrent intracellular protein binding, the erythrocytes represent an easily accessible target organ for quantitative chromium determination after occupational exposure to Cr(VI) compounds. The results of an earlier experimental study indicate that human plasma too is capable of spontaneous reduction of Cr(VI) ions of up to 2 ppm to Cr(III). This plasma reduction capacity (PRC) can be increased and accelerated considerably by adding ascorbic acid (AA). These findings were supported in this investigation by proving a decreased binding of Cr(VI) inside the erythrocytes under the effect of AA. This leads to the assumption that only those Cr(VI) concentrations can penetrate the membrane of the erythrocytes and enter the cell which either come into contact with the membrane during the reduction process or exceed this limit concentration of 2 ppm. Only in these two instances can corresponding chromium findings be analyzed in isolated and washed erythrocytes. These results are compared with those obtained by conventional methods, such as Cr determination in the blood and/or urine. Our findings indicate that a single determination of chromium concentration in the erythrocytes will permit the monitoring of critical cases of Cr(VI) exposure. This is a new type of biological monitoring in the sense of a condensed longitudinal study, in order to find out whether threshold concentrations have been respected over a given period.

Modification of the erythrocyte anion carrier by chromate.

Abstract: It was confirmed that chromate is taken up by human erythrocytes via the general anion carrier. The chromate flux is unidirectional and chromium is accumulated within red cells presumably due to intracellular reduction of Cr(VI) to Cr(III). The analysis of the initial rates of uptake of chromate revealed two distinct uptake mechanisms at low (0.001-0.01 mM) and at high (0.05-1.0 mM) chromate concentrations. After prolonged incubation with 1 mM chromate, the subsequent rate of uptake of chromate was decreased. It is suggested that the decreased uptake is due to a modification of the anion-transport protein by chromate.

Corrosion of stainless steels in chloride solution: An XPS investigation of passive films

Abstract: Five commercial steels ranging from the martensitic stainless steel containing 12% chromium to the superferrite containing 29% chromium, 4% molybdenum, and 2% nickel have been studied by XPS. In addition, a pure iron-chromium alloy containing 7% chromium has been investigated. Armco iron and pure chromium (99.99%) were included as references. The formation of the passive films (or corrosion) occurred in deoxygenated 0.1 M NaCl solution (pH=5.6), from which the samples were transferred directly to the XPS chamber under controlled atmosphere (Ar). Concentration profiles (at.-%) of the alloy constituents in their oxidized and metallic states have been determined separately from the measured XPS depth profiles. Forc≳= 12% chromium the passive films have the following structure: there is a depletion of Cr in the inner region, followed by an enrichment (concentration maximum) in the central region of the films. The height of this maximum increases, and its position shifts towards the surface with increasing chromium content in the alloy. The outermost monolayers are rich in water and hydroxyl groups. Various significant properties of the films change drastically at the critical chromium concentration of about 12%. This behaviour is rather independent of the other components (Mo, Ni, Cu) present in the alloys and is discussed in terms of a phase transition in the films which is controlled by the chromium concentration.

The Effect of Nitrogen on the Sensitization of AISI 304 Stainless Steel

Abstract: Thermodynamic calculations have been used to construct time temperature sensitization (TTS) diagrams for AISI 304 stainless steels (SS). The quantitative Stawstrom and Hillert model, which is based on chromium diffusion control of sensitization, was used to calculate the TTS diagrams. Electrochemical potentiokinetic reactivation (EPR) tests were performed on these steels, with various nitrogen additions, to obtain the experimental TTS curves. The calculated and experimental TTS diagrams have been compared to better understand nitrogen addition effects on the sensitization kinetics of AISI 304 SS. These results indicate that nitrogen additions below 0.16 wt% retard the sensitization kinetics; one possible mechanism by which this is achieved is an increase in chromium concentration adjacent to the grain boundaries, which decreases the chromium concentration gradient between the austenite matrix and the grain boundaries and hence retards carbide growth.

