Gallium

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60.1%

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39.9 %

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Risk

Gallium is a rare occurring chemical element with the chemical symbol Ga and atomic number 31 in the periodic table it is in the 4th period and is the third element of the third main group ( group 13) or boron group. It is a silvery white, easy to liquefying metal. Gallium does not crystallize in one of the otherwise frequently encountered in metals crystal structures, but in its most stable modification in an orthorhombic structure with gallium dimers. In addition, six other modifications are known to form during special crystallization conditions or under high pressure. In its chemical properties, the metal is very similar to the aluminum.

In nature, gallium occurs only to a small extent and mostly as an addition to aluminum, zinc or germanium ores; Galliumminerale are very rare. Accordingly, there is also obtained as a byproduct in the production of aluminum or zinc. The largest part of the gallium is further processed to the semiconductor gallium arsenide, which is used mainly for light emitting diodes.

9.1 compounds with elements of the nitrogen group
9.2 halides
9.3 Other compounds

History

For the first time a later gallium corresponding element in 1871 by Dmitri Mendeleev was predicted. He predicted using the periodic table he developed a new, eka- aluminum element called and said some properties of this element ( atomic mass, specific gravity, melting point location and type of salts ) operating systems.

The French chemist Paul Émile Lecoq de Boisbaudran that Mendeleev did not know predictions had found that certain laws prevail in the line of succession in the line spectrum of element families, and tried to confirm this for the aluminum family. He realized that between aluminum and indium another, yet unknown element must stand. In 1875 he succeeded finally, in the emission spectrum of sphalerite ore which he had dissolved in acid and treated with metallic zinc to prove two violet spectral lines, which he then assigned to the unknown element.

Then could win a larger amount gallium Lecoq de Boisbaudran from a few hundred kilograms sphalerite. From this he made by dissolving in a potassium carbonate solution and electrolysis for the first time the use of gallium dar.

For the naming, there are two theories. After the first named Boisbaudran the element to Gaul, the Latin name of his home country France. The second is also the Latin word gallus ( tap) as the source of the name, which in French is called Le Coq. Paul Émile Lecoq de Boisbaudran would have named the new element thus after his own name.

Once the properties of the new element were determined Mendeleev quickly realized that this is to the of him precomputed must act eka- aluminum. Many properties corresponded very exactly with the predicted values. So deviated from the theoretically calculated value of the density of 5.9 only very little from the experimental of 5,904.

Occurrence

Gallium is a rare element on Earth, with a content of 19 ppm in the Earth's continental crust is its frequency comparable to that of lithium and lead. It's not elementary, but only bound before, mainly in aluminum, zinc or germanium ores. The galliumreichsten ores are Bauxite, sphalerite ores and germanite.

The gallium budgets are usually small, the bauxite found in Suriname with the highest known content contains only 0.008% gallium. The present in bauxite Galliumreserven worldwide are estimated to be 1.6 × 106 tons. Higher levels of up to 1% gallium occur in germanite. Only in the Apex Mine in Utah occur so high contents in the ores that degradation was attempted to Galliumgewinnung. However, this failed after a short period of time for reasons of profitability.

Few Galliumminerale are known, these include the predominantly found in Tsumeb in Namibia Gallit ( CuGaS2 ) Söhngeit (Ga (OH ) 3) and Tsumgallit ( GaO (OH ) ).

Production and representation

Gallium is recovered as a by-product in the production of aluminum from bauxite in the Bayer process. The starting material which serves in sodium hydroxide dissolved mixture of sodium and Natriumgallat. Gallium thereof can be separated from the aluminum by various methods. One possibility is the fractional crystallization by means of carbon dioxide, wherein first preferred aluminum hydroxide precipitates, while the more readily soluble Natriumgallat enriched in sodium hydroxide solution. Only after further process steps gallium hydroxide is precipitated, mixed with aluminum hydroxide. Then the mixture is dissolved in sodium hydroxide solution and extracted gallium by electrolysis. Since this process is energy -and labor- consuming, it is only in countries with a low cost for about the People's Republic of China applied.

Gallium may also be obtained directly by electrolysis of caustic soda. Mercury cathode to be used, thereby forming a Galliumamalgam during electrolysis. Also adding to said solution sodium amalgam is possible.

Using special hydroxyquinoline chelating ligands, it is possible to extract gallium from the sodium hydroxide solution with kerosene and so be separated from aluminum. Other elements which are likewise extracted can be removed with dilute acid. Subsequently, the remaining gallium is dissolved in concentrated hydrochloric or sulfuric acid and electrolytically reduced to the metal.

