# chrome

properties
[ Ar ] 3 d 5 4 s 1
24 Cr
General
Name , symbol , atomic number Chromium, Cr, 24
Element category Transition metals
Group , period , block 6 , 4 , d
Appearance silvery metallic
CAS number 7440-47-3
EC number 231-157-5
ECHA InfoCard 100.028.324
Mass fraction of the earth's envelope 0.019%
Atomic
Atomic mass 51,9961 (6) et al
Atomic radius (calculated) 140 (166) pm
Electron configuration [ Ar ] 3 d 5 4 s 1
1. Ionization energy 6th.76651 (4) eV652.87 kJ / mol
2. Ionization energy 16.486 305 (15) eV 1 590.69 kJ / mol
3. Ionization energy 30th.959 (25) eV2 987.1 kJ / mol
4. Ionization energy 49.16 (5) eV4 743 kJ / mol
5. Ionization energy 69.46 (4) eV6 702 kJ / mol
Physically
Physical state firmly
Crystal structure body-centered cubic
density 7.14 g / cm 3 (20 ° C )
Mohs hardness 8.5
magnetism antiferromagnetic ,
paramagnetic ( Χ m = 3.1 · 10 −4 )
Melting point 2180 K (1907 ° C)
boiling point 2755 K (2482 ° C)
Molar volume 7.23 · 10 −6 m 3 · mol −1
Heat of evaporation 347 kJ / mol
Heat of fusion 16.93 kJ mol −1
Speed ​​of sound 5940 m s −1 at 293.15 K.
Specific heat capacity 449 J kg −1 K −1
Work function 4.5 eV
Electric conductivity 7.87 · 10 6 A · V −1 · m −1
Thermal conductivity 94 W m −1 K −1
Chemically
Oxidation states 6, 3, 2, 1
Normal potential • −0.744 V (Cr 3+ + 3 e - → Cr)

• −0.913 V (Cr 2+ + 2 e - → Cr)

Electronegativity 1.66 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
48 Cr {syn.} 21.58 h ε 1.659 48 V
49 Cr {syn.} 42.3 min ε 2.631 49 V
50 Cr 4,345% > 1.8 · 10 17 a 2 εε 1.037 50 Ti
51 Cr {syn.} 27.7025 d ε 0.753 51 V
52 Cr 83.789  % Stable
53 Cr 9.501% Stable
54 Cr 2.365% Stable
55 Cr {syn.} 3.497 min β - 2.603 55 mn
56 Cr {syn.} 5.94 min β - 1.617 56 mn
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
53 Cr 3/2 −1.515 10 7 0.000903 11.3
safety instructions
GHS labeling of hazardous substances
 no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
Toxicological data
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Chromium ( ancient Greek χρῶμα chrṓma , German 'color' ) is a chemical element with the element symbol Cr and the atomic number 24. It is one of the transition metals , in the periodic table it is in the 6th subgroup (6th  IUPAC group ) or chromium group . The compounds of chromium have many different colors and are often used as pigments in paints and varnishes.

## history

Louis-Nicolas Vauquelin , first manufacturer of elemental chrome

In 1763 Michail Wassiljewitsch Lomonossow reported from his trip to Saint Petersburg that Johann Gottlob Lehmann , who worked there as a professor of chemistry, had discovered a “red lead ore” in Berezovsk and was investigating it.

In 1766 Johann Gottlob Lehmann described an orange-red mineral from the Urals as “red lead ore”, which forms a green solution in hydrochloric acid . Since he thought it was an iron-containing lead compound that crystallized out with calcium sulfate, chromium remained undiscovered.

In 1770 Peter Simon Pallas found a red lead mineral in the same place, which François Sulpice Beudant initially referred to as crocoise and later as crocoite (cf. Greek κρόκος krókos “saffron”). The use of red lead ore as a color pigment increased rapidly. A bright yellow obtained from crocoite, chrome yellow , became a fashionable color as postal yellow .

