Alums

 Alums are compounds with the formula AB(SO4)2.12H2O with A=+1 alkali/ammonium and B=+3 hexaaquo ion. The larger A is, or the smaller B is, the better stability crystal has.

Most members are not that stable as Tutton's salts and sometimes only solid solutions and cesium alums are stable. Cubic system, octahedral crystals.

They are honest in colors, as aqueous solutions of such a highly charged ion usually undergoes hydrolysis.

Known examples:

Al: Colorless. Best known, with K and NH4 common and stable. Na also possible but very unstable.

Sc: Colorless. Unstable.

Ti: Purple? Highly reducing both in solid and solution.

V: Blue or green or what? Reducing. My procedure gave nothing but alum with solution. Better make pure compound.

Cr: Dark purple, very famous. K mixing with KAl is the most common way with ratio variable.

Mn: Red. Extremely unstable as any form except a dilute solution in NH4Al in which it is flesh-like orange. See my blog for more details.

Fe: Purple?

Co: Greyish-blue. Extremely oxidizing but its solution in acid can be prepared by electrolysis.

Ga: ?

Mo: Yellow? Reducing and unstable.

Ru: Yellow?

Rh: Yellow?

In: ?

Ir: Yellow?