For many technical applications very pure gallium is needed for semiconductors, for example, it may sometimes only a hundred millionth of foreign substances. Purification methods are vacuum distillation, fractional crystallization or melting zone.

The amount of produced gallium is low, 2008, the global primary production was 95 tons. Another important source is the re-processing of gallium-containing waste, from 2008, a further 135 tonnes of gallium was recovered. Main producing countries are the People's Republic of China, Germany, Kazakhstan and Ukraine, for the Galliumrecycling the United States, Japan and the United Kingdom.

On a laboratory scale gallium can by electrolysis of a solution of gallium in sodium hydroxide solution at platinum or tungsten electrodes pose.

Properties

Physical Properties

Gallium is a silvery white, soft (Mohs hardness: 1.5 ) metal. It has a for metals unusually low melting point is 29.76 ° C. It is thus according to the metal mercury and cesium having the lowest melting point which is well below those of the adjacent elements and aluminum, and indium. This is probably responsible for the unusual crystal structure, in contrast to the structures of other metals does not have a high degree of symmetry and is therefore not very stable. Because of the relatively high boiling point of 2400 ° C, gallium has an unusually large area in which it is liquid. Due to the difficult crystallization, liquid gallium can easily be below the melting point to cool ( hypothermia ) and crystallized abruptly in the formation of crystallization nuclei.

Gallium possesses such as silicon, a number of other elements and water a density anomaly in the liquid state its density is higher by about 3.2% as in solid form. This is typical for substances that have molecular bonds in the solid state.

Gallium is diamagnetic in a solid state, but is in a liquid state paramagnetic ( = 2.4 x 10-6 at 40 ° C)

Characteristic of his structures is the formation of gallium - gallium bonds. There are known various modifications, which are under different crystallization conditions ( four modifications known, α -bis δ gallium, under atmospheric pressure ), and pressure forming ( total of three high-pressure modifications, GA II, Ga III, Ga -IV). The most stable at room temperature modification is the α - gallium, which crystallizes in an orthorhombic layer structure. Each have two via a covalent bond to each other bound atoms form a dimer. Each gallium atom is adjacent in addition to six other atoms of other dimers. Between the individual dimers prevail metallic bonds. The Galliumdimere are so stable that they remain first obtained during melting and also in the gas phase can be detected.

Further modifications formed during the crystallization of supercooled liquid gallium. At -16.3 ° C, β - gallium, which has a monoclinic crystal structure forms. In the structure are arranged in parallel zigzag chains of gallium atoms. If the crystallization at a temperature of -19.4 ° C., is trigonal δ - gallium present in the comparable α -boron distorted icosahedron of twelve gallium forms. These are connected to one another via individual gallium. At -35.6 ° C eventually produced γ - gallium. In this modification, orthorhombic form of tubes of interconnected Ga7 -rings, a linear chain is in the midst of other gallium atoms.

If gallium set at room temperature under high pressure so formed during pressure increase successively various high-pressure modifications. From 30 kbar cubic gallium modification II is stable, wherein each atom is surrounded by eight other. The pressure is increased to 140 kbar, the metal crystallizes as tetragonal gallium III, in a structure corresponding to that of indium. If the pressure is further increased kbar to about 1200, eventually forming the face-centered cubic structure of the gallium -IV.

Chemical Properties

The chemical properties of gallium resemble those of aluminum. As this is gallium passivated by the formation of a dense oxide layer on the air and not responding. Only in pure oxygen at high pressure, the metal burns with a bright flame to form the oxide. Similarly, it also does not react with water, as this forms the insoluble gallium hydroxide. If, however, gallium alloyed with aluminum and liquid by lowering the melting point at room temperature, it reacts violently with water. Even with halogens react quickly gallium to form the corresponding salts GaX3.

Gallium is amphoteric and soluble in both acids and bases in the evolution of hydrogen. In acids form analogous to aluminum salts with Ga3 ions in alkali gallates the form [ Ga ( OH ) 4] -. In dilute acids it dissolves it slowly in aqua regia and concentrated sodium hydroxide solution quickly. Nitric gallium is passivated.

Most metals are attacked by liquid gallium, so that it can be stored in containers made of quartz, glass, graphite, alumina, tungsten and tantalum to 800 ° C. to 450 ° C..

Isotopes

There are a total of 30 known Galliumisotope between 56Ga and 86Ga and seven Kernisomere. Of these two, 69Ga and 71Ga are stable and are also found in nature. In the natural isotopic composition 69Ga predominates, with 60.12 %, 39.88 % are 71Ga. Of the unstable isotope has 67Ga 3.26 days, the longest half-life, the other half-lives ranging from seconds to a maximum of 14.1 hours at 72Ga.

