Chemical formula: NH4Al(SO4)2.12H2O
Synonyms: ammonium aluminium sulphate dodecahydrate, double salt of ammonium sulphate and aluminium sulphate, aluminium sulphate (misnomer).
Amount: 100 g
Appearance and odour: mostly medium size white crystals, without appreciable odour
Packaging: professional wide necked, white HDPE bottle with blue conical seal cap and tamper evidence ring. Labelled. Bottle and cap are UN approved and reusable.
Description and uses: a double salt of aluminium sulphate and ammonium sulphate, NOT to be confused with its potassium homologue - alum - or with ferric ammonium alum - both also on sale from us. Used as a dye mordant and water treatment agent (flocculant) it is a primary source of soluble aluminium salt. Poorly soluble in iced water but highly soluble in hot water it is an ideal substance for crystal growing. It forms regular octahedral crystals with flattened corners and can be grown into very larger crystals with some patience. Mixed crystals with chromium alum or ferric alum can also be grown.
It belongs to the class of MM’(SO4)2.12H2O alum double salts, here with M = NH4 and M’ = Al.
Add ammonia solution to an alum solution and hydrated aluminium hydroxide precipitates: the basis of most other aluminium salts. Add strong alkali (NaOH or KOH) to the precipitate and it dissolves to aluminate, a phenomenon known as amphoterism.
Pyrolysis of this compound leaves pure alumina (Al2O3) behind.
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