Metal chromates

Kinetic characteristics for the different compounds are given by Darrie et al. [912]. It was concluded, from comparisons of the values of E found with spectroscopically determined charge transfer energies, that the activa- 

The ammonium chromates, which have been studied more extensively than any other salts of these anions, are considered in the next section. 

The grouping of ammonium salts in a separate section serves to emphasize the similarities of behaviour which are apparent in reactions yielding the volatile NH3 molecule, following removal of a proton from the NH4 cation. This property is not unique indeed, many cations are volatile and numerous salts leave no residue on completion of decomposition. Few kinetic investigations have, however, been reported for other compounds, in contrast to the extensive and detailed rate measurements which have been published for solid phase decompositions of many ammonium salts. Comparisons with the metal salts containing the same anion are sometimes productive, so that no single method of classification is altogether satisfactory. 

Determination of the influence of crystal structure and reactant environment on deammination and dehydration processes is complicated by the several solid phase transformations that are a characteristic feature of many ammonium salts. Sublimation and/or melting may also occur. Deammination and dehydration steps are generally reversible. At high temperatures, however, particularly in the presence of a residual oxide 

Thermal analyses of 26 ammonium salts are reported by Erdey et al. [915] and more references to ammonium salts are to be found in Duval s book [916]. Calculations of the enthalpies of formation have been used to predict the thermal properties of these compounds [917]. (See also refs. 918, 919.) 

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Soluble pigments The most important pigments in this class are the metallic chromates, which range in solubilities from 17 0 to 0-00005 g/1 CrO . An examination has recently been carried out of the mechanism of inhibition by chromate ions and it has been shown by chemical analysis of the stripped film, Mdssbauer spectroscopy and electron microprobe analysis that the air-formed film is reinforced with a more protective material in the form of a chromium-containing spinel (Chapter 17). The situation is, however, complicated by the possibility that some chromates, particularly the basic ones, may inhibit through the formation of soaps. There is evidence that lead chromate can function in this way. 

Metal Chromates Nitrites Benzoates Borates Phosphates Silicates Tannins... 

For coloured smoke puffs, pigments based on metal chromates are utilised, the pigment being intimately mixed with a fuel such as magnesium. On combustion in free air, a smoke cloud is produced that has residual colour due to the chromate. 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Dialkylnitriloosmium chromates, rteu4 N=0sR2l(CrO4) (1). Preparation.1 These metal chromates are effective catalysts for oxygenation of primary and secondary alcohols.2 Primary alcohols are oxidized exclusively to aldehydes and no esters are formed from secondary alcohols. Primary alcohols are oxidized more rapidly than secondary ones. 

In order to limit the reduction of perborate at the cathode alkali metal chromate or bichromate (to achieve a concentration of about 0.3 g. Cr03 per litre) is added to the electrolyte. Apart from this, a too low current density, below 10 A/sq.dm, should not be used at the cathode. 

I carbonates Co-Na, Co-K, Ni-Na, Ni-K nitrates Zn-alkali metals sulphates (schoenites) Mg-Na carbonates Al-K, Ce-Na nitrates rare earth-alkali metals sulphates (alums) Me111-alkali metals carbonates Ce-alkali metals nitrates Ce-alkali metals sulphates U-alkali metals alkali metals vanadates alkali metal chromates, Ag chromate, alkali metal molybdates and tungstates... 

Note Many other metals can be used to replace the lead anode electrode. Such metals include, aluminum, zinc, iron, nickel, copper, and various other metals forming the corresponding metal chromates. 

IV.33 CHROMATES, CrOj", AND DICHROMATES, Cr20 " The metallic chromates are usually coloured solids, yielding yellow solutions when soluble in water. In the presence of dilute mineral acids, i.e. of hydrogen ions, chromates are converted into dichromates the latter yield orange-red aqueous solutions. The change is reversed by alkalis, i.e. by hydroxyl ions. 

Solubility The chromates of the alkali metals and of calcium and magnesium are soluble in water strontium chromate is sparingly soluble. Most other metallic chromates are insoluble in water. Sodium, potassium, and ammonium dichromates are soluble in water. 

Alkali metal chromates can be crystallised only from alkaline solutions at low pH dichromates are obtained ... 

The flammable part of these materials is not the pigment, although certain metallic chromate pigments are, in themselves, flammable. 

Lattice potential energies for alkali metal chromates have been determined. Values... 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

PHENYLHYDRAZINE HYDROCHLORIDE or PHENYLHYDRAZINE MONOHYDROCHLORIDE or PHENYLHYDRAZINIUM CHLORIDE (59-88-1) Combustible solid (flash point about 194 F/90°C). Dust or powder forms explosive mixture with air. A strong reducing agent Reacts violently with strong oxidizers, alkalis, ammonia, ammonium persulfate, bromine dioxide, lead dioxide, nitric acid, perchlorates, permanganates, peroxides, sulfuric acid. Incompatible with alkali metals, chromates, copper salts. Corrosive to metals, nickel. 

EXPLOSION and FIRE CONCERNS combustible, but solid form is difficult to ignite powder form may ignite spontaneously and can continue burning under water powder form is very explosive when mixed with oxidizing agents dangerous explosion hazard in form of dust by chemical reaction with air, alkali hydroxides, alkali metal chromates, dichromates, sulfates,... 

The absorption is slightly greater in the more dil. soln., and this points to slight dissociation with formation of some dichromate, which, however, is only produced in very minute quantity. H. Becquerel found that soln. of potassim chromate are transparent for the ultra-red rays and G. Massol and A. Faucon studied the transmission of ultra-violet rays. D. Brewster investigated the absorption spectrum of soln. of ammonium chromate and J. H. Gladstone, soln. of metal chromates. H. von Halban and H. Siedentopf found that a soln. of potassium chromate in 0-05A -KOH had the extinction coefE., e, for light of wave-length, A, when where C denotes the cone, in mols per litre. 

The metallic chromates are usually coloured solids, yielding yellow solutions when soluble in water. In the presence of dilute mineral acids, i.e. of... 

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