Two Galliumisotope, 67Ga and the short-lived with half-life of 67.71 minutes 68Ga be used in nuclear medicine as tracers for positron emission tomography. 67Ga is generated in a cyclotron, while for the production of 68Ga no cyclotron is required. Instead, the longer- germanium isotope 68Ge produced by irradiation with protons of 69Ga. This decays to 68Ga, 68Ga with the resultant can be extracted in a gallium -68 generator. For investigations the gallium is usually tied in a complex with a strong chelating ligands such as 1,4,7,10 -tetraazacyclododecane -1 ,4,7,10 - tetraacetic acid ( DOTA ).

→ List of gallium isotopes

Use

Due to the rarity of the element gallium is used only to a limited extent. Different gallium compounds are produced from the largest part of the gallium produced. Are the economically by far the most important with the elements of the fifth main group, especially gallium arsenide, which is needed inter alia for solar cells and light emitting diodes. In 2003, 95 % of the gallium produced were processed this. In addition, it also serves as a material for doping silicon (p- doping).

The wide temperature range, in which the element is liquid, and the same low vapor pressure can be exploited for the construction of thermometers (as part of galinstan ). Gallium can be used up to temperatures of 1200 ° C. Liquid gallium can be used as sealing liquid to measure the volume of gases at higher temperatures as well as the liquid electrode material in the production of high-purity metals such as indium.

Gallium has a high wettability and good reflectivity and is therefore used as a coating for the mirror. It is also used for heat exchangers in nuclear reactors and as a substitute for mercury in lamps in melting alloys.

Alloys of gallium with other metals have various areas. Magnetic materials created by alloying with gadolinium, iron, yttrium, lithium, and magnesium. The alloy having vanadium in the composition V3Ga is a superconductor having a comparatively high transition temperature of 16.8 K. nuclear it is alloyed with plutonium, in order to prevent phase transformation. Many gallium alloys as galinstan are liquid at room temperature and can replace the toxic mercury or the very reactive sodium-potassium alloys.

Proof

Gallium can be detected qualitatively different typical color reactions. These include the reaction with rhodamine B in benzene, the orange yellow upon addition of gallium to red-violet fluorescence, Morin, the green fluorescence shows as in the reaction with aluminum, and Kaliumhexacyanidoferrat (III ), with the gallium a white precipitate of Galliumhexacyanidoferrat (III ) forms. Furthermore, a spectroscopic detection of the characteristic violet spectral lines at 417.1 and 403.1 nm is possible.

Quantitative evidence can be made through complexometric titrations, for example, with ethylenediaminetetraacetic acid or atomic absorption spectrometry.

Toxicology and biological significance

For gallium, there are no toxicological data; However, it is irritating to skin, eyes and respiratory tract. The compounds of gallium (III ) nitrate Ga ( NO3) 3 and gallium (III ) oxide Ga2O3 have oral LD50 values in the gram range: 4.360 g / kg for the nitrate and 10 g / kg for the oxide. Gallium is therefore considered to have low toxicity and plays, as far as known, no role as a trace element for humans.

Compounds

Gallium compounds is in almost exclusively in the oxidation state 3. In addition, rare and usually very unstable gallium ( I) compounds are known and those that contain both single-and trivalent gallium (formal gallium ( II) compounds ).

Connections with elements of the nitrogen group

The technically most important compounds of gallium are the ones with the elements of the nitrogen group. Gallium nitride, gallium phosphide, gallium arsenide and gallium antimonide typical semiconductors ( III-V semiconductors) and used for transistors, diodes and other components of the electronics. In particular, light-emitting diodes of different colors are produced as gallium - nitrogen group compounds. Which depends on the band gap of color can be adjusted by varying the ratio of the anion, or by the replacement of aluminum by gallium or indium. Gallium arsenide is also used for solar cells. In this particular satellite can be used as gallium arsenide is more resistant to ionizing radiation than silicon.

Halides

Gallium halides of the form GaX3 are similar in many properties to the corresponding aluminum compounds. With the exception of gallium ( III) fluorides they come in a Aluminiumbromidstruktur as a dimer. As the only halide gallium (III ) chloride has a low economic significance. It is used as Lewis acid in a Friedel-Crafts reactions.

Gallium

History

Occurrence

Production and representation

Properties

Physical Properties

Chemical Properties

Isotopes

Use

Proof

Toxicology and biological significance

Compounds

Connections with elements of the nitrogen group

Halides

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