Chromium hydroxide, dehydration

The chromium oxides were prepared according to the following procedure [6-8]. Chromium oxide resulted from the dehydration of chromium hydroxide obtained by the addition of an ammonia solution (5M) to a solution of chromium nitrate (0.5M) The final pH was equal to 7.5 and the hydroxide formed was kept constantly stired and heated at 80°C for 1 h so as to obtain complete precipitation. This solid was filtered and washed three times with hot distilled water and dried for 16 h in an oven at 90°C. It was then submitted to a dynamic thermal treatment under nitrogen at 380°C for 8 h. The chromium oxide formed was cooled down under the same vector gas. 

During prebaking for the internal coating, dehydration of chromium hydroxide (decrease in n-value) has occurred. If surface oil is oxidized at the same time, the adhesion loss of UVC lacquer due to the dehydration is made up for by the oxidation of the oil. 

The adhesion of UVC lacquer is likely to deteriorate in wicket-contact portions where the dehydration of chromium hydroxide occurs but only limited oxidation and vaporization of oil take place. 

The kinetic stability of these compounds towards dehydration is quite low. They usually transform spontaneously as early as 60 °C or through ageing at room temperature into (he green amorphous pha.se characteristic of chromium gels [24], and later into oxyhydroxidcs CrOOH and into the more or less hydrated oxide Cr203 [21,23,51 -54]. Chromium hydroxide can also be obtained by acidification of strongly alkaline solutions of chromite [Cr(OH)4(OH2)2] [23]. From a thermodynamic point of view, the instability of hydroxides appears to be due to the positive charge on the water molecule (Table 2.1). 

The mixed chromium and nickel (5% and 10% Ni atomic) catalysts were prepared by dehydration of mixed chromium and nickel hydroxides prepared by adding an ammonia solution to a solution of chromium and nickel nitrate in order to maintain a pH = 7 1. The final pH was equal to 7.S. The hydroxyde treatment was the same as the one already described for chromium oxide. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

A conventional wastewater treatment system with an average flow rate of 160,000 gpd produces effluent suitable for NPDES discharge. Metal hydroxide sludges are dewatered in a 15 cu. ft filter press producing more than one half ton of filter cake per day. The filter cake is further dewatered in a 7 cu. ft, batch-type sludge dryer. Based upon recommendations by their consultant, the firm also uses the sludge dryer to dehydrate nickel strip solutions. Two reverse osmosis systems are used for partial nickel recovery. Trivalent chromium is recovered by drag-out control and evaporation. 

In the presence of catalysts such as are used for the synthesis of methanol from mixtures of hydrogen and carbon monoxide and which have been promoted by the addition of an alkali oxide, ethanol may be dehydrated to form butanol in a high pressure process. Catalyst mixtures composed of chromium and zinc oxides to which either barium hydroxide or potassium oxide has been added have been specified.6 ... 

The effect will be clear from a comparison of the magnetic properties of chromium oxide gel with those of massive crystalline chromic oxide. Chromium oxide gel may be made by precipitation of the hydroxide from a nitrate solution, followed by slow dehydration. Several other processes are available, of which slow reduction from a basic chromate solution is one. On ignition, these gels generally undergo the glow-phenomenon during which they revert to Crystalline chromic oxide. 

Under stronger alkalinization, chromium forms a series of increasingly condensed polycations [3], and then a hydrated hydroxide gel Cr(OH)3(bH2)3 [4]. Different behavior is observed for iron(lll) and aluminum(lll). Aluminum forms the stable hydroxide AlfOH), (gibbsite, bayerite) [5], whereas the Fe(OH)3 hydroxide has never been identified- It transforms very rapidly to an oxyhydroxide through spontaneous dehydration [6j. 

It is clear that, in spite of a few analogies, Al(IlI), Fe(III) and Cr(III) each have their own behavior in solution. The spontaneous tendency of the hydroxylated form of iron to undergo rapid dehydration probably explains the poor solubility in an alkaline medium. With aluminum or chromium, for which oxolaiion is less likely, hydroxo bridges are relatively stable with respect to dehydration in the hydroxides. However, they are easily dissolved in an alkaline medium because free hydroxyls are better nucleophiles than hydroxo bridges in the solid. However, iron and aluminum oxides are poorly soluble in an alkaline medium because oxo bridges are... 

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