Chromic ions

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

Table 3.S9 Concentration of chromic ion in passive films formed on amorphous alloys and stainless steels in I n HCI at ambient temperature...

Since the dichromate ion on the left side of the equation has been reduced to chromic ion, Cr+ on the right side, the conversion of methanol to formaldehyde must involve oxidation. To show more clearly that methanol has been oxidized, let us balance this reaction by the method of half-reactions. We have encountered the halfreaction involving dichromate and chromic ions before (Problem 20b in Chapter 12). It is... 

As an example of the problem of species in solution, consider the case of a solution made by dissolving some potassium chrome alum, KCrfSO s-12H20, in water. On testing, the solution is distinctly acidic. A currently accepted explanation of the observed acidity is based upon the assumption that, in water solution, chromic ion is associated with six H20 molecules in the complex ion, Cr(H20) a. This complex ion can act as a weak acid, dissociating to give a proton (or hydronium ion). Schematically, the dissociation can be represented as the transfer of a proton from one water molecule in the Cr(H20) 3 complex to a neighboring H20 to form a hydronium ion, H30+. Note that removal of a proton from an H20 bound to a Cr+3 leaves an OH- group at that position. The reaction is reversible and comes to equilibrium ... 

The following equations summarize the steps believed to occur when NaOH is slowly added to a solution of chromic ion. Step (4c) corresponds to formation of solid hydrated chromium hydroxide step (4d) corresponds to its dissolving in excess NaOH. 

Fio. 16. Spectra of chromic oxide and of chromic ion in a hydrated salt, in anion and in cation complexes. 

This includes the oxidation of sulfite (56) and of hydrogen peroxide (45, 65), as well as the oxidation of organic ligands like acetylacetonate (16, 72). The reaction of chromic ions with hydrogen peroxide has been also claimed to proceed via Mechanism 5 (93). Analogous to the behavior of ferric ions (56), the reduction of Cu+2 by sulfite ions (89) very likely proceeds by... 

When a second sulfite or SO2, if you like, or H2SO3 attacks 03Cr0S02 2, it goes all in one fell-swoop to chromic ion, to which sulfate is attached at the end of the reaction, and an SCV - radical. One can t precipitate sulfate with barium immediately after the reaction. Coordination of the chromium (111) stabilizes the radical until it can react with another as was mentioned by Dr. Halpern. This mechanism accounts for both the stoichiometry and kinetics. 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

In acidic solution, orange dichromate ion is a powerful oxidant that is reduced to chromic ion ... 

Ruby is a variety of corundum (A1203) with a Mohs hardness of 9. Its red color results when chromic ions (Cr3+) replace some of the aluminum ions in the crystal. The finest rubies come from Myanmar (formerly Burma) or Kenya. Star rubies are stones with a special starlike appearance. They usually have a cabochon cut and appear to show a 6-pointed star due to microscopic inclusions. Rubies of very high quality are sometimes made synthetically. 

When a stoichiometric amount of chromic ion was added to a boiling N,N-dimethylformamide solution of bis (8-hydroxy-5-quinolyl) methane, the chromium coordination polymer precipitated immediately. Elemental analysis analyzed for [Cr2(Ci9Hi2N202)3 2H20]W. 

Chromic acid use is not recommended if you are doing experiments in conductivity because the chromic ion can coat the walls of the glassware. 

Geometry of Metal Catechol Complexes. The coordination geometries of [Fe(cat)3]3" and [Cr(cat)3]3" have been determined by single crystal studies of the salts K3[M(cat)3] 1.5 H20 (M = Cr, Fe) in order to explore the crystal field effect of chromic ion on the coordination geometry and, indirectly, to determine the coordination geometry of enterobactin itself (26). 

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