It was not until 1797 that Louis-Nicolas Vauquelin discovered an unknown metallic substance in red lead ore, which he compared in the form of an acid ( chromic acid ) with molybdic acid . He first received chromium (III) oxide Cr 2 O 3 from the mineral and hydrochloric acid. In 1798 he succeeded in producing contaminated elemental chromium by reducing chromium (III) oxide with charcoal . At around the same time, Martin Heinrich Klaproth also dealt with the mineral and also discovered the new element, but Vauquelin published his research earlier. René-Just Haüy , a friend of Vauquelin, suggested naming the newly discovered metal chromium (from the Greek χρῶμα chrṓma "color") because of the many colors of its salts in different oxidation states. At first Vauquelin did not like the proposed name because the metal he discovered had an inconspicuous gray color and did not justify the name, but his friends convinced him of this name. Vauquelin was able to detect traces of the new element in precious stones such as ruby and emerald .

In the 19th century, chrome compounds were mainly used as color pigments and in chrome tannery . At the end of the 20th century, chromium and chromium compounds were mainly used for the production of corrosion and heat-resistant alloys ( chrome plating , chrome steel ).

## Occurrence

Development of chrome production over time

Chromium is recognized as a mineral by the International Mineralogical Association (IMA) ( system number according to Strunz: 1.AE.05 or formerly I / A.06-10), but occurs only very rarely in natural form . So far, only ten sites are known. For the most part, chromium is therefore only mined in bound form, especially as the mineral chromite (chrome iron stone) FeCr 2 O 4 with a chromium content of around 46% in open-cast mining or at shallow depths. Some other minerals, for example ferchromide (~ 87%) or grimaldiite (~ 61%), contain more chromium, but are much less common than chromite. A total of around 100 minerals containing chromium are currently (as of 2010) known.

Turkey was the main chrome supplier to the Third Reich during World War II .

In 2000 approximately 15 million tons of marketable chromite ore were extracted. This could produce 4 million tons of ferrochrome with a market value of 2.5 billion dollars. Metallic chromium is very rare in deposits. In the Udachnaya pipe -Mine in Russia a diamond-containing is kimberlite - "Pipe" exploited. Diamonds and metallic chromium formed in the reducing matrix .

Chromite ore production in thousands of tons (2018)
1 South Africa 16,000 200,000
2 Turkey 6,500 26,000
3 Kazakhstan 4,600 230,000
4th India 3,500 100,000

## Extraction and presentation

Chromium crystals are
refined by thermal decomposition of chromium iodide

The extracted chromite ore is freed from the dead rock. The second step is an oxidizing digestion at approx. 1200 ° C to give the chromate:

${\ displaystyle \ mathrm {4 \ FeCr_ {2} O_ {4} +8 \ Na_ {2} CO_ {3} +7 \ O_ {2} \ longrightarrow}}$  ${\ displaystyle \ mathrm {8 \ Na_ {2} CrO_ {4} +2 \ Fe_ {2} O_ {3} +8 \ CO_ {2} \}}$

The sodium chromate is extracted with hot water and converted into dichromate with sulfuric acid :

${\ displaystyle \ mathrm {2 \ Na_ {2} CrO_ {4} + H_ {2} SO_ {4} \ longrightarrow}}$  ${\ displaystyle \ mathrm {Na_ {2} Cr_ {2} O_ {7} + Na_ {2} SO_ {4} + H_ {2} O \}}$

The sodium dichromate crystallizes out of the solution as a dihydrate on cooling. A subsequent reduction with carbon gives chromium (III) oxide:

${\ displaystyle \ mathrm {Na_ {2} Cr_ {2} O_ {7} \ cdot 2 \ H_ {2} O + 2 \ C \ longrightarrow}}$  ${\ displaystyle \ mathrm {Cr_ {2} O_ {3} + Na_ {2} CO_ {3} +2 \ H_ {2} O + CO \}}$

This is followed by the aluminothermic reduction of chromium (III) oxide to chromium:

${\ displaystyle \ mathrm {Cr_ {2} O_ {3} +2 \ Al \ longrightarrow Al_ {2} O_ {3} +2 \ Cr \}}$
Pure, aluminothermically obtained chrome piece

Chromium cannot be obtained from oxidic ores by reduction with coal, as this results in chromium carbide . Purer chromium is produced by electrolytic deposition of the Cr 3+ ion from a sulfuric acid solution. Corresponding solutions are made by dissolving chromium (III) oxide or ferrochromium in sulfuric acid. Ferrochrome as a starting material, however, requires the iron to be separated off beforehand.

Extremely pure chromium is produced by further purification steps using the Van Arkel de Boer process .

