Exercise 55. Potassium Chrome Alum

As a preliminary, ferric sulfate is made by the oxidation of ferrous sulfate. Dissolve 100 g. of ferrous sulfate in 100 cc. of boiling water, to which has been added before heating 10 cc. of sulfuric acid. Add concentrated nitric acid portionwise to the hot solution, until a diluted sample gives a reddish-brown (not black) precipitate with ammonia. This will require about 25 cc. Boil the solution down to a viscous liquid to get rid of excess nitric acid, dilute to about 400 cc., and add the calculated weight of ammonium sulfate. The crystallization is conducted as in the former exercise, preferably under 20°. By the addition of potassium sulfate, the corresponding potassium iron alum may be secured. In this case, it is necessary to concentrate the solution until there is about four parts of water to one of the hydrated alum and cool to about zero to secure crystallization. Both of these alums are amethyst in color, the potassium salt being much less stable and having a rather low transition point. 

The formation of complex salts is particularly common when the acid radical is any one of the following CN, CNS-, S2O3, C2O4, N02 and when the metal asso- 

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