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Titanium dioxide (TiO2) (CAS Number 13463-67-7) is a non-combustible, white, crystalline, solid, odourless powder. It is insoluble in water, hydrochloric acid, nitric acid, and alcohol, and it is soluble in hot concentrated sulphuric acid, hydrogen fluoride, or alkali. TiO2 has several naturally occurring mineral forms, or polymorphs, which have the same chemical formula and different crystalline structure. Common TiO2 polymorphs include rutile (CAS Number 1317-80-322 2) and anatase (CAS Number 1317-70-0). While both rutile and anatase belong to the tetragonal crystal system, rutile has a denser arrangement of atoms. Both anatase and rutile are used as white pigment. Rutile TiO2 is the most commonly used white pigment because of its high refractive index and relatively low absorption of light. Anatase is used for specialised applications (e.g., in paper and fibres). TiO2 does not absorb visible light, but it strongly absorbs ultraviolet (UV) radiation. Commercial rutile TiO2 is prepared with an average particle size of 0.22 μm to 0.25 μm. Pigment-grade TiO2 refers to anatase and rutile pigments with a median particle size that usually ranges from 0.2 μm to 0.3 μm. Particle size is an important determinant of the properties of pigments and other final products.