Chromium (VI) compounds: Overview
Description
Is used to make bricks and linings for furnaces. Compounds are used for chrome plating (chromic acid), manufacture of dyes (soluble chromates), wood treatment and water treatment.
Substance details
Substance name: Chromium (VI) compounds
CASR number: 7440-47-3 (Cr metal), 18540-29-9 (Cr (VI))
Molecular formula: Cr6+
Synonyms: Chromium (VI) is an oxidation state of the element chromium. Other common oxidation states of chromium include 0 (the metallic element chromium), and III (the state in which chromium is found in nature. Chromium (III) compounds are dealt with elsewhere in the NPI). Hexavalent chromium compounds (VI) include ammonium dichromate, barium chromate, calcium chromate, chromium trioxide, lead chromate, sodium dichromate, strontium chromate, potassium chromate, potassium dichromate, sodium chromate, and zinc chromate.
Physical properties
Vary widely depending on the particular compound, for example:
Most Cr(VI) compounds are soluble in water, however, barium chromate (a yellow powder with specific gravity = 4.49) and lead chromate (an orange-yellow powder with specific gravity = 6.3) are insoluble in water, and calcium chromate (a yellow powder) is slightly soluble in water. Sodium dichromate is in the form of red to orange crystals, with specific gravity = 2.35, and is soluble in water. Chromic acid (H2CrO4) exists only as salts (e.g. lead chromate, barium chromate) or in solution.
Chemical properties
Chromic acid is very reactive. Most Cr(VI) solutions are powerful oxidising agents in acidic conditions, but much less oxidising under alkaline conditions. Chromium trioxides are soluble in alcohol, ethanol, sulfuric acid, and nitric acid; when heated to decomposition chromium trioxides emit smoke and irritating fumes. Lead chromate is insoluble in water, acetic acid, and ammonia, but is soluble in acid and alkalies; when heated to decomposition emits toxic fumes of lead. Potassium chromate and potassium dichromate are soluble in cold and hot water and insoluble in alcohol. Sodium dichromate is soluble in cold and hot water and insoluble in alcohol; toxic fumes of sodium monoxide are emitted when this compound is heated to decomposition. Strontium chromate is soluble in cold and hot water, hydrochloric acid, nitric acid, acetic acid, and ammonium salts. Zinc chromate is insoluble in cold water and acetone, dissolves in hot water, and is soluble in acid and liquid ammonia.
Further information
The National Pollutant Inventory (NPI) holds data for all sources of chromium (VI) compounds emissions in Australia.
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