Chromate ions reduction

Figure 3.13. Illustration of the determination of concentration by the method of standard additions. Curve A Polarogram of chromate ion reduction from the chromium in an aliquot of a digested steel sanv-pie. Curve B A polarogram of a similar aliquot to which has been added a known volume of a standard chromium solution.

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

None of the Cr(III) products from Equations 6 or 7 are effective crosslinkers since a chromic aqua ion must be hydrolyzed first to form olated Cr to become reactive. Colloidal and solid chromium hydroxides react very slowly with ligands. In many gelation studies, this critical condition was not controlled. Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). 

Figure 6.8 (a) Polarogram of the reduction of chromate ion at a dropping-mercuiy electrode, with [Cr04 ] = 1 mmol dm in degassed 0.1 mol dm NaOH solution, and using a scan rate v of 20 mV s. The small lower trace represents the residual current obtained at the same DME but in the absence of chromate, (b) The sequence of operation and the way that drop size dictates the current note that current is negative since reduction is involved. From Bard, A. J. and Faulkner, L. R., Electrochemical Methods Fundamentals and Applications, Wiley, 1980. Reprinted by permission of John Wiley Sons, Inc. 

M. Branca, A. Dessi, H. Kozlowski, G. Micera, and J. Swiatek, Reduction of chromate ions by glutathione tripeptide in the presence of sugar ligands, J. Inorg. Biochem., 39 (1990) 217-226. 

Only chrome yellow and chrome orange (lead chromates) exhibit an additional peak at -1-0.45 V, attributable to the reduction of chromate ions ... 

Polyethylene (chromium catalyst). The chromium on silica catalyst is quickly reduced from Cr(VI) to Cr(II). The active site consists of a single chromium ion present as silyl chromate before reduction with ethylene. Ethylene adds to the chromium as indicated. 

Figure 4.5 illustrates the titration curve that is obtained with an amperometric indicating system for the titration of lead ion with dichromate ion. If the applied potential is set on the plateau for the reduction of lead ion (approximately -0.5 V vs. SCE), curve a in Figure 4.5, will result. In contrast, if the applied potential is set at 0 V versus SCE, no current will flow until the point when excess chromate ion exists in the solution curve b is indicative of the titration curve that would be obtained. 

Cr2C>7 + 141T+ + 6e = 2Cr3+ +7H20 E° = 1.33 V The mechanism of oxidation of Fe2+ and other common ions by Crvl has been studied in detail with one- and two-electron reductants, respectively, Crv and Criv are initially formed.32 The chromate ion in basic solution, however, is much less oxidizing Cr0 - + 4H2O + 3e = Cr(0H)3(s) + 5OH- E° = -0.13 V... 

In all three experiments, Cr(VI) underwent a substantive decrease in concentration, and the distribution of Cr(T) was modified, indicating the widespread mobilization of chromium and the extensive coeval reduction of Cr(VI) to Cr(III). Accumulation of chromium in the anode chamber shows that electromigration was the predominant driving force for the transport of ions (e.g. Mukhopadhyay, Sundquist, and Schmitz, 2007). Generally, under neutral or high pH conditions, Cr(VI) exists as the soluble and mobile CrOi ion (Reddy et al., 2003) and,to a lesser extent, the dichromate ion Cr207. Reaction of the dichromate ion and the chromate ion to chromic acid occurs only under strongly acidic conditions (e.g. Mukhopadhyay, Sundquist, and Schmitz, 2007) and were most likely not attained in this study. 

Gustavsson J, Li G, Hummelgard C, Backstrom J, Cornell A (2012) On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions. J Electrochem Sci Eng 2 185-198. doi 10.5599/jese.2012.021... 

Primary halides (and some secondary cases) may be oxidized directly to carbonyl compounds by reaction with chromate ion, either in the presence of crown compounds, or with the chromate associated with a polymer matrix. Reduction of low molecular weight halides to hydrocarbons has been reported in superacid media [e.g. HF-TaFs), and a hydride transfer pathway is suggested. Allyl iodides may be reduced with triphenylphosphonium hydriodide (equation 10), but acid sensitive groups may not survive these conditions. Vinylic and aryl halides are converted into the parent hydrocarbons by reaction with Grignard reagents and a catalytic quantity of manganese salts a reaction mechanism has been proposed. 

Since H2S and possibly other sulfur compounds rapidly reduce the chromate ion. the use of this inhibitor is uneconomical in the presence of appreciable quantities of such impurities. In addition to increasing operating costs by destroying the inhibitor, the reduction reaction results in the formation of insoluble precipitates, which cause erosion of equipment, fouling of heat exchanger surfaces, and other operating difficulties. Fortunately, H2S itself appears to... 

Other types of wastes and waste repositories have redox-related performance issues. Figure 5 shows redox potentials associated with (1) reductive precipitation of chromate ion and (2) the progressive reductive dechlorination of tetra-chloromethane to methane. These two contaminants are selected as illustrative compounds to represent components of landfill leachate and hazardous industrial waste. 

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