27 августа, 2015 
Pokraskin Selenium is positioned between sulfur and tellurium in Group VIb (chalcogens) of the periodic table, and exhibits properties similar to both these elements. It is more metallic than sulfur but less so than tellurium.
Like other members of the group it exhibits allotropy and exists in red and gray forms.
It exhibits several valency states, -2 (selenide), -4 (selenite) and -6 (selenate). Pigment chemistry is concerned solely with the -2 state.
Selenium is quite volatile, with the gray form melting at 217 °C and boiling at 685 °C. This causes some selenium to be lost when pigments are calcined during manufacture.
Hydrogen selenide, like hydrogen sulfide, is gaseous, toxic and extremely pungent. Although selenides can be precipitated using hydrogen selenide, this route is never used in pigment manufacture because of its toxic nature.
As with sulfur, selenium is very soluble in alkali sulfides. This provides a convenient way to dissolve selenium and finely precipitate it in the manufacture of cadmium reds.
3.3.3
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