The anhydrous chloride, which has some important uses, is made by heating aluminium turnings in a current of chlorine.
Alums. Aluminium sulphate can be prepared by the action of sulphuric acid upon aluminium hydroxide. It has the property of combining with the sulphates of the alkali metals to form compounds called alums. Thus, with potassium sulphate the reaction is expressed by the equation
K2SO4 + Al2(SO4)3 + 24H2O = 2(KAl(SO4)2·12H2O).
Under similar conditions ammonium sulphate yields ammonium alum:
(NH4)2SO4 + Al2(SO4)3 + 24H2O = 2(NH4Al(SO4)2·12H2O).
Other trivalent sulphates besides aluminium sulphate can form similar compounds with the alkali sulphates, and these compounds are also called alums, though they contain no aluminium. They all crystallize in octahedra and contain twelve molecules of water of crystallization. The alums most frequently prepared are the following:
| Potassium alum | KAl(SO4)2·12H2O. |
| Ammonium alum | NH4Al(SO4)2·12H2O. |
| Ammonium iron alum | NH4Fe(SO4)2·12H2O. |
| Potassium chrome alum | KCr(SO4)2·12H2O. |
An alum may therefore be regarded as a compound derived from two molecules of sulphuric acid, in which one hydrogen atom has been displaced by the univalent alkali atom, and the other three hydrogen atoms by an atom of one of the trivalent metals, such as aluminium, iron, or chromium.
Very large, well-formed crystals of an alum can be prepared by suspending a small crystal by a thread in a saturated solution of the alum, as shown in Fig. 84. The small crystal slowly grows and assumes a very perfect form.