Iron confirmatory tests
For both the tests a preliminary operation is needed. The intention is to bring Fe3+ in solution; in order to achieve this results we solubilize ferric hydroxide with dilute hydrochloric acid (2N).
The hydroxide is easily dissolved:
Fe(OH)3 + 3H+ Fe3+ + 3H2O
In fact, the Fe 3+ ion in solution coordinates around 6 water molecules and it is pretty stable.
Fe+3 + 6H2O Fe(H2O)63+
The solution should be now colorless, but if the coordinate complex undergoes partial hydrolysis it is possible to obtain a slightly colored solution (from yellow to red rust). Obviously we are talking of a clear but colorful solution, there should be no precipitate.
Fe(H2O)63+ [Fe(H2O)5(OH)]2+ + H+
Identification with potassium ferrocyanide K4[Fe(CN)6]
Fe+3 + K4[Fe(CN)6] KFe[Fe(CN)6]
We add a few drops of reagent to the solution containing Fe3+ . If present, Ferric ferrocyanide gives a blue precipitate.
Identification with potassium thiocyanate KCNS
We create a two-phase system by adding ether to our solution. Then put in two spatula of potassium thiocyanate. A complex forms and it goes in the organic layer.
[Fe(H2O)5(OH)]2+ + NCS- [Fe(H2O)5NCS] 2+ + OH -
It is ferric thiocyanate, red precipitate.
The complex is not very strong and then is advisable to operate using a large excess of reagent (KCNS) to shift the balance of complex formation to the right. But if we use a too large excess, what we are going to get it is a new complex, red colored and soluble in the aqueous phase (K3Fe(CN)6).