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Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al 2 (SO 4) 3 ·16H 2 O and octadecahydrate Al 2 (SO 4) 3 ·18H 2 O are the most common. The heptadecahydrate, whose formula can be written as [Al(H 2 O) 6] 2 (SO 4) 3 ·5H 2 O, occurs naturally as the ...
2 (SO 4) 4 ·22 H 2 O, MgAl 2 (SO 4) 4 ·22 H 2 O, NaAl(SO 4) 2 ·6 H 2 O, MgSO 4 ·7H 2 O and Al 2 (SO 4) 3 ·17 H 2 O. [3] [4] The Ancient Greek Herodotus mentions Egyptian alum as a valuable commodity in The Histories. [5] The production of potassium alum from alunite is archaeologically attested on the island Lesbos. [6]
The only stable chalcogenides under normal conditions are aluminium sulfide (Al 2 S 3), selenide (Al 2 Se 3), and telluride (Al 2 Te 3). All three are prepared by direct reaction of their elements at about 1,000 °C (1,832 °F) and quickly hydrolyse completely in water to yield aluminium hydroxide and the respective hydrogen chalcogenide .
The following chart shows the solubility of various ionic compounds in water at 1 atm pressure and room temperature (approx. 25 °C, 298.15 K). "Soluble" means the ionic compound doesn't precipitate, while "slightly soluble" and "insoluble" mean that a solid will precipitate; "slightly soluble" compounds like calcium sulfate may require heat to precipitate.
Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO 4) 2 ·12H 2 O (sometimes written Na 2 SO 4 ·Al 2 (SO 4) 3 ·24H 2 O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking powder and as a food additive. Its official mineral name is alum-Na (IMA symbol: Aum-Na [3]).
Ammonium iron(III) sulfate, NH 4 Fe(SO 4) 2 ·12 H 2 O, or NH 4 [Fe(H 2 O) 6](SO 4) 2 ·6 H 2 O, also known as ferric ammonium sulfate (FAS) or iron alum, is a double salt in the class of alums, which consists of compounds with the general formula AB(SO 4) 2 · 12 H 2 O. [2] It has the appearance of weakly violet, octahedrical crystals.
Other related compounds, sometimes called sodium aluminate, prepared by reaction of Na 2 O and Al 2 O 3 are Na 5 AlO 4 which contains discrete AlO 4 5− anions, Na 7 Al 3 O 8 and Na 17 Al 5 O 16 which contain complex polymeric anions, and NaAl 11 O 17, once mistakenly believed to be β-alumina, a phase of aluminium oxide.
Ammonium aluminium sulfate, also known as ammonium alum or just alum (though there are many different substances also called "alum"), is a white crystalline double sulfate usually encountered as the dodecahydrate, formula (NH 4)Al(SO 4) 2 ·12H 2 O. It is used in small amounts in a variety of niche applications.