Ferrochrome is produced by reducing chromite in an electric arc furnace at 2800 ° C.

${\ displaystyle \ mathrm {FeCr_ {2} O_ {4} +4 \ C \ longrightarrow Fe + 2 \ Cr + 4 \ CO}}$

## properties

Oxidation states of chromium
−2 Na 2 [Cr (CO) 5 ]
−1 Na 2 [Cr 2 (CO) 10 ]
0 Cr (C 6 H 6 ) 2
+1 K 3 [Cr (CN) 5 NO]
+2 CrCl 2 , CrO
+3 CrCl 3 , Cr 2 O 3
+4 CrF 4 , CrO 2 , K 2 CrF 6
+5 CrF 5 , K 3 CrO 8
+6 CrF 6 , CrO 3 , K 2 CrO 4
Chrome, refined. You can also see microcrystals of the chromium, which evaporates very easily at the high melting temperature of 1907 ° C, which are deposited again during the melting process.

Chromium is a silver-white, corrosion and tarnish-resistant hard metal that is tough, malleable and forgeable in its original state. It is antiferromagnetic with a Néel temperature of 311 K (about 38 ° C). Chromium dissolves in hydrochloric acid and sulfuric acid after some time with evolution of hydrogen when the protective oxide layer has been removed. Common oxidation states of chrome are +2, +3 and +6, with +3 being the most persistent.

Cr (II) with d 4 - configuration unstable . The Jahn-Teller effect occurs with this configuration . As a result, Cr (II) complexes are often distorted octahedral or square coordination. Cr 2+ - solutions are only stable when it (for example, electrolytic chromium.) Of pure chromium can be obtained. Cr (II) compounds are powerful reducing agents .

Cr 3+ is the most stable form. This is achieved by the crystal field theory explains, at a d after 3 - configuration all t 2g orbitals ( Oktaederfeldaufspaltung ) with a single, unpaired electron are occupied. This configuration is particularly favorable in terms of energy and is therefore stable.

Cr (VI) as chromate (CrO 4 2− ) or dichromate (Cr 2 O 7 2− ) is used as a strong oxidizing agent. It is toxic and carcinogenic . In aqueous solutions, there is a chemical equilibrium between the two ions that is pH- dependent. Acidified to a dilute yellow chromate - solution in, so you are H 3 O + ions to do so, shifts to the Le Chatelier's principle , the equilibrium in the direction of the dichromate , the solution becomes orange.

${\ displaystyle {\ ce {2CrO4 ^ 2- + 2H3O + <=> Cr2O7 ^ 2- + 3H2O}}}$

## proof

Green chromium (III) chloride: In the alkaline bath, the trivalent green chromium belonging to the ammonium sulfide group is oxidized to yellow chromate ( redox reaction ).
Chromium (VI) peroxide: chromium determination

Chromium (III) salts produce yellow chromate solutions in the cation separation process in the detection reaction with the "alkaline bath" (NaOH in conc. Hydrogen peroxide solution) , which react in acidic conditions to form orange dichromate :

${\ displaystyle \ mathrm {3 \ H_ {2} O_ {2} +2 \ Cr ^ {3 +} + 10 \ OH ^ {-} \ longrightarrow}}$  ${\ displaystyle \ mathrm {2 \ CrO_ {4} ^ {2 -} (yellow) +8 \ H_ {2} O \}}$
(Warning: chromates and dichromates are carcinogenic, see below.)

A characteristic proof of chromium is the formation of blue chromium (VI) peroxide , CrO (O 2 ) 2 (often also described as CrO 5 ). For this purpose, dilute nitric acid is mixed with hydrogen peroxide and covered with diethyl ether . Then the solution to be tested is carefully brought under the ether layer without mixing the liquids. If chromium is present, a blue ring of chromium (VI) peroxide forms at the interface. (The ether serves as a stabilizer, since otherwise the chromium peroxide will decompose again after a short time with evolution of oxygen .)

