Chromium

Chromium, Cr, is a transition metal element, found in Group VIa of the periodic table.

Atomic Number : 24
Relative Atomic Mass : 51.996

Discovery

Chromium was discovered by L N Vauquelin in 1797AD.

Occurrence

Chromium does not occur in elemental form, but is found in Chrome Ochre, Cr2O3.

The principal ores are Chromite, Fe(CrO2)2, and Crocoisite, PbCrO4, is also important as a source. Chromium is a trace component in semiprecious stones, emerald, jade, serpentine, etc..

Extraction

Chromium is made commercially by the Thermite Process, where Chromic Oxide is reduced by the stoichiometric amount of Silicon and Quick Lime in an electric arc furnace.

2 Cr2O3 + 3 Si + 3 CaO ==> 4 Cr + 3 CaSiO3

Properties

Chromium is a bluish-white metal, which can take a brilliant polish.
Chromium is a soft metal.
Chromium containing a small amount of Carbon is one of the hardest metals.
Chromium is very stable in air, and is used as an electroplated surface to protect more reactive metals from corrosion.

Reactions

Chromium is attacked slowly by dilute hydrochloric acid to form blue solutions of chromous salts.

Cr + HCl ==> CrCl2 + H2

The chromous salt turns green on standing due to its oxidation in air to chromic salts.

4 Cr + 4 HCl + O2 ==> 4 CrCl3 + 2 H2O

Uses

Chromium, Cr, is an important component

in steels, in particular Ferrochrome (i.e. an alloy containing 40% to 80% Chromium, Cr with Iron, Fe),
in Chrome Vanadium Steels, used for tools,
in Nichrome (i.e. an alloy containing 10% to 25% Chromium and 50% to 70% Nickel),
in Stellite (i.e. an alloy of Chromium, Cobalt, and Tungsten) used for surgical instruments,
as a plating metal on account of its resistance to corrosion, and
as its salts in the glass industry, in pottery and as a mordant in dyeing.

Detection and Analysis

Chromium is detected by the deep blue colouration imparted to ethereal solution over an aqueous test solution containing the substance under test with hydrogen peroxide which has been acidified with sulphuric acid.

Start of Hypertext .... Elements .... Compounds .... Index