Polish that Chrome!
Chromium is the first of the group 6 transition metals. It is denoted by the symbol Cr and atomic number 24. Chromium is a grey coloured, hard and very lustrous metal. It takes a high polish, resists tarnishing, and has a high melting point. The name of the element is derived from the Greek word “chrōma” (χρώμα), meaning colour.
Chromium is highly corrosion resistant, a character that it brings with it when added to steel to create stainless steel. Another popular use for chromium is electroplating, which gives hubcaps, bumpers and other shiny bits of cars their street-cred.
Many of its compounds are intensely coloured. Besides providing the familiar non-fading “chrome yellow” colour used on American school buses and by the German postal service, there is the bright red pigment, chrome red (PbCrO4·Pb(OH)2), a bright green (Cr2O3), a pale green ([CrCl(H2O)5]Cl2), and rich violet ([Cr(H2O)6]Cl3). Trace amounts of chromium also gives rubies and emeralds their characteristic colours.
Below is the relatively rare mineral, crocoite (PbCrO4), the state mineral of Tasmania.
Chromium is the 24th most abundant element in Earth’s crust with an average concentration of 100 ppm. Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 300 mg/kg, in sea water 5 to 800 µg/litre, and in rivers and lakes 26 µg/litre to 5.2 mg/litre. Chromium is mined as chromite (FeCr2O4) ore. About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa
In the laboratory Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatus. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively). The use of dichromate cleaning solutions is now phased out due to the high toxicity and environmental concerns. Modern cleaning solutions are highly effective and chromium free. Potassium dichromate is a chemical reagent, used as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
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