Reaction:
${\ displaystyle {\ ce {Cr2O7 ^ 2- + 4H2O2 + 2H3O + -> 2CrO (O2) 2 + 7H2O}}}$
Decomposition:
${\ displaystyle {\ ce {4CrO (O2) 2 + 12H3O + -> 4Cr ^ 3 + + 7O2 + 18H2O}}}$

In the preliminary samples , too , when the salt bead is melted with phosphorus salt NaNH 4 HPO 4 or borax (disodium tetraborate) Na 2 B 4 O 7, a characteristic color with heavy metal salts occurs (with Cr 3+ green). In the oxidation melt with soda and saltpeter, on the other hand, chromium (III) is oxidized to chromate (yellowing). Chromium (VI) can be determined quantitatively by means of iodometry , whereby the green color of the resulting chromium (III) aqua complex makes the visual recognition of the equivalence point difficult. Traces of chromium compounds can be determined by methods of atomic spectrometry . The detection limit given is 2 µg / l for the flame AAS and 0.02 µg / l for the graphite tube AAS. In polarography , dichromate in 1 M potassium chloride solution shows several levels at −0.28, −0.96, −1.50 and −1.70 V (versus SCE ). Trivalent chromium (as Hexaminkomplex ) results in a 1 M ammonia - ammonium chloride - buffer a stage at -1.42 V.

## Safety instructions and biological significance

The role of chromium (III) (Cr 3+ ions) in the human body is currently the subject of controversy. There are indications that chromium (III) could be important in the carbohydrate and fat metabolism of mammals. This information is currently being followed up. Earlier evidence that the popular dietary supplement chromium (III) picolinate has a beneficial effect on body structure could not be confirmed in later studies. A study with hamster cells showed that chromium (III) picolinate is mutagenic and can cause cancer.

The data currently available indicate that it is extremely unlikely to suffer from a chromium deficiency. Even higher doses of chromium (III) trigger a toxic effect with difficulty, since the solubility product of chromium (III) hydroxide is extremely low (6.7 · 10 −31 ). It is therefore presumably very difficult to absorb in the human intestine. In the US, the recommended intake of chromium (III) has been reduced from 50–200 µg / day to 35 µg / day in adult men and to 25 µg / day in adult women.

In 2014, the European Food Safety Authority removed chromium from the list of essential minerals. The agency concluded that the ingestion of chromium has no beneficial effects on human health.

Chromium (VI) compounds are extremely toxic. They are mutagenic and damage DNA . They enter the body through the airways and damage the lung tissue. People chronically exposed to such compounds are at an increased risk of developing lung cancer . The poisonous effect increases with the insolubility of the salt. Since 1998, processes for the treatment of commodities made of leather that, when used as intended, such as watch straps, do not only touch the human body temporarily, are no longer allowed in Germany , provided that chromium (VI) can then be detected in the product. For the European Union, the RoHS directives have been increasingly restricting the use of Cr (VI) compounds in electrical and electronic equipment since 2004 . Since May 2015, leather products that come into contact with the skin may no longer be placed on the EU market if the content of chromium (VI) compounds exceeds 3 mg / kg leather. The use and placing on the market of cement or cement mixtures that contain more than 2 mg of soluble chromium (VI) compounds per kg after solidification and that pose a risk of skin contact was already prohibited.

## use

Chromium and chromium compounds are used for a wide variety of applications in which its resistance is used:

• Alloy element : in corrosion- and heat-resistant stainless steels and non-ferrous alloys
• Hard chrome plating : galvanic application of a wear protection layer up to 1 mm thick directly onto steel, cast iron, copper. Aluminum can also be chrome-plated after an intermediate layer has been applied (hard chrome-plated aluminum cylinders in engine construction). Since September 21, 2017, the use of chromium (VI) requires an authorization from the EU if chromium (VI) electrolytes are to continue to be used for chromium plating.
A chrome-plated alloy wheel: very popular, especially in the tuning scene .
• Decorative chrome plating : galvanic application of a <1 μm thick Cr layer as a decoration with an anti-corrosion intermediate layer made of nickel or nickel-copper. Plastic parts are also very often chrome-plated. The achrolyte process is a substitute for decorative chrome plating .
• Passivation of galvanic zinc layers ( chromating )
• Catalyst : to enable or accelerate chemical reactions
• Chrome tanning : the most important method for the manufacture of leather

Chromium oxide green pigment consists of chromium (III) oxide

Chromium oxide green: Chromium (III) oxide Cr 2 O 3 , is used as enamel paint and for coloring glass (green bottles) (also Cologne bridge green ). This is not to be confused with the toxic chrome green .

