Aluminates

Aluminates are salts of the aluminum acid HAlO ₂ · H ₂ O, which is identical to aluminum hydroxide (which is amphoteric ), in which aluminum forms a complex anion [Al (OH) ₄ ] ^- with hydroxide ions as ligands , and salts in which the anion is present as a condensate of the aluminate ion.

The general composition of such compounds is M ^I [Al (OH) ₄ ] with M as the monovalent cation. Completely condensed (anhydrous) compounds have the general composition M ^I AlO ₂ with AlO ₂ ^- as the anion.

There are numerous naturally occurring mineral compounds with divalent cations , such as B. Magnesium aluminate (MgAl ₂ O ₄ , see Spinel ), zinc aluminate (ZnAl ₂ O ₄ , see Gahnite ), iron aluminate (FeAl ₂ O ₄ , see Hercynite ) and manganese aluminate (MnAl ₂ O ₄ , see Galaxite ). In all of these compounds, aluminum is in the oxidation state (III).

Examples

A well-known aluminate is sodium aluminate , which is obtained by dissolving aluminum hydroxide in sodium hydroxide solution, for. B. The Bayer process for the production of aluminum from bauxite produces:

${\displaystyle \mathrm {Al(OH)_{3}+NaOH\longrightarrow NaAl(OH)_{4}} }$

In the ion notation :

${\displaystyle \mathrm {Al(OH)_{3}+OH^{-}\longrightarrow [Al(OH)_{4}]^{-}} }$

Tetrahydroxidoaluminate ion

The tetrahydroxoaluminate ion (for example in: sodium tetrahydroxoaluminate) tends to condense with the formation of dialuminate ions [(HO) ₃ Al − O − Al (OH) ₃ ] ²⁻ and higher polymers. The polymerization is similar to that of orthosilicic acid . The end products of such a condensation are obtained by fusing aluminum oxide and metal oxides.

${\displaystyle \mathrm {Al_{2}O_{3}+Na_{2}O\longrightarrow 2\ NaAlO_{2}} }$

Further aluminates are the calcium aluminates , which are formed either by melting aluminum oxides with lime or by reacting sodium aluminate with milk of lime :

${\displaystyle \mathrm {2\ NaAl(OH)_{4}+3\ Ca(OH)_{2}\longrightarrow Ca_{3}[Al(OH)_{4}]_{2}(OH)_{4}\ +2\ NaOH} }$

Calcium aluminates can form salts with anions, for example different calcium aluminate sulfates with sulfates , depending on the reaction conditions calcium aluminate monosulfate or calcium aluminate trisulfate, which is also known as ettringite mineral. All calcium aluminates play an important role in cement chemistry .

Chromium aluminate is obtained by heating a mixture of clay, calcium fluoride and ammonium dichromate and used as a ceramic paint.

Barium aluminate is obtained from bauxite, barite and coal and is used to clean industrial waste water and as a scale solvent .

Lead aluminate is produced by heating a mixture of black lead and alumina, which was used as a white, solid pigment and used to make bricks and refractory linings.

literature

• Aluminates . In: Meyers Konversations-Lexikon . 4th edition. Volume 1, Verlag des Bibliographisches Institut, Leipzig / Vienna 1885–1892, p. 429.
• Aluminates . In: Brockhaus Konversations-Lexikon 1894-1896, Volume 1, p. 482.

Individual evidence

1. ↑ Erich Ammedick, Heinz Kadner: Textbook of Chemistry . 2nd Edition. 1974, p. 329, VLN 152-915 / 59/75.
2. ↑ Federal Customs Administration: Chapter 28: Inorganic Chemical Products; inorganic or organic compounds of precious metals, radioactive elements, rare earth metals or isotopes , accessed on June 17, 2017.