Sulfoselenide Cadmium Red

Pure cadmium selenide [1306-24-7], CdSe, is brownish black and has no pigment properties. Like cadmium sulfide, it is dimorphous and occurs in hexagonal and cubic modifications. Cadmium selenide is insoluble in dilute acid. It readily liberates hydrogen selenide in concentrated hydrochloric acid. It dissolves completely in fuming nitric acid, the Se2 - ions being converted to SeO2 ions. Cadmium selenide is an n-type semiconductor. 

Cadmium red consists of cadmium sulfoselenide [12656-57-4], [58339-34-7], Cd(S,Se), and is formed when sulfur is replaced by selenium in the cadmium sulfide lattice. With increasing selenium content, the color changes to orange, red, and finally dark red. The density of these pigments increases correspondingly from 4.6 to 5.6 g/cm3 and the refractive index from 2.5 to 2.8. The crystals have cubic or spheroidal habits, the prevalent particle size is 0.3-0.4 pm. 

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 cadmium red pigment intermediate is obtained as a precipitate which is filtered off, washed, and calcined at approx. 600 °C. As with cadmium yellow, calcination yields the red pigment and determines the particle size, particle size distribution, and color shade. Analogously to the cadmium yellow process, cadmium red can be produced by direct reaction of cadmium oxide or cadmium carbonate with sulfur and the required amount of selenium at approx. 600 °C. 

Products in which selenium is totally or partially replaced by tellurium [3.108], [3.109] have not had any commercial application owing to their poor coloristic properties. 

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Cadmium sulfide selenides (cadmium sulfoselenide, cadmium red ) were originally commercialised in 1910, the manufacturing process involving heating cadmium sulfide, sulfur and selenium to about 600°C in an inert atmosphere. An improved process developed by the German company Bayer in 1919 involved the use of aqueous precipitation from solutions of alkaline sulfides and selenides the precipitate was yellow, the red shade developing on calcination at 300°C. 

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