Chemical elements
    Physical properties
    Chemical properties
      Aluminium subfluoride
      Aluminium trifluoride
      Aluminium trichloride
      Aluminium tribromide
      Aluminium iodide
      Aluminium chlorate
      Aluminium perchlorate
      Aluminium bromate
      Aluminium periodate
      Aluminium suboxide
      Aluminium sesqui-oxide
      Aluminium peroxide
      Aluminium hydroxides
      Tricalcium aluminate
      Sodilim aluminate
      Aluminium sesqui-sulphide
      Aluminium selenide
      Aluminium telluride
      Aluminium sulphite
      Aluminium sulphate
      Sodium alum
      Potassium alum
      Ammonium alum
      Hydroxylamine alum
      Silver alum
      Aluminium dithionate
      Aluminium selenite
      Aluminium selenate
      Aluminium chromate
      Aluminium molybdate
      Aluminium silicomolybdate
      Aluminium tungstate
      Aluminium silicotungstate
      Aluminium phosphotungstate
      Aluminium nitride
      Aluminium phosphide
      Aluminium arsenide
      Aluminium nitrate
      Aluminium Phosphates
      Basic aluminium arsenite
      Aluminium carbide
      Aluminium carbonate
      Aluminium thiocyanate
      Aluminium oxalate
      Aluminium alkyls
      Aluminium Hydrocarbon
      Aluminium acetylacetonate
      Aluminium silicide
      Aluminium silicates
      Aluminosilicic acids aluminosilicates
      Aluminium Borides
      Aluminium Boride
      Aluminium Boride
      Aluminium borocarbides
      Aluminium borate
      Aluminium sodium perborate
    PDB 1a6e-1zca
    PDB 2b8w-3i62
    PDB 3kql-5ukd

Aluminium trifluoride, AlF3

Aluminium trifluoride, AlF3, is not found in nature in the anhydrous state, but the hydrated fluoride occurs as the mineral fluellite, AlF3.H2O. It crystallises in the rhombic system (bipyramidal; a:b:c = 0.770:1:1.874); its density is 2.17.

Aluminium is completely converted into the fluoride when heated in fluorine (Moissan). The fluoride is also formed when aluminium or alumina is heated to redness in hydrogen fluoride. Deville, who made an exhaustive study of the fluoride, obtained it first by heating aluminium in silicon fluoride, and afterwards prepared it by treating alumina with hydrofluoric acid and subliming the product in hydrogen at a white heat. He also obtained the fluoride by heating a mixture of fluorspar and alumina in a stream of hydrogen fluoride, and by fusing cryolite with anhydrous aluminium sulphate. The impure fluoride is best purified by sublimation at 1100°.

Anhydrous aluminium fluoride forms colourless, transparent crystals which are described as rhombohedra, but, according to Poulenc, are probably triclinic. It is exceedingly refractory, being insoluble in water and unattacked by alkalies and acids, including concentrated sulphuric acid. It may be decomposed by prolonged fusion with an alkali carbonate.

When 42 grams of basic aluminium acetate, 54 grams of alumina, 100 grams of water, and 66 grams of 40 per cent, hydrofluoric acid are mixed and warmed, complete solution results, and a gelatinous, hydrated aluminium fluoride separates on standing. If the solution be somewhat diluted, crystals of the trihydrate, AlF3.3H2O, slowly separate; if only 75 grams of water are employed in the preparation, crystals of another hydrate, 2AlF3.17H2O, separate. The latter hydrate effloresces in air, passing into the former. The monohydrate, AlF3.H2O, can also be obtained in sparingly soluble, silky needles, and likewise the hydrate, 2AlF3.7H2O, which slowly changes to the hexahydrate, AlF3.6H2O, on standing in water.

The hydrate 2AlF3.7H2O has been obtained by Baud in two forms. The first, practically insoluble in water, is obtained by dissolving aluminium hydroxide in aqueous hydrofluoric acid and evaporating the solution at 100°; the second, easily soluble in water, by concentrating a solution of aluminium hydroxide in hydrofluoric acid and adding twice its volume of alcohol. An aqueous solution of the latter form is acid to litmus. The hydrate 2AlF3.7H2O loses water at 140°, leaving the hemihydrate 2AlF3.H2O, which decomposes at a bright red heat.

According to Deville, the compounds AlF3.3HF, 3AlF3.2HF.5H2O, and 2AlF3.HF.5H2O may be prepared. Aluminium fluoride enters into the composition of a large number of double compounds. With each of the alkali fluorides it forms a compound of the type AlF3.3XF stable at its melting-point. The melting-points are given by Puschin and Baskov as follows: -

Alkali MetalLiNaKRbCs
Melting-point of Salt, ° C800°1020°1035°985°823°

According to the same authors, compounds of the type 2AlF3.3XF are also formed when X = Na, K, or Rb; but this conclusion is not justified from their experiments and is probably erroneous. In the case of the sodium aluminium fluorides, the second compound has the formula 3AlF3.5NaF, and has no melting-point, but dissociates at 723° into AlF3.3NaF and aluminium fluoride.

The equilibrium diagram, as far as it has been worked out, is shown in fig. Calcium and aluminium fluorides form no compound; the eutectic point is 815° to 820.°

Hydrated double fluorides of the formulse 2AlF3.6KF.7H2O, 2AlF3.6NaF. 7H2O, and 2AlF3.4NH4F.3H2O are obtained as gelatinous precipitates when the requisite alkali fluoride solutions are added to an aqueous solution of Baud's soluble hydrate of aluminium fluoride. They are slightly soluble in water, 100 parts of which dissolve 0.385 of the potassium, 0.352 of the sodium, and 1.0 of the ammonium compound at 16° (Baud, loc. cit.). Berzelius has described an insoluble double fluoride AlF3.3NH4F, and Petersen and Helmolt have prepared a soluble double fluoride of the same composition.

The double fluoride 3AlF3.5NaF occurs in nature as the tetragonal mineral chiolite; the compound AlF3.3NaF is found as the mineral cryolite. Cryolite (i.e. ice-stone) occurs at Ivigtut, an Esquimaux hamlet on the southwest coast of Greenland, in one huge deposit, contaminated with siderite, zincblende, galena, etc. Density, 2.96; hardness, 2.5; melting-point, c. 1000°. It forms monoclinic prisms (Holohedral; a:b:c = 0.9662:1:1.3882, β = 90°11') and at c. 565° is converted into a cubic modification. At 15°, 100 parts of water dissolve 0.034 of cryolite (Baud), but the latter is appreciably soluble in aqueous solutions of aluminium salts. It is decomposed by sulphuric acid. In the purification of crude cryolite, advantage is taken of the superior density of its impurities to effect a first purification, and other impurities are then removed by means of an electromagnet. Cryolite is used as a solvent for alumina in the process of manufacturing aluminium, in the preparation of opaque white glass, and of an enamel for steel. It was formerly used as raw material for the Danish alkali industry, and ha been powdered and used by the Esquimaux as snuff.

The following double salts may be prepared by dissolving the requisite hydroxides in hydrofluoric acid and evaporating the solution: -

AlF3.ZnF2.7H2O; 2AlF3.3CuF2.18H2O; AlF3.2CuF2.11H2O; AlF3.CuF2.HF.8H2O.

Aluminium subchloride has been said to be produced by heating aluminium trichloride with aluminium in a sealed tube, but the statement is in all probability erroneous.

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