Chromium: bright and dangerous

Chromi­um is a hard, sil­very shiny met­al that is re­sis­tant to cor­ro­sion. It has so many col­ored com­pounds that even its name in Greek means “col­or”. Some of its com­pounds are well-known in demon­stra­tive chem­i­cal ex­per­i­ments. In in­dus­try and ev­ery­day life chromi­um can be en­coun­tered in var­i­ous al­loys, pig­ments, abra­sives and many oth­er things.

The most im­por­tant min­er­als con­tain­ing chromi­um are chromite (Fe­Cr₂O₄) and cro­coite (PbCrO₄).

Oth­er min­er­als, for ex­am­ple es­ko­laite, are quite rare. Min­er­als con­tain­ing chromi­um are pri­mar­i­ly mined in South Africa and Kaza­khstan.

The his­to­ry of chromi­um

In 1761, the Ger­man min­er­al­o­gist and ge­og­ra­pher Jo­hann Got­t­lob Lehman dis­cov­ered a pre­vi­ous­ly un­known beau­ti­ful or­ange-red min­er­al. He called it cro­coite. Al­though cro­coite was quite rare, and it was hard to mine, it swift­ly be­came pop­u­lar as a com­po­nent of paints. Soon car­riages of the no­bil­i­ty of Eng­land and France were paint­ed a fash­ion­able yel­low col­or, for which cro­coite was used. In 1797, the French chemist and phar­ma­cist Louis-Nico­las Vauquelin got his hands on some cro­coite ore.

He be­gan to ex­per­i­ment with this min­er­al, try­ing to dis­cov­er its chem­i­cal com­po­si­tion. By treat­ing the cro­coite with al­ka­li and acid, adding com­pounds of mer­cury and lead to the so­lu­tions, he ob­tained com­pounds of dif­fer­ent col­ors–or­ange, yel­low and green. A year lat­er, Vauquelin iso­lat­ed a pre­vi­ous­ly un­known met­al from the min­er­al. As the new el­e­ment could form com­pounds of dif­fer­ent col­ors, Vauquelin and his col­leagues de­cid­ed to call it Chromi­um from the Greek “χρῶμα”–“col­or”. Vauquelin lat­er dis­cov­ered that chromi­um gave ru­bies and emer­alds their col­or.

Chem­i­cal prop­er­ties of chromi­um

Am­mo­ni­um dichro­mate (NH₄)₂Cr₂O₇ is well-known from the “chem­i­cal vol­cano” ex­per­i­ment. A mound is made from or­ange-red pow­der of am­mo­ni­um dichro­mate, and a drop of al­co­hol is dripped on to the top, and then ig­nit­ed. A re­ac­tion of the de­com­po­si­tion of am­mo­ni­um dichro­mate is ini­ti­at­ed, which then sup­ports it­self be­cause of the great re­lease of heat. In the re­ac­tion, green chromi­um(III) ox­ide is formed, with a vol­ume which great­ly ex­ceeds the orig­i­nal dichro­mate. The flakes of ox­ide fly in dif­fer­ent di­rec­tions like vol­canic ash. The chem­i­cal re­ac­tion of this process:

(NH₄)₂Cr₂O₇→Сr₂O₃ + N₂↑+ 4H₂O

Chromi­um(III) ox­ide is a cat­a­lyst of the ox­i­da­tion of am­mo­ni­um. If heat­ed chromi­um(III) ox­ide is placed in an at­mos­phere of am­mo­ni­um, the fol­low­ing re­ac­tion takes place:

4NH₃ + 5O₂ → 4NO + 6H₂O

2NO + O₂ → 2NO₂

Be­tween the chro­mate ion (CrO₄²⁻) and the dichro­mate ion (Cr₂O₇²⁻) in the so­lu­tion there is equi­lib­ri­um that de­pends on the pH of the medi­um. If the so­lu­tion is al­ka­line, a yel­low chro­mate ion forms, and if the so­lu­tion is acid­i­fied, a dichro­mate ion forms once more.

2CrO₄²⁻ + 2H₃O⁺ ↔ Cr₂O₇²⁻ + 2H₂O

In the acidic medi­um, sodi­um py­ro­sul­fate (Na₂S₂O₅) re­duces the dichro­mate ion (Cr₂O₇²⁻) to a chromi­um(III) ion, which gives the so­lu­tion a green col­or. Ini­tial­ly sodi­um py­ro­sul­fate dis­so­ci­ates in this mix­ture, and S₂O₅²⁻ ions turn to HSO₃⁻ions:

Na₂S₂O₅ + H₂O → 2NaH­SO₃

Then the re­duc­tion of dichro­mate ions takes place:

