What Is Zinc Oxide?
Zinc oxide is an oxide of zinc with the chemical formula ZnO.
It occurs naturally as red zinc ore in limited areas. Zinc oxide is often referred to as zinc flower or zinc white.
Uses of Zinc Oxide
Zinc oxide is used in white pigment paints, paints, and emulsion glass. It is also used as an additive in rubber products, such as tires, to improve durability and heat dissipation.
Widely used in cosmetics, it is available as a sunscreen due to its property of reflecting ultraviolet rays. It is used in antiperspirants and body sheets for its deodorant and sweat-reducing effects. It also has astringent, anti-inflammatory, and antiseptic effects, and is used in pharmaceutical products, such as ointments and liquids.
Zinc oxide is also used as a catalyst for the synthesis of methanol.
Properties of Zinc Oxide
At room temperature, zinc oxide is a white powder that does not decompose when heated; it turns yellow when heated to about 300°C and returns to yellow when cooled; it begins to sublimate at about 1,300°C when heated at 1 atm. When heated at 1 atmospheric pressure, sublimation begins at about 1,300°C, which is lower than the temperature at which zinc oxide is reduced with carbon.
Zinc oxide is a true semiconductor. It reflects light over a wide wavelength range. With the addition of a small amount of transition metal, it becomes a phosphorescent material. It reflects visible light and can absorb ultraviolet light. It emits phosphorescent light in sunlight, emits green or purple light on anode or cathode rays, and emits white thermoluminescence when heated.
Zinc oxide is an amphoteric oxide. It dissolves in acid to form zinc salts and in aqueous alkaline hydroxide solutions to form zincates. It is soluble in ammonium carbonate solution and ammonia water, forming complex salts. However, it is not soluble in water.
Structure of Zinc Oxide
Zinc oxide is a compound of zinc and oxygen. Red zinc ore will form hexagonal crystals.
Zinc oxide crystallizes in two forms: urutzite (hexagonal) and sphalerite (cubic). The wurtzite-type structure, which is the most stable, is the most common. The sphalerite structure can be stabilized by growing zinc oxide on a substrate with a cubic lattice structure. Both crystals are tetrahedral, a common Zn(II) shape.
Other Information on Zinc Oxide
1. Synthesis of Zinc Oxide
Natural red zinc ore is a rare mineral found in only two mines in the United States. Crystals precipitated in the chimneys of a zinc factory in Poland are available for sale.
Zinc oxide is obtained by pyrolysis of zinc oxalate, whose chemical formula is ZnC2O4, at 400°C. It can also be produced by pyrolysis of basic zinc carbonate, ZnCO3-3Zn(OH)2. It can also be produced by the thermal decomposition of zinc nitrate or zinc sulfate.
Industrially, zinc oxide is obtained by heating metallic zinc, vaporizing it, and burning it with air.
2. Reaction of Zinc Oxide
When free fatty acids from sebum react with zinc oxide, a small amount of zinc ions are produced. When zinc oxide comes into contact with the earth’s atmosphere, it absorbs a small amount of carbon dioxide and water.
When scorched with cobalt oxide, the pigment cobalt green can be produced.
When zinc oxide is treated with phosphoric acid, it forms a cement-like substance used in dentistry. Its main component is called hopestone, and its chemical formula is Zn3(PO4)2・4H2O.
3. Hazards of Zinc Oxide
Zinc oxide itself is not toxic. However, it is dangerous to inhale zinc oxide fumes, which are produced when zinc or zinc alloys are oxidized at high temperatures. This occurs when alloys containing brass are melted because the melting point of brass is close to the boiling point of zinc. Exposure to airborne zinc oxide can also occur during welding of galvanized steel and can cause metal fume heat.
Combination formulations of zinc oxide and UV absorbers, such as those found in sunscreens, have been reported to be toxic because of the photolysis of zinc oxide with small molecule UV absorbers.