Crystalline iron(III) nitrate nonahydrate, Fe(NO3)3.9H2O, has a very pale violet color. When added to water, the crystals dissolve to form a brown solution. Treatment of this brown solution with concentrated nitric acid yields a very pale violet solution while treatment with HCl yields a yellow solution.

I could identify that the brown solution is due to [Fe(OH)(H2O)5]2+, [Fe(OH)2(H2O)4]+, and on reaction with HNO3, we obtain [Fe(H2O)6]3+, but am unable to identify the yellow colored solution which is obtained on reaction with concentrated HCl.



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