A review of the role of short-circuit diffusion in the oxidation of nickel, chromium, and nickel-chromium alloys

Abstract: The evidence for short-circuit diffusion during the oxidation of nickel, chromium, and nickel-chromium alloys is reviewed. On the basis of experimental evidence for the Ni/NiO and Cr/Cr2O3 systems, inferences are made about the role of short-circuit diffusion through the oxide scales forming on the binary nickel-chromium alloys. The review highlights the importance of scale microstructure in governing oxidation rates.

Low level chromium (VI) inhalation effects on alveolar macrophages and immune functions in Wistar rats.

Abstract: In inhalation chambers, 5-week-old male Wistar rats of the strain TNO-W-74 were continuously exposed to submicron aerosols of sodium dichromate in concentrations from 25 (low level) to 200 micrograms/m3 Cr (high level). Subacute exposure (28 days) to 25 and 50 micrograms/m3 Cr resulted in "activated" alveolar macrophages with stimulated phagocytic activities, and significantly elevated antibody responses to injected SRBC's. After subchronic (90 days) low level exposure there was a more pronounced effect on activation of the alveolar macrophages, with increased phagocytic activities. However, at high Cr (VI) exposure level (200 microgram/m3), inhibited phagocytic function of the alveolar macrophages was seen. In rats which were exposed to this chromium aerosol concentration for 42 days, the lung clearance of inert iron oxide was reduced significantly. The humoral immune system was still stimulated at subchronic low chromium aerosol concentrations of 100 micrograms/m3, but significantly depressed at 200 micrograms/m3 Cr. These results show that respiratory defence and immunologic functions were stimulated or inhibited depending on dose and time of chromium (VI) inhalation.

Oxochromium compounds. 2. Reaction of oxygen with chromium(II) and chromium(III) porphyrins and synthesis of a .mu.-oxo chromium porphyrin derivative

Abstract: Reaction de l'oxygene avec des complexes porphyriniques de Cr(II). On a isole le complexe (TPP) CrOCr(TPP) de la reaction CrO(TPP) avec Cr(II)(TPP). La formation de complexe μ-oxo similaires a ete demontree spectroscopiquement entre autre CrO(P)/Cr(P') ou P et P' peuvent etre semblables ou differents. TPP=la tetraphenylporphyrine

Chromium(V) oxidations of organic compounds

Abstract: Oxydation de composes organiques (diacides carboxyliques, hydroxy- et cetoacides, alcools, aldehydes et divers) par le complexe bis-[ethyl-2 hydroxy-2 butanoato (2-)] oxochromate(V) de sodium

Thermal spray iron alloy powder containing molybdenum, copper and boron

Abstract: A novel iron based alloy is disclosed which is characterized by high resistance to wear and corrosion. The alloy consists essentially of 0 to 40% chromium, 1 to 40% molybdenum, 1 to 15% copper, 0.2 to 5% boron, and 0.01 to 2% carbon; the balance being incidental impurities and at least 30% iron, with the molydenum being at least 10% if the boron is greater than 2%. The alloy is preferably in the form of a powder for thermal spraying, and coatings produced thereby may have an amorphous structure.

Mechanism of the oxidation of alkyl aryl and diphenyl sulfides by chromium(VI)

Abstract: On etudie les vitesses d'oxydation de 17 alkyl aryl sulfures et de 6 diphenyl sulfures dans des solutions aqueuses d'acide acetique de concentration variable

Chemical and Electrochemical Behavior of the Cr(III)/Cr(II) Half‐Cell in the Iron‐Chromium Redox Energy Storage System

Abstract: The Cr(III) complexes present in the acidified chromium solutions used in the iron-chromium redox energy storage system have been isolated and identified as Cr(H2O)6(3+) and Cr(H2O)5Cl(2+) by ion-exchange chromatography and visible spectrophotometry. The cell reactions during charge-discharge cycles have been followed by means of visible spectrophotometry. The spectral bands were resolved into component peaks and concentrations of the Cr(III) species calculated using Beer's law. During the charge mode, Cr(H2O)5Cl(2+) is reduced to Cr(H2O)5Cl(+), and during the discharge mode Cr(H2O)5Cl(+) is oxidized back to Cr(H2O)5Cl(2+). Electrode potential measurements also support this interpretation. Hysteresis effects in the charge-discharge curves can be explained by the slow attainment of equilibrium between Cr(H2O)6(3+) and Cr(H2O)5Cl(2+).