Chromium water exchange

Table 77 Kinetic Data for Water Exchange at Chromium(III)...

Adsorption of vanadium and chromium are of direct significance in geo-chemical cycles. Hydroxide surfaces largely determine their transformation and complex formation. Such surfaces are additionally built up by a thin adherent water film. The adsorption kinetics of vanadyl (IV) and chromium (III) depend on the surface OH ligands (Wehrli Stumm 1988). For the interpretation of the results the relation between the adsorption rate of different ions and the rate of water exchange is based on the data obtained by Hachiya et al. (1984). 

The formation of malonate and oxalate complexes from [Cr(OH2)6] + involves rate-determining chromium-water bond breaking in an ion-pair in each case. Activation parameters for these two reactions, for formation of the monoglycine complex, and for water exchange at chromium(iii) are all rather similar. The formation of edta complexes of chromium(iii) is more complicated, involving several water displacement and chelation steps. Rates and activation parameters are reported for the tridentate to quinquedentate conversion ... 

Complex reducing agents are V(ll), Cr(Il) and Fe(ll). The aqua ions of iron(II) and chromium(Il) with and t2g Cg configurations respectively (high spin d and high spin d ) are kinetically labile undergoing rapid water exchange... 

The cw-labilizing effect of oxoanions such as nitrate or acetate when co-ordinated to chromium(iii) was discussed in Volume 4 of this series. This is believed to occur through the formation of transient bidentate species through chelation of the oxoanion. From studies of the nitrate substitution reactions of [Cr(H20)50N02] + in aqueous DMSO, the rate constant at 298.2 K for water exchange with [Cr(HaO)5 ONOa] " is reported to be ca. 8 x 10 s , which is several orders of magnitude larger than for [Cr(H20)e] +, The rate constant for the aquation of [Cr(HaO)6-ONOa] + is 10 A = 8.8 s i at 298.2 K. 

The literature available from the end of the last report (December 1989) to September 1991 is covered in this chapter. A complete revision of the International Union of Pure and Applied Chemistry (lUPAC) " Nomenclature for Inorganic Chemistry has appeared and lUPAC-recommended ligand abbreviations will be used wherever possible. Research activity in chromium chemistry continues at about the same level as in the past, but there are odd surges as new techniques " or complexes become available. As in previous years, the general chemistry of chromium has been reviewed. l Other, more specialist reviews include the spectroscopy of Cr(VI), organochromium(III) chemistry,and macrocyclic complexes of chromium in various oxidation states.Closer to the mechanistic area is a review of the photophysics of chromium(III) complexes and, more specifically, the photochemical water-exchange process in chromium(III) complexes. A summary of new insights into the mechanism of spontaneous and base-catalyzed substitution reactions of inert-metal amine complexes has also appeared. ... 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Reported concentrations of chromium in open ocean waters range from 0.07 to 0.96 xg/l with a preponderance of values near the lower limit. Methods used for the determination of chromium at this concentration have generally used some form of matrix separation and analyte concentration prior to determination [170-173], electroreduction [174,175] and ion exchange techniques [176,177]. 

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