Aluminium fluoride refers to inorganic compounds with the formula AlF₃·''x''H₂O. They are all colorless solids. Anhydrous AlF₃ is used in the production of aluminium metal. Several occur as minerals.

Occurrence and production

Aside from anhydrous AlF₃, several hydrates are known. With the formula AlF₃·''x''H₂O, these compounds include monohydrate (''x'' = 1), two polymorphs of the trihydrate (''x'' = 3), a hexahydrate (''x'' = 6), and a nonahydrate (''x'' = 9). The majority of aluminium fluoride is produced by treating alumina with hydrogen fluoride at 700 °C: Hexafluorosilicic acid may also be used make aluminum fluoride. :H₂SiF₆ + Al₂O₃ + 3 H₂O → 2 AlF₃ + SiO₂ + 4 H₂O Alternatively, it is manufactured by thermal decomposition of ammonium hexafluoroaluminate. For small scale laboratory preparations, AlF₃ can also be prepared by treating aluminium hydroxide or aluminium metal with hydrogen fluoride. Aluminium fluoride trihydrate is found in nature as the rare mineral rosenbergite. The anhydrous form appears as the relatively recently (as of 2020) recognized mineral óskarssonite. A related, exceedingly rare mineral, is zharchikhite, Al(OH)₂F.

Structure

According to X-ray crystallography, anhydrous AlF₃ adopts the rhenium trioxide motif, featuring distorted AlF₆ octahedron, octahedra. Each fluoride is connected to two Al centers. Because of its three-dimensional polymeric structure, AlF₃ has a high melting point. The other trihalides of aluminium in the solid state differ, Aluminium chloride, AlCl₃ has a layer structure and aluminium bromide, AlBr₃ and aluminium iodide, AlI₃, are molecular dimers. Also they have low melting points and evaporate readily to give dimers. In the gas phase aluminium fluoride exists as trigonal molecules of ''D_3h'' symmetry group, symmetry. The Al–F bond lengths of this gaseous molecule are 163 picometre, pm.

Applications

Aluminium fluoride is an important additive for the production of aluminium by electrolysis. Together with cryolite, it lowers the melting point to below 1000 °C and increases the conductivity of the electrolyte, solution. It is into this molten salt that aluminium oxide is dissolved and then electrolyzed to give bulk Al metal. Aluminum fluoride complexes are used to study the mechanistic aspects of phosphoryl transfer reactions in biology, which are of fundamental importance to cells, as phosphoric acid anhydrides such as Adenosine triphosphate, ATP and Guanosine triphosphate, GTP control most of the reactions involved in metabolism, growth and differentiation. The observation that aluminum fluoride can bind to and activate heterotrimeric G proteins has proven to be useful for the study of G protein activation in vivo, for the elucidation of three-dimensional structures of several GTPases, and for understanding the biochemical mechanism of GTP hydrolysis, including the role of GTPase-activating proteins.

Niche uses

Together with zirconium fluoride, aluminium fluoride is an ingredient for the production of fluoroaluminate glasses. It is also used to inhibit fermentation (biochemistry), fermentation. Like magnesium fluoride it is used as a low-index optical thin film, particularly when far UV transparency is required. Its deposition by physical vapor deposition, particularly by evaporation, is favorable.

Safety

The reported oral animal lethal dose (LD50, LD₅₀) of aluminum fluoride is 0.1 g/kg. Repeated or prolonged inhalation exposure may cause asthma, and may have effects on the bone and nervous system, resulting in bone alterations (Skeletal fluorosis, fluorosis), and nervous system impairment. Many of the Neurotoxicity, neurotoxic effects of fluoride are due to the formation of aluminum fluoride complexes, which mimic the chemical structure of a phosphate and influence the activity of ATP phosphohydrolase (H+-transporting), ATP phosphohydrolases and phospholipase D. Only micromolar concentrations of aluminum are needed to form aluminum fluoride. Human exposure to aluminum fluoride can occur in an industrial setting, such as emissions from aluminum reduction processes, or when a person ingests both a fluoride source (e.g., fluoride in drinking water or residue of fluoride-based pesticides) and an aluminum source; sources of human exposure to aluminum include drinking water, tea, food residues, infant formula, aluminum-containing antacids or medications, deodorants, cosmetics, and glassware. Fluoridation chemicals may also contain aluminum fluoride. Data on the potential neurotoxic effects of chronic exposure to the aluminum species existing in water are limited.Aluminum Compounds Review of Toxicological Literature Abridged Final Report
Prepared for National Institute of Environmental Health Sciences. ''NTP.gov Nomination Summary for Aluminum contaminants of drinking water (N20025).'' October 2001

References

External links

MSDS

ToxNet Profile

PubChem
{{Authority control Aluminium compounds Fluorides Metal halides