Cadmium salts, colors

Plastics and Synthetic Products. To prevent degradation of plastics at elevated processing temperatures, it is necessary to use suitable heat stabilizers. Eor example, flexible poly(vinyl chloride) (PVC) manifests uncontroUed color development in the absence of stabilizers. Accordingly, cadmium salts of organic acids are typically used in a synergistic combination with corresponding barium salts, in about a 1 3 cadmium barium ratio, to provide a cost-competitive heat stabilizer for flexible PVC. 

Toxicological studies on direct food additives have revealed toxic and harmful actions. Food dyes and preservatives have been used since ancient Roman times to improve the color of wine or to disinfect wine containers. The development of chemistry led to many unwise experiments, such as the dying of food with copper, chrome, lead, mercury, arsenic, and cadmium salts. In the U.S. in 1906, over 300 food dyes were officially tested, of which only seven passed and were allowed to be used in food. Only two of them - erythrosine and idigotine - are permitted now. The lists of preservatives are also constantly modified in different countries. Quite recently, formic acid, which is used to preserve semi-products, was banned in Poland due to its deleterious effects. 

Cadmium nitrate is used for coloring glass and porcelain (historically) as a flash powder in photography and in the manufacture of many other cadmium salts. 

Production. Cadmium red pigments are produced in a similar way to the cadmium yellow pigments. The cadmium salt solution is prepared by dissolving the metal in mineral acid and then sodium sulfide is added. A certain amount of selenium powder is dissolved in the sodium sulfide solution to obtain the desired color shade. In an alternative procedure, the cadmium solution is mixed with sodium carbonate solution to precipitate cadmium carbonate which is reacted with the selenium-containing sodium sulfide solution. 

The nitrates, sulfates, perchlorates, oxalates, carbonates, and sulfides of zinc and cadmium need not be described here. It should be recalled that all zinc salts, except for those having a colored anion, are colorless ZnS and GeSo are, in fact, the only nonhydrotyzable white metallic sulfides. Cadmium salts, except for the largely covalent bromide, iodide, and sulfide are also colorless. 

Cadmium yellow pigments (CdS) are manufactured by adding sodium sulfide to dissolved metallic cadmium or a cadmium salt solution to precipitate the fine particle raw colorant, which does not yet have pigment properties. Calcination at temperatures between 600 and 700°C induces particle growth to ca. 0.2 pm at which optimum tinting strength and hiding power are attained. 

Use Cadmium salts, photographic emulsions, coloring glass and porcelain, laboratory reagent, cadmium salts. 

Cadmium salts, like some other ions, form a colored chelate with glyoxal bis-(2-hydroxyanil) (see page 401). By masking and selective adsorption on exchange resins, the test becomes specific for cadmium. ... 

If a drop of the test solution is added to a solution of a few eg ammonium molybdate in 50 % sulfuric acid, a blue color appears immediately due to the reduction of Mo i to Mo. Alkali sulfides react similarly but sulfites, even in large amounts, do not. For this reason, it may be assumed that free (unstable) H2S2O8 acts as a reductant towards Mo i through its SH groups. Sulfides can be eliminated by previous treatment of the test solution with a cadmium salt, to precipitate CdS, and centrifuging. 

Reaction (36) was discovered by Arbuzov more than a century ago (not with PVC). It reflects the strength of P(iii) as a reducing agent, the product, P(v) bearing a P=0 bond. This is the primary color retention contribution of the phosphite component. Reaction (36) is catalyzed by zinc and cadmium salts, and probably by stannous compounds. 

Titanate Pigments. When a nickel salt and antimony oxide are calcined with mtile titanium dioxide at just below 1000°C, some of the added metals diffuse into the titanium dioxide crystal lattice and a yellow color results. In a similar manner, a buff may be produced with chromium and antimony a green, with cobalt and nickel and a blue, with cobalt and aluminum. These pigments are relatively weak but have extreme heat resistance and outdoor weatherabihty, eg, the yellow is used where a light cadmium could not be considered. They are compatible with most resins. 

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. 

Coloring agents can be either organic or inorganic, but the former can adversely affect heat stability. Examples of suitable inorganic colorants are oxides and salts of iron, chromium, cobalt, titanium, and cadmium. Some not only provide color, but can also have some beneficial heat-aging effects. 

Aside from these three classes (species with unfilled inner subshells, with unpaired electrons, or with two different oxidation states of the same element), there are a number of colored inorganic substances about which generalizations may be set up only with difficulty. Among these are many of the elementary nonmetals, a large number of covalent salts (such as mercuric iodide, cadmium sulfide, silver phosphate and lithium nitride), a number of nonmetal halides (iodine monochloride, selenium tetrachloride, antimony tri-iodide, etc.), and the colored ions, chromate, permanganate, and Ce(H20) v, whose central atoms presumably have rare-gas structures. 

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