Alum
Related compounds

Many trivalent metals are capable of forming alums. The general form of an alum is XY (SO) · n H O, where X is an alkali metal or ammonium, Y is a trivalent metal, and n often is 12. The most important example is chrome alum, KCr(SO) ·12 H O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.

In general, alums are formed more easily when the alkali metal atom is larger. This rule was first stated by Locke in 1902, who found that if a trivalent metal does not form a caesium alum, it neither will form an alum with any other alkali metal or with ammonium.
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