Cr2O3 is an inorganic compound that goes by the chemical name Chromic Oxide. The alternative names of the compound are Dichromium Trioxide or Chromium Sesquioxide or Chromium (III) Oxide or Chrome green or Chromia. This is one of the major Oxides of Chromium. Here, the metal has a +3 oxidation state. The monoisotopic mass of the Chromic Oxide (Cr2O3) is 151.866 Da. The molar mass is 151.9904 g/Mol. It appears as crystals or in fine crystalline powder form of light to dark green colouration. The rare natural mineral of Cr2O3 is Eskolaite. It is named after a Finnish geologist Pentti Eskola.
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The mineral is extracted from Chromium rich tremolite skarns, Metaquartzite, and Chlorite veins. A trace amount can also be found in Chondrite meteorites.
Properties of Chromic Oxide (Cr2O3)
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The compound has a hexagonal crystal structure with nearly spherical morphology. Cr2O3 is a hard and brittle material. The density of the compound is 5.22 g/cm3.
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It is insoluble in water, alcohol, and acetone and not very reactive to acids. It is very slightly soluble in alkalis.
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It has a boiling point of 40000 C or 4270 K and a melting point of 24350C or 2708 K.
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It is canonicalized with five covalent bonds. There are three hydrogen bond acceptors and zero hydrogen bond donors.
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The chemical composition of chromic oxide has 68.46% of Chromium and 31.58% Oxygen content.
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It has a refractive index of 2.551. The magnetic susceptibility is +1960.0×10-6 cm3/mol. It is antiferromagnetic up to 307K.
Production of Chromic Acid
The very first transparent hydrated form of Chromium (+3) Oxide was prepared by The Parisians Pannetier and Binet in 1838. The mineral Chromite like (Fe, Mg)CR2O4 is used to derive Cr2O3. The conversion process takes place via Na2Cr2O7, which gets reduced with Sulfur at high temperatures.
Na2Cr2O7 + S → Na2SO4 + Cr2O3
Chromia can also be formed by the decomposition of Chromium salts like Chromium nitrate or Ammonium dichromate by an exothermic reaction.
(NH4)2Cr2O7 → Cr2O3 + N2 + 4H2O
Since the traditional process used for chromic oxide production discharges large quantities of solid waste and has low energy efficiency, a cleaner process is developed by the Chinese Academy of Sciences in Beijing. This new cleaner process promises advancement for industrial production is based on the 3Rs principle.
Uses of Chromic Oxide (Cr2O3)
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Chromic oxide is quite stable and is used as a green pigment in paints, inks, and glasses for its stability. Originally, it was called viridian.
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It is also used as a colourant for ceramics and produces a green tinge in ‘chrome green’ and ‘institutional green’.
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It is used as a surface coating on food-processing and food packaging equipment to prevent abrasive wear.
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In refractory materials, electric semiconductors it is used as a pigment. At times, the compound has also been used in printing banknotes and fabrics.
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It is a catalyst for organic and inorganic reactions.
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It has metallurgical uses, like manufacturing chromium metal and aluminium-chromium master alloys.
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Another special use is in colouring cement and granules for asphalt roofing and camouflage painting.
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Anhydrous chromic oxide is used for its heat, light, and chemical resisting properties in applications.
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It is a catalyst in the preparation of methanol, butadiene, and high-density polyethene.
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Stainless steel polishing is also done by chromic oxide.
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It is used as a green pigment in automotive finishes.
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Chemical Nature of Cr2O3
Chromium being a d-block element exhibits a variation in oxidation numbers of its oxides. Many d-block elements exhibit both basic or acidic properties in different oxide forms. The higher the oxidation number the more acidic its corresponding oxide. Chromium has three different oxides. CrO has an oxidation number +2, and is a basic oxide, while CrO3 is acidic with a +6 oxidation number. Cr2O3 is amphoteric. Even if it is insoluble in water, it dissolves in acid to give hydrated chromium ions [Cr(H2O)6]3+. It reacts with alkali to yield chromite ions.