A colored solution of a salt gives following reactions:

I. It gives white precipitate with sodium hydroxide which becomes brown on exposure to air. 

II. It gives white precipitate with ammonia solution which is soluble in ammonium salts.

 III. Its acidified solution containing a few drops of dilute ${\mathrm{AgNO}}_3$ on boiling with ammonium peroxodisulphate gives a reddish violet solution. The cation of the salt is: