Oxidation states chromium

Low-temperature treatment of Cr(VI) /silica by ethylene can also produce Cr(II) as was shown in 1968 by Baker and Carrick [239], who reported the valence of a catalyst containing 0.5 wt% chromium, initially hexavalent, after it reacted with ethylene at 135 °C. Afterward, the catalyst was treated with deoxygenated dilute HC1 to dissolve the chromium. The solution was then analyzed by polarography to determine the chromium oxidation state. The authors measured 85-96% conversion to Cr(II) within a few minutes of exposure to ethylene. Formaldehyde was released as the major (probably only) by-product. More recently, formaldehyde was again confirmed as the by-product of Cr(VI) reduction by ethylene [232],... 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

In this vein, many appHcations have appeared in the literature on speciation of oxidation states, such as Cr(m)/Cr(VI), Fe(n)/Fe(m), As(III)/As(V), Se(IV)/ Se(VI). Also, FIA techniques have been used to preconcentrate Sn, Hg, and Pb organometallics mainly from natural waters, sediment, and soil extracts. In particular great interest has been focused into the speciation of chromium oxidation states in water samples at very low level (nanograms per liter). A FI system with a minicolumn of acidic preconcentration and inductively coupled plasma optical emission spectrometry (ICP-OES) for final detection was developed for a rapid speciation of Cr(VI) and Cr(III) in waters. On sample injection, Cr(VI) is retained in the alumina column whilst Cr(III) is not passing directly to the atomic detector. Afterwards, the retained Cr(VI) is eluted by injection of ammonium hydroxide, as shown in Figure 5, and its analytical signal of emission in the ICP-OES is registered. 

The calcination temperature plays an important role in the stabilization of a particular chromium oxidation state. This state seems to be responsible of the improvement of the catalytic performances of sulfated zirconia aerogels doped with chromium (Figure 6.5) [45]. 

Figure 6.5. influence of chromium oxidation state on the catalytic activity of sulfated zirconia aerogels doped with chromium A. AZ873/Cr(III)-Al B. AZ873/Cr(V)-Al [45],... 

It can be seen from Tables 1 and 2 that the diffusion coefficients complexes decrease when the chromium oxidation state increases and that the values found by different methods are in a good agreement. 

Figure 6.13 Synchrotron radiation microprobe mapping of potassium and chromium oxidation states in a single human epithelial cell exposed to particulate chromate, and corresponding optical microscopy view. Beam spatial resolution (VxH) 0.5x1 pm, color scale in counts per pixel. Cr(VI) distribution shows a perinuclear localization. 2005 American Chemical Society.

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