Chromium , equilibrium

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

The hydrolysis equilibria for H2Cr04 given in Table 3 are only valid in HNO or HCIO solutions. Other acids yield complexes such as those shown for chloride and bisulfate ions. The exact composition of chromate(VI) anion(s) present in aqueous solution is a function of both pH and hexavalent chromium concentration (68). However, at pH values above 8, virtually all the Cr(VI) is present as the CrO anion. When the pH is between 2 and 6, an equilibrium mixture of HCrO and Ci2 is present when the pH is below 1, the principal species is H2Cr04 (68,69). At very high Cr(VI) concentrations... 

To fully understand the formation of the N13S2 scale under certain gas conditions, a brief description needs to be given on the chemical aspects of the protective (chromium oxide) Ci 203/(nickel oxide) NiO scales that form at elevated temperatures. Under ideal oxidizing conditions, the alloy Waspaloy preferentially forms a protective oxide layer of NiO and Ci 203 The partial pressure of oxygen is such that these scales are thermodynamically stable and a condition of equilibrium is observed between the oxidizing atmosphere and the scale. Even if the scale surface is damaged or removed, the oxidizing condition of the atmosphere would preferentially reform the oxide scales. 

In the case of chromium in 1 N H2SO4 transpassivity occurs at about 1 1 V (below the potential for oxygen evolution, since the equilibrium potential in acid solutions at pH 0 is 1 23 V and oxygen evolution requires an appreciable overpotential) and is associated with oxidation of chromium to dichromate anions ... 

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

Relaxation experiments. Use the relaxation times for the equilibrium shown to calculate the forward and reverse rate constants. The values are expressed in terms of the total concentration of chromium(VI), or [Cr(VI)]i = [HCrOj] + 2[Cr202 ] ... 

The reaction between arsenic(III) and chromium(VI) in 0.01-0.05 M sulphuric acid was studied by DeLury . On recomputing his data" taking into consideration the dichromate-hydrogen chromate equilibrium... 

The fact that the rate of reaction is inversely proportional to the concentration of iron(iri) is explained by Wagner and Preiss on the basis of an equilibrium between chromium(VI) and iron(II), viz. 

In a similar fashion, chromium ions Cr will reduce dissolved acetylene to ethylene and then are regenerated at the cathode from the Cr + ions that were formed in the reaction. Or, at a platinum electrode in a solution of AsO and AsO ions, the equilibrium potential of this redox system is not established. After the addition... 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

The data of Loukidou et al. (2004) for the equilibrium biosorption of chromium (VI) by Aeromonas caviae particles were well described by the Langmuir and Freundlich isotherms. Sorption rates estimated from pseudo second-order kinetics were in satisfactory agreement with experimental data. The results of XAFS study on the sorption of Cd by B. subtilis were generally in accord with existing surface complexation models (Boyanov et al. 2003). Intrinsic metal sorption constants were obtained by correcting the apparent sorption constants by the Boltzmann factor. A 1 2 metal-ligand stoichiometry provides the best fit to the experimental data with log K values of 6.0 0.2 for Sr(II) and 6.2 0.2 for Ba(II). 

Loukidou MX., Zouboulis AI, Karapantsios TD, Matis KA (2004) Equilibrium and kinetic modeling of chromium(VI) biosorption by Aeromonas caviae. Colloid Surface A 242 93-104... 

Chromium(II) chloride, 6 528t, 531, 564t Chromium(III) chloride, 6 532 physical properties, 6 528t Chromium(IV) chloride, 6 535 Chromium(III) chloride hexahydrate, physical properties, 6 528t Chromium chromate coatings, 76 219—220 Chromium complexes, 9 399 Chromium compounds, 6 526-571 analytical methods, 6 547-548 economic aspects, 6 543-546 environmental concerns, 6 550—551 health and safety factors, 6 548-550 hydrolysis, equilibrium, and complex formation constants, 6 530t manufacture, 6 538-543... 

Schauble EA, Rossman GR, Taylor Jr. HP (2001) Theoretical estimates of equilibrium Fe-isotope fractionations from vibrational spectroscopy. Geochim Cosmochim Acta 65 2487-2497 Schauble EA, Rossman GR, Taylor Jr. HP (2003) Theoretical estimates of equilibrium chlorine-isotope fractionations. Geochim Cosmochim Acta 67 3267-3281 Schauble EA, Rossman GR, Taylor Jr. HP (2004) Theoretical estimates of equilibriimi chromium-isotope fractionations. Chem Geol, in press. 

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