Chrome yellow
Lead (II) chromate PbCrO 4 , used to be used as a brilliant yellow color pigment (“post yellow”). Due to its toxicity , it is now almost completely replaced by organic color pigments. In analysis it is used for the iodometric determination of lead .

The chrome yellow, which is used as an artist's paint , is a lead sulfate / lead chromate (about 2 PbSO 4  · PbCrO 4 ) , depending on the manufacture . The pigment was discovered by Louis-Nicolas Vauquelin in 1809 and has been produced commercially in Germany since 1820. Chrome yellow has a high hiding power , its light stability depends on the yellow tone . Chrome yellow is rarely used in oil painting . Vincent van Gogh, however, used chrome yellow a. a. in the famous sunflower paintings executed in oil on canvas. Today, however, some of these suffer from discoloration of the yellow tones.

The art technologist Christoph Krekel from the Stuttgart Art Academy on the use of the pigment chrome yellow : “The painters threw themselves on chrome yellow because it is a very brilliant yellow - it has a great color intensity, which means that you could use this new yellow shade create a painting that is much brighter ”.

Chrome yellow is also an important color in the forgery analysis of “old” paintings.

Chromium dioxide
Chromium (IV) oxide CrO 2 , is a black ferromagnetic powder for the production of magnetic tapes with a better signal-to-noise ratio than conventional iron oxide magnetic tapes , as chromium dioxide has a higher coercivity .
Chromic acid
with the hypothetical structure H 2 CrO 4 only exists in dilute aqueous solution. It's very poisonous. It exists as an anion in some chromates and dichromates .

The anhydride of chromic acid, the very toxic chromium (VI) oxide CrO 3 , is known as chromium trioxide .

The orange-colored, very poisonous potassium dichromate K 2 Cr 2 O 7 is a powerful oxidizing agent : In sulfuric acid solution, primary alcohols are easily converted into the relevant aldehydes , which can be used for the semi-quantitative detection of alcohol in the breath. In the laboratory, it was used in the form of chromosulfuric acid to clean glass appliances. However, on contact with chloride ions, the volatile, carcinogenic chromyl chloride CrO 2 Cl 2 is formed (deduction!). Potassium dichromate is also used as a titrant and as a fixative in industrial dye baths. Potassium dichromate and ammonium dichromate (NH 4 ) 2 Cr 2 O 7 , which is also very poisonous, are the light-sensitive substances in chrome gelatin layers in early photography (see fine printing process ).

Chromite
(Chrome iron stone; see above) FeCr 2 O 4 is used to make molds for burning bricks.

## Complex compounds

A large number of complex compounds of chromium are known, in particular chromium (III). The ammine complexes play an important role here. Between the hexaammine chromium (III) ion [Cr (NH 3 ) 6 ] 3+ and the pure aqua complex [Cr (H 2 O) 6 ] 3+ there are several transitions from aquapentaammine chromium (III) - to tetraaquadiammine chromium (III) - ion. There are also a number of mixed ammine complexes with organic ligands, such as. B. ethylenediamine . Complexes of the type [CrX 6 ] 3− exist with the ligands fluorides , chlorides , thiocyanates and cyanides . Again there are several mixed forms. One example is the Reinecke salt NH 4 [Cr (SCN) 4 (NH 3 ) 2 ], which plays a role in analytical chemistry for precipitating cations . Mixed aquachloro complexes occur in chromium (III) chloride solutions and have coined the term hydration isomerism . Aquasulfato complexes can form in chromium (III) sulfate solutions . Polynuclear complexes are preferably formed via oxo or hydroxo bridges, e.g. B. [(NH 3 ) 5 Cr (OH) Cr (NH 3 ) 5 ] 5+ . Complexes with other oxidation states of chromium are partly unstable. Chromium (II) compounds are powerful reducing agents. Ammine complexes such as [Cr (NH 3 ) 6 ] 2+ , complexes with hydrazine , ethylenediamine, bipyridines and the thiocyanate ion are known here, for example . Peroxo complexes are formed by chromium in higher oxidation states, such as potassium peroxochromate, which is formed by the reaction of potassium dichromate solution with hydrogen peroxide .

## literature

Commons : Chromium  album with pictures, videos and audio files
Wiktionary: Chromium  - explanations of meanings, word origins, synonyms, translations

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