Chromium salt solutions

The hydrolysis reaction is very slow at ambient temperatures and is accelerated by boiling chromium salt solutions (5). The hydrolysis reaction is characterized by the transformation of the deep blue colored CrtHgOJg to green colored hydrolyzed olates. Another indication is tnatan aged or boiled Cr(III) salt solution has a higher neutralization equivalent than a fresh one due to the hydrolytically produced protons. One way to establish hydrolytic equilibria quickly is to add appropriate equivalents of bases such as NaOH to Cr(III) salt solutions. 

Figure 1.1.19 illustrates such hematite particles coated with chromium hydroxide in which the latter was produced by the hydrolysis of a chrome alum solution in the presence of a-Fe203 cores on aging the dispersion at 85°C. For greater efficiency the chromium salt solution was either preheated or preheated with KOH (but avoiding precipitation of chromium hydroxide) (144). 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Polyethylene. Low pressure polymerization of ethylene produced in the Phillips process utilizes a catalyst comprised of 0.5—1.0 wt % chromium (VI) on siUca or siUca-alumina with pore diameter in the range 5—20 nanometers. In a typical catalyst preparation, the support in powder form is impregnated with an aqueous solution of a chromium salt and dried, after which it is heated at 500—600°C in fluid-bed-type operation driven with dry air. The activated catalyst is moisture sensitive and usually is stored under dry nitrogen (85). 

Baeumel, A. and Tramposch, O, Investigation of Intercrystalline Grain Boundary Corrosion of Austenitic Manganese-chromium Steels by Water and Aqueous Salt Solutions , Werkst. Korros., 17, 110 (1966) C.A., 64, 13847d... 

Standardise the ammonium iron(II) sulphate solution against the 0.02/Vf potassium dichromate, using /V-phenylanthranilic add as indicator. Calculate the volume of the iron(II) solution which was oxidised by the dichromate originating from the chromium salt, and from this the percentage of chromium in the sample. 

Various experimental conditions have been used for oxidations of alcohols by Cr(VI) on a laboratory scale, and several examples are shown in Scheme 12.1. Entry 1 is an example of oxidation of a primary alcohol to an aldehyde. The propanal is distilled from the reaction mixture as oxidation proceeds, which minimizes overoxidation. For secondary alcohols, oxidation can be done by addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the alcohol. Oxidation normally occurs rapidly, and overoxidation is minimal. In acetone solution, the reduced chromium salts precipitate and the reaction solution can be decanted. Entries 2 to 4 in Scheme 12.1 are examples of this method. 

Chromate conversion coatings for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

Reactions with salts. This procedure is more limited and is illustrated by the use of chromium chloride solutions under reflux for partial dealumination of Y and X zeolites (19), as well as of erionite (20). It is assumed that in this case a partial substitution of chromium for aluminum takes place, leading to the formation of Si-O-Cr bonds in the framework (19). Up to 40 percent of aluminum was removed by this method. Zeolites can also be dealuminated with solutions of ammonium fluorosilicate (107). 

The combined dichloromethane solutions may be washed with dilute hydrochloric acid, sodium bicarbonate solution, and water to remove excess traces of pyridine and chromium salts, or they may be filtered directly through a filter aid or passed through a (ihromatographic column. The product is obtained by removal of dicihloromethaiU any pyridine that remains can oft m be removed under redtuied pressure. 

Another work on the Hquid phase hydrogenation of acetophenone is that of Casagrande et al. The reaction was studied over a series of silica-supported bimetallic catalysts with various Ru/Cr atomic ratios, which were prepared by reduction at room temperature with aqueous sodium tetrahydroborate. The nanostructured catalysts are very active in the low-pressure hydrogenation of acetophenone, although the selectivity towards 1-phenylethanol did not surpass 22% at 90% conversion. The addition of chromium salts to the starting solution gave rise to... 

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

In plating of metals a film of metal such as chromium is deposited from a salt solution onto a reactive metal electrochemicaUy,... 

The formation of rnonodisperse amorphous, spherical particles of chromium hydroxide is offered here as an example. These dispersions are readily obtained by simply heating chrome alum solutions (35). It was established experimentally that the sulfate ion was essential in order to obtain uniform dispersions thus, when chromium nitrate solutions were aged, monodispersed particles were produced only if a sulfate salt was introduced into the reaction. 

The reduction of aqueous chromium(III) solutions can be carried out electrolytically o chemically with zinc amalgam, zinc and acid or a Jones reductor.2,24 Electrolytic procedures ca be cumbersome, and with chemical reductants contamination with other products can occur Chromium metal and acid can be used to reduce chromium(III) salts, and this requires less c the metal than in the method described in Section 35.3.1.1.i. 

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