Alums are the special type of double salts of certain compounds with the general formula
`M_(2)SO_(4)M_(2)(SO_(4))24H_(2)O`
Here M: monovalent cation link `Na^(+) ,K^(+),Rb^(+),NH_(4)^(+)`
M: trivalent cation like `Al^(3+),Fe^(3+)` or `Cr^(3+)`.
Potash alum : `K_(2)SO_(4),Al_(2)(SO _(4)) _(3) .24 H_(2)O`
Ferric alum: `(NH_(4))_(2)SO_(4).Fe_(2)(SO_(4))_(3).24H_(2)O`
Chrome alum : `K_(2)SO_(4)Cr_(2)(SO_(4))_(3).24 H_(2)O`.
