1770 Peter Simon Pallas found in the same place a red lead mineral,
because of its red color Crocoite (from the Greek krokos "saffron")
was called. The use of red lead ore as a color pigment increased
rapidly. A product derived from Crocoite bright yellow, chrome yellow,
became the fashionable color yellow as a post.
Louis-Nicolas Vauquelin in 1797 won chromium (III) oxide, Cr2O3 from
Crocoite and hydrochloric acid. In 1798 he received contaminated
elemental chromium by reduction of chromium (III) oxide with charcoal.
This newly isolated element was named chromium (Greek chroma,
"color"), because of the variegation of its salts in different
oxidation states. Could detect traces of the new element Vauquelin in
gemstones such as ruby and emerald.
In the 19th Century were mainly chromium compounds used as pigments
and chromium in the tannery. End of the 20th Century, chromium and
chromium compounds mainly used to produce corrosion-and heat-resistant
alloys (chrome plating, stainless steel).
Occur
Temporal evolution of the chrome support
Chromium is the International Mineralogical Association (IMA)
recognized as a mineral (System-Nr. after Strunz: 1.AE.05 I/A.06-10 or
former), but occurs only rarely in nature in pure form on . So far,
only 10 localities are known [9]. Mostly chrome is therefore only in
bound form, especially as the mineral chromite (chrome iron ore) mined
FeCr2O4 with a chromium content of about 46% in surface or at shallow
depths. Some do contain other minerals such as chromium more
Ferchromid (~ 87%) or Grimaldiit (~ 61%), but come to the chromite
much rarer. In total there are currently (as of 2010) approximately
100 known minerals containing chromium [10]
2003 promoted South Africa 50% of world demand for chromite. Other
significant producing countries are Kazakhstan (15.2%), India (12.1%),
Zimbabwe (3.7%) and Finland (3%). According to the 2006 ICDA South
Africa promoted 36%, India 19%, Kazakhstan 17%, and Brazil, Zimbabwe,
Turkey and Finland 13%.
In 2000, funded approximately 15 million tonnes marketable chromite
ore. From this it can be gained 4 million tons of ferrochrome with a
market value of $ 2.5 billion. Metallic chromium is very rare in
deposits. In Udatschnaja mine in Russia is a diamond-bearing
kimberlite "pipe" exploited. In the reducing matrix were formed
diamonds and metallic chromium.
Extraction and representation
Chromium crystals by thermal decomposition of Chromiodid refined.
The extracted chromite ore is freed from the gangue. In the second
step, an oxidative decomposition takes place at about 1200 ° C for
chromate:
The sodium chromate is extracted with hot water and transferred into
dichromate with sulfuric acid:
The sodium dichromate dihydrate crystallize upon cooling from the
solution. By a subsequent reduction with coal obtained chromium (III)
oxide:
This is followed by the aluminothermic reduction of chromium (III)
oxide to chromium:
pure aluminothermic chromium recovered pieces
Chromium can not be obtained by reduction with coal from the oxide
ores, as this chromium carbide is formed. Pure chromium is represented
by electrolytic deposition of Cr 3 + ion from sulfuric acid solution.
Corresponding solutions are prepared by dissolving chromium (III)
oxide or ferro-chrome produced in sulfuric acid. Ferrochrome as a
starting material, however, requires prior removal of the iron.
Extremely pure chromium is produced by further purification steps
after the van Arkel-de Boer method.
Ferrochrome is produced by reduction of chromite in the arc furnace at 2800 ° C.
Properties
Chromium, refined. See also the microcrystals are very easily with the
high melting temperature of 1907 ° C evaporating chromium, which is
deposited again during the melting process.
Chromium is a silver-white, corrosion-and tarnish-resistant hard metal
that is in original condition tough, malleable and malleable. It is
antiferromagnetic with a Néel temperature of 475 K. [11] chromium
dissolves in hydrochloric acid and sulfuric acid after some time on
the evolution of hydrogen when the protective oxide layer is gone.
Common oxidation states of chromium are +2, +3 and +6, +3 is the most
stable being.
Cr (II) is unstable with d4 configuration. There is hardly a reducing
agent as quickly absorbs oxygen from the air such as Cr (II). But even
without exposure to air Cr2 + solutions are only stable for a short
time when they are made from pure chromium (eg, electrolytic
chromium).
Cr3 + is the stable form. This is explained by the crystal field
theory, according to the d3 configuration is stabilized by a
half-filled shell.
Chromium (VI) oxide
Cr (VI) as chromate (CrO42) or dichromate (Cr2O72) is used as a strong
oxidizing agent. It is toxic and carcinogenic. In aqueous solutions
exists between the two ions, a chemical equilibrium that is pH
dependent. Acidifying a dilute yellow chromate solution is added so
that H + ion to do so, according to LeChatelier shifts the equilibrium
toward the dichromate, the solution turns orange.
Evidence
Green chromium (III) chloride: In the alkaline bath, the ammonium
sulfide group belonging to trivalent chromium green to yellow chromate
is oxidized (redox)
Chromium (VI)-peroxide: chrome-determination
Chromium (III) salts, resulting in cation separation transition in the
detection reaction with the "alkaline bath" (caustic soda in
concentrated hydrogen peroxide solution.) Yellow chromate, which react
in the acid to orange dichromate:
(Caution: Chromates and dichromates are carcinogenic, see below.)
Chromium is a characteristic evidence of the formation of blue
chromium (VI)-peroxide, CrO (O2) 2 (often described as CrO5). For this
purpose, dilute nitric acid is mixed with hydrogen peroxide and
layered with diethyl ether. Then you bring the solution to be tested
carefully under the ether, without mixing the liquids. In the presence
of chromium is formed at the interface of a blue ring of chromium
(VI)-peroxide. (The ether is used as a stabilizer, since otherwise the
Chromperoxid decomposes with evolution of oxygen after a short time
back.)
