Chromium reducers

Divalent chromium reduces triple bonds to double bonds (trans where applicable) [195], enediones to diones [196], epoxides to alkenes [192] and aromatic nitroso, nitro and azoxy compounds to amines [190], deoxygenates amine oxides [191], and replaces halogens by hydrogen [197,198],... 

Interpretation of these data suggests a question What is the oxidation process leading to the formation of pyrite (analytically determined as chromium-reducible sulfide, CRS) instead of simple precipitation of FeS (analytically determined as acid-volatile sulfide, AVS) AVS constituted less than 10% in the study of White et al. (35). 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

Bomberger, D.R., "Hexavalent Chromium Reduces Corrosion in Coal-Water Slurry Pipeline. Materials Protection, Vol. 4, pp. 43-49, 1965. 

Ferrous ion (Fe +) reacts with hexavalent chromium, reducing the chromium to a trivalent state and oxidizing the ferrous ion to the ferric state. The reaction occurs as follows ... 

The catalytic activity of the catalysts prepared by coimpregnation is similar (Fig. 7) the hypothesis is that clusters of calcium can be formed and the effect of this poorly dispersed element is independent from its concentratioa The introduction of calcium determines only a small increase of the catalytic activity compared with the sample containing chromium, but the product distribution shifts to the formation of heavy products. Moreover, all the data confirm that the presence of chromium reduces the formation of CFC-1113. 

In the experiment represented in Figure 33, methanol was again used as a poison (in the amount of 1 CH3OH molecule/Cr atom), but this time with a catalyst containing divalent chromium (reduced in CO at 350 °C). There was no retardation of the development of polymerization rate, as the catalyst was already reduced, but the methanol nonetheless did affect the final polymerization rate. 

Chromium reduction to measure reduced inorganic sulfur compounds in sediments was proposed by Zhabina and Volkov (1978). Since then it has found wide use internationally (Sullivan etal. 1999), particularly when pyritic sediments and acid volatile mono-sulfides are expected. The method is not measurably affected by sulfur in organic matter or sulfates (Canfield etal. 1986 Morse Cornwell 1987). Accordingly, this chromium reducible sulfur method (SCr) is especially useful on samples with appreciable organic matter and also for sandy soils where the %S action criterion is very low (e.g. as low as 0.03%S). 

Pyrite and other iron disulfides and acid volatile sulfides (chromium reducible sulfur)... 

Figure 5.1 Schematic of the apparatus used in the chromium reducible method for determination of SCr. Nitrogen (N2) gas flow rate is regulated at 2-3 bubbles/second. Allow the N2 gas to purge the system for about 3 minutes between samples.

Stumm, W. Morgan, J.J. (1996) Aquatic Chemistry. 3rd Edn. Wiley-Interscience, New York, NY. Sullivan, L.A., Bush, R.T. McConchie, D.M. (2000) A modified chromium-reducible sulfur method for reducing inorganic sulfur optimum reaction time for acid sulfate soil. Aust. J. Soil Res., 38,... 

Fig. 8.15 Sulfur geochemistry of a 4-m deep sediment core from the upper slope of the western Black Sea. Left frame SO , H S, CH and Fe + (notice scales) in the pore water. The smooth curves are model fits to the data based on the PROFILE model (Berg et al. 1998). Right frame Chromium reducible sulfur (CRS) and acid volatile sulfide (AVS), the latter showing the black band of iron sulfide at 250-300 cm depth due to the downward progressing sulfidization front. From Jorgensen et al. (2004).

Reduction processes are important for the treatment of wastewaters from the plants for the surface finishing of metals, containing salts of hexavalent chromium. Reducing agents, such as FeSO, SO2, Na2S03, NaHS03 reduce hexavalent chromium in acid media (pH 3-5) to the trivalent form, and this is precipitated in the form of chromium trihydroxide. 

Chromium reduces the sulfur diffusion mobility in the Cr-Fe-S system [1974Mok]. 

Nickel and high-nickel alloys tend to oxidize along grain boundaries when subject to alternate oxidation and reduction. Alloying with chromium reduces this tendency. Also, in contact with sulfur or sulfur atmospheres at elevated temperatures, nickel and high-nickel alloys are subject to intergranular attack. Consequently, nickel is not usefully resistant to such atmospheres above about 315 °C... 

Urokinase-type plasminogen activator (EC 3.4.21.73) is synthesised in a single-chain form and is converted to an active, two chain form after it binds to its cell surface receptor, uPAR. Treatment human of alveolar type II cells (A549) with 1 to 5 jUM chromium(Vl) for 4 and 12 h decreased both the specific activity and the amount of uPA protein (Shumilla and Barchowsky 1999). Chromium reduced uPA protein levels by inhibiting protein synthesis and had no effect on uPA mRNA levels or the rate of uPA protein degradation. In contrast, both mRNA and protein levels for the uPA receptor were increased by treatment with concentrations of chromium(VI) that did not completely inhibit protein synthesis. 

Test results are conveniently presented as a graph of exposure temperature on a linear scale versus the time for 50% failure on a logarithm scale. It is shown in Figure 6.3, for example, that the metal having the most deleterious effect on the heat aging life of polypropylene is copper, followed by brass, nickel, and chromium, not in order of severity. Nickel and chromium reduce the heat aging life by three- and... 

Bomberger, D. R. 1965. Hexavalant Chromium Reduces Corrosion in a Coal-Slurry Pipeline. Materials Protection, 4,1, 41-48. 

The chromium atom departs with a pair of electrons that formerly belonged to the alcohol the alcohol is thereby oxidized and the chromium reduced. 

It is generally conceded that steels containing only very low amounts of copper are particularly susceptible to severe atmospheric corrosion. In one test over a 3%-year period in both a marine and an industrial atmosphere, a steel containing 0.01 percent copper corroded at a rate of 80 //m/y, whereas increasing the copper content by a factor of five reduced the corrosion rate to only 35 //m/y. Further additions of small amounts of nickel and chromium reduced the corrosion rate to 10... 

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