Chrome green is a mixture of chrome yellow and Prussian blue.Ĭhromium is used to harden steel, to manufacture stainless steel, and to form many useful alloys. The chromates are generally yellow, the best known being lead chromate (PbCrO 4), an insoluble solid widely used as a pigment called chrome yellow. Potassium dichromate (K 2Cr 2O 7) is a red, water-soluble solid that, mixed with gelatin, gives a light-sensitive surface useful in photographic processes. In chromates and dichromates, chromium has a valence of +6. Chromic oxide (Cr 2O 3) is a green solid. Most of these compounds are green, but some are red or blue. In chromites and chromic salts, chromium has a valence of +3. The extensive use of chromite as a refractory is based on its high melting point, its moderate thermal expansion, and the stability of its crystalline structure. For this reason it is widely used as body trim on automobiles and other vehicles. When deposited electrolytically, chromium provides a hard, corrosion-resistant, lustrous finish. Because of its hardness, an alloy of chromium, cobalt, and tungsten is used for high-speed metal-cutting tools. In the stainless steels, chromium makes up 10 percent or more of the final composition. The addition of chromium imparts hardness, strength, and corrosion resistance to the alloy. The chief use of chromium is to form alloys with iron, nickel, or cobalt. It is an ingredient in several important catalysts. More than half the production of chromium goes into metallic products, and about another third is used in refractories. The Cr-Cr bond distance in a range of these quadruply bonded species has been found to vary between 195-255 pm. Each Cr(II) ion has 4 d electrons but the complex is found to be diamagnetic which is explained by the formation of a quadruple bond between the two metal ions. 2H 2O is an example of a Cr(II) complex which is reasonably stable in air once isolated. See the laboratory manual for this course for a range of other Cr(III) complexes for which you should know the structure. This allows a small amount of the Cr(II) ion to be formed, which is very labile but unstable with respect to oxidation back to Cr(III). the dark green trans-Cl.2H 2O salt shown above, etc.Īnhydrous CrCl 3 reacts with pyridine only in the presence of Zinc powder.Hydrated chromium chloride, 'CrCl 3.6H 2O', exists as hydrate isomers, including: Further deprotonation and polymerization can occur and, as the pH is raised, the final product is hydrated chromium(III) oxide or 'chromic hydroxide'.Īnhydrous CrCl 3 and hydrated 'CrCl 3.6H 2O', The ease with which the proton is removed can be judged by the fact that the hexaaquo ion (pKa ~ 4) is almost as strong as acetic acid. This is thought to occur by the loss of a proton from coordinated water, followed by coordination of the OH - to a second cation: One of the most obvious characteristics of Cr(III) is that it is acidic i.e it has a tendency to hydrolyse and form polynuclear complexes containing OH - bridges in a process known as OLATION.

pH 2-6 HCrO 4 - and Cr 2O 7 2- orange-red.World production of chromite ores approached 12 million tonnes in 1995. The sodium chromate produced in the isolation of chromium is itself the basis for the manufacture of all industrially important chromium chemicals. Alternatively, the Cr 2O 3 can be dissolved in sulfuric acid to give the electrolyte used to produce the ubiquitous chromium-plating which is at once both protective and decorative. The main use of the chromium metal so produced is in the production of nonferrous alloys, the use of pure chromium being limited because of its low ductility at ordinary temperatures.

Mass of chromium sulfate