Chromium III pigments

Chromium(III) Pigments. There are two green pigments based on chromium in the +3 oxidation state. The first one is chromium oxide [1308-38-9]) CTO, the second is hydrated chromium oxide, Ct203 wH O. Worldwide production is about 20,000 metric tons. Principal producers are American Chrome and Chemicals in the United States, Bayer in Germany, British Chrome and Chemicals in the United Kingdom, and Nihon Denko in Japan. 

Chromium(III) Pigments. There are two green pigments based on chromium in the +3 oxidation state. The first one is chromium oxide, Cr203 the second is hydrated chromium oxide, Cr203 JCH2O,... 

The +3 state of chromium is best represented by chromium(III) oxide, Cr2Os, which is a green, inert solid used as a green pigment. It can be made in rather spectacular fashion by heating ammonium dichromate. Once started, the reaction... 

Chromium(III) hydroxide is used as green pigment as mordant as a tanning agent and as a catalyst. 

Oxidative dissolution processes can also be used for identifying solid materials. This is the case of chromium (III) oxide—a green pigment that produces a well-defined anodic signal in acidic electrolytes near to -fl.O V, as can be seen in Fig. 2.15. This corresponds to the electrochemical process [147, 149] ... 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

Since chromium(III) oxide is virtually inert, chromium oxide green pigments are remarkably stable. They are insoluble in water, acid, and alkali and are thus extremely stable to sulfur dioxide and in concrete. They are light, weather, and temperature resistant. A change of the tint only occurs above 1000 °C due to particle growth. 

Reduction of Ammonium Dichromate. Chromium(III) oxide can be obtained by thermal decomposition of ammonium dichromate. Above ca. 200 °C, a highly voluminous product is formed with elimination of nitrogen [3.48]. The pigment is obtained after addition of alkali salts (e.g., sodium sulfate) and subsequent calcination [3.49]. 

Chromium(III) oxide pigments are thermally stable and insoluble in water. They are not classified as harzardous materials and are not subject to international transport regulations. As long as they are kept dry their utility as a pigment is practically unlimited. 

The use of chromium(III) oxide as a pigment for toys, cosmetics, and in plastics and paints that come in contact with food is permitted in national and international regulations [3.62] -[3.68]. Maximum limits for heavy metals or their soluble fractions are usually a prerequisite. Because pure starting materials are used, these limits are satisfied by most types of chromium oxide. 

Zinc Iron Brown. Variation of the zinc to iron ratio in red-brown iron spinel (ZnFe204), or replacement of some of the iron ions by aluminum and titanium ions, gives light to medium brown pigments. Inclusion of a small proportion of lithium ions considerably improves stability towards reducing agents, e.g., when the pigments are used to color plastics [3.87]. The partial replacement of iron ions by chromium(III) ions yields dark brown products [3.88]. 

Sodium chromate and sodium dichromate are both starting points for the production of pigments, corrosion inhibitors, fungicides, and ceramic glazes. Chromium(III) is essential to human health and may play a role in the regulation of glucose metabolism. 

The preparation of chromium(III) and cobalt(III) complexes of tetradentate formazans in which the coordination sphere of the metal is completed by a variety of neutral ligands has been reported116 and these (185) were claimed to be very stable towards acids. By analogy with copper complexes, however, some doubt must exist regarding the structures assigned to those complexes derived from l,5-bis(2-carboxyphenyl)formazans. Various dyestuff and pigment applications have... 

Chromium compounds, mostly in chromium(III) or chromium(VT) forms, produced by the chemical industry are used for chrome plating, the manufacture of dyes and pigments, leather tanning, and wood preserving. Smaller amounts are used in drilling muds, rust and corrosion inhibitors, textiles, and toner for copying machines. For more information on the physical and chemical properties and on the production and use of chromium, see Chapters 3 and 4. 

Studies of chromate production workers, who are exposed to a variety of chromium compounds both hexavalent and trivalent, and chromate pigment industries, where exposure is mainly to chromium(VI), have consistently demonstrated an association with respiratory system cancer. Studies in chrome platers, who are exposed to chromium(VI) and other agents, including nickel, generally support the conclusion that certain chromium(VI) compounds are carcinogenic. Studies in stainless steel welders exposed to chromium(VI) and other chemicals, and in ferrochromium alloy workers, who are exposed mainly to chromium(O) and chromium(III), but also to some chromium(VI), were inconclusive. Studies in leather tanners, who are exposed to chromium(III), were consistently negative. 

Alkali dichromates are used as starting materials for the production of chromium(III) oxide pigments. They are available as bulk industrial products in the required purity. High impurity levels have an unfavorable effect on the hue. 

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