Hydronium ion formation during the hydrometallurgical formation of valentinite (Sb₂O₃)

Cubic antimony trioxide or senarmontite (Sb₄O₆) is used as an additive to facilitate the flame retardancy of plastics and other commercial materials. This retardancy is due to the propensity of antimony trioxide to form gas-phase antimony trihalide under flame conditions, which is in turn able to stabilize the free radicals in the flame. Orthorhombic Sb₂O₃ or valentinite shows similar properties as senarmontite, but is not normally used because it oxidizes from a white to gray color when exposed to air and sunlight [1]. Such graying can impact on the coloring of plastics, paints and textiles, and so photo stable white senarmontite is the preferred choice.