Reaction:
Decomposition:
Even with the test samples occurs during the melting of the salt of
phosphorus bead with borax or salt NaNH4HPO4 (disodium) Na2B4O7 a
characteristic staining with heavy metal salts on (with Cr3 + green).
In the oxidation with sodium carbonate and potassium nitrate melt,
however chromium (III) is oxidized to chromate (yellow staining).
Quantitatively, chromium (VI) can be detected by iodometry, whereas
the green color of the resulting chromium (III)-aqua complex visual
recognition complicates the equivalence point. Traces of chromium
compounds can be determined by methods of atomic spectrometry. Here
are a detection limit of 2 mg / l for the flame AAS and 0.02 mg / l
for the graphite furnace AAS has been given. [12] In the polarography
gives dichromate in 1 M potassium chloride solution at several levels
-0.28, -0 , 96, -1.50 and -1.70 V (vs. SCE). Trivalent chromium (as
Hexaminkomplex) results in a 1 M ammonia-ammonium chloride buffer, one
step at -1.42 V [13]
Safety and biological significance [Edit]
The role of Cr (III) (Cr 3 + ions) in the human body is being
discussed controversially. There is evidence that could have Cr (III)
has a meaning in the carbohydrate and fat metabolism in mammals. This
information is followed at the time. Previous evidence has, that the
popular nutritional supplement Cr (III)-picolinate has a favorable
influence on body composition could not be confirmed in later studies.
In a study with hamster cells showed that Cr (III) picolinate is
mutagenic and can cause cancer.
The currently available data indicate that it is extremely unlikely to
suffer a chromium deficiency. Solve a toxic effect, higher doses of Cr
(III) is difficult, since the solubility of Cr (III) hydroxide is
extremely low (6.7 · 10-31). It is therefore probably absorbed in the
human intestine is very difficult. In the U.S., the recommended intake
of Cr (III) of 50-200 mg / day to 35 mcg / day for adult men and 25
micrograms / day was decreased in adult women.
Cr (VI) compounds are extremely toxic. They are mutagenic, damaging
the DNA. You get damage on the airways in the lungs and the body
tissue. People who are chronically exposed to such compounds are at
increased risk for lung cancer. The toxic effect increases with the
insolubility of the salt. [14] The RoHS directive restricts the use of
Cr (VI) compounds in a strong Europe.
Use [Edit]
Chromium and chromium compounds are used for various applications
where its resistance is used.:
Hard Chrome Plating: galvanic deposition of a thick up to 1 mm wear
layer directly on steel, cast iron, copper. Aluminum may also after
the application of an intermediate layer can be chrome (hard
chrome-plated aluminum cylinders in the engine).
A chrome-plated alloy wheels: especially in the tuning scene very popular.
Dekorverchromung: galvanic deposition of a <1 micron thick Cr layer as
a decor with a corrosion-inhibiting intermediate layer of nickel or
nickel-copper. Very often plastic parts are chrome plated. A
replacement for the Dekorverchromung Achrolyte process.
Passivation of electroplated zinc coatings (chromating)
Alloying element: in corrosion and heat resistant stainless steels and
nonferrous alloys
Catalyst: to enable or accelerate chemical reactions
Tanning: the most important method for the manufacture of leather
Compounds [Edit]
Chrome Oxide
Chromium (III) oxide is Cr2O3, as enamel paint and glass staining
(green bottle) is used (also Cologne Green Bridge). This is not to be
confused with the poisonous chromium green.
Chrome yellow
Lead (II) chromate PbCrO4, served as brilliant yellow pigment ("yellow
post"). Replaced due to its toxicity, it is now almost entirely due to
organic color pigments. In the analysis it is used for the iodometric
determination of lead.
The chrome yellow, the color is as an artist application, depending on
production, a lead sulfate / chromate (about 2 PbSO4 · PbCrO4). The
pigment was discovered in 1809 by Louis-Nicolas Vauquelin and produced
commercially in Germany since 1820. Chrome yellow has a high opacity,
its stability depends on the light yellow color. Chrome yellow is
rarely used in oil painting. Vincent van Gogh, however, include chrome
yellow used in the famous sunflower paintings, which are executed in
oil on canvas. These suffer, however, now partly under a yellow
discoloration of the notes. [15] [16]
The art technologist Prof. Christoph Krekel from the Art Academy in
Stuttgart to the use of the pigment chrome yellow: "The painters have
seized on the chrome yellow, because it is a very brilliant yellow is
- it has a great color intensity, which means you could with the help
of this new bright yellow hue, a much stronger painting produce. "
Chrome yellow is also an important color in the counterfeit analysis
of "old" paintings.
Chromium dioxide
Chromium (IV) oxide is CrO2, a black ferromagnetic powder for the
manufacture 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 H2CrO4 exists only in dilute aqueous
solution. It is 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 referred to as chromium trioxide.
The orange-colored, highly poisonous potassium dichromate is a strong
oxidizing agent K2Cr2O7: In sulfuric acid solution, primary alcohols
easily converted into the corresponding aldehydes, which can be to use
semi-quantitative detection of alcohol in the breath. In the
laboratory it has been used in the form of chromic acid to clean
glassware. Upon contact with chloride ions, however, the volatile,
carcinogenic CrO2Cl2 chromyl chloride is formed (fume-chamber).
Potassium is also used as a titrant and as a fixative in industrial
dye baths. Potassium dichromate and also very toxic ammonium
dichromate (NH 4) 2Cr2O7 are the light-sensitive substance in chrome
gelatin layers of early photography (see Noble printing process).
Chromite
(Chromite, see above) FeCr2O4 is used to produce molds for the firing of bricks.
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