2HCrO₄⁻ + 4H­SO₃⁻ + 6H⁺ → 2Cr⁺ + 3SO₄²⁻ + S₂O₆²⁻ + 6H₂O

2HCrO₄⁻ + 3H­SO₃⁻ + 5H⁺ → 2Cr⁺ + 3SO₄²⁻ + 5H₂O

The process can be re­versed by adding an al­ka­line so­lu­tion of hy­dro­gen per­ox­ide–then or­ange dichro­mate ions form again:

2Cr⁺ + 3H₂O₂ + 10OH⁻ → 2CrO₄²⁻ + 8H₂O

If a so­lu­tion of potas­si­um dichro­mate, hy­drochlo­ric acid and zinc is put in a test tube, the so­lu­tion first turns green, then blue. This is be­cause in the re­ac­tion of zinc with hy­drochlo­ric acid, atom­ic hy­dro­gen is re­leased, which re­duces the Cr(IV) in the dichro­mate ion first­ly to green Cr(III) and then to blue Cr(II):

Zn + 2HCl →Zn­Cl₂ + H₂

Cr₂O₇²⁻ + 3H₂ + 8HCl → 2Cr­Cl₃ + 7H₂O + 2Cl⁻

2Cr³⁺ + H₂ + 4HCl → 2Cr­Cl₂ + 6H⁺

If an al­ka­line so­lu­tion is added to a so­lu­tion of chromi­um(III) salt, gray-blue amor­phous chromi­um (III) hy­drox­ide hy­drate pre­cip­i­tates:

Cr³⁺ + 3OH⁻ → Cr(OH)₃↓

Where chromi­um is used

The ma­jor­i­ty of chromi­um is used for cre­at­ing al­loys. Al­loys con­tain­ing chrome are durable and heat-re­sis­tant. They are used, for ex­am­ple, for the man­u­fac­ture of plane en­gines, gas tur­bines and cut­ting mech­a­nisms. Ma­te­ri­als con­tain­ing chromi­um are used as heat-re­sis­tant coat­ings for smelt­ing fur­naces.

Chromi­um is re­sis­tant to cor­ro­sion, and it is used to coat oth­er met­als. Ad­di­tion­al­ly, chrome-plat­ed parts serve as dec­o­ra­tive el­e­ments for cars and mo­tor­cy­cles.

Chromi­um com­pounds are tox­ic. There was even a film made about chromi­um(VI) poi­son­ing, “Erin Brock­ovich” di­rect­ed by Steven Soder­bergh (2000). On the one hand, the tox­i­c­i­ty of chromi­um com­pounds re­stricts their use and in­volves spe­cial safe­ty re­quire­ments in work­ing with them. On the oth­er hand, be­cause of their tox­i­c­i­ty cer­tain chromi­um com­pounds, for ex­am­ple chro­mat­ed cop­per ar­se­n­ate, are used to pro­tect wood­en items from pests.

The less tox­ic chromi­um(III)-potas­si­um sul­fate and chromic acid are used as tan­ning agents in treat­ing leather.

Chromi­um, liv­ing up to its name which re­flects the di­ver­si­ty of col­ors of its com­pounds, has found ap­pli­ca­tion in mak­ing paints and pig­ment ad­di­tives. Paints on the ba­sis of cro­coite were not only used to paint car­riages in the 18th-19th cen­turies, but also school bus­es in the UK and the USA.

Bus­es paint­ed this col­or could be seen more clear­ly in the dawn and twi­light. Lat­er, how­ev­er, chromi­um-based paints were re­placed with less tox­ic or­gan­ic paints. But chromi­um is still used for mak­ing ar­ti­fi­cial ru­bies. The crys­tal of alu­minum ox­ide with ad­di­tions of Cr(III) ions is a ruby! This crys­tal was used by Theodore Maiman, who cre­at­ed the first work­ing laser in the world in 1960.

Sources

• Paul Par­sons, Gail Dixon — The Pe­ri­od­ic Ta­ble A vis­ual guide to the el­e­ments
• Nor­man N. Green­wood, Alan Earn­shaw — Chem­istry of the El­e­ments (2nd ed.). But­ter­worth-Heine­mann, 1997
• Guertin, Jacques; Ja­cobs, James Alan & Avakian, Cyn­thia P. Chromi­um (VI) Hand­book. CRC Press, 2005
• Maiman, T. H. "Stim­u­lat­ed op­ti­cal ra­di­a­tion in ruby". Na­ture. 187 (4736). 1960. P. 493–494.
• Wikipedia. Chromi­um
• Wikipedia. An­ti­fer­ro­mag­netism;
• Wikipedia. Es­ko­laite
• Wikipedia. Jo­hann Got­t­lob Lehmann
• Wikipedia. Chrome plat­ing