Iron is a group 3 cation, and we may find it in solution as Fe3+.
For both the tests we will see, a preliminary operation is needed. The purpose is to move Fe3+ in solution (notice that iron, if present, had been previously precipitated as Fe(OH)3 ). To achieve this result we just use diluted hydrochloric acid (2N).
The hydroxide is dissolved according to the following acid-base neutralization:
Fe(OH)3 + 3H+ Fe3+ + 3H2O
In fact, the Fe 3+ ion in solution is quite stable and exists as an exa-coordinated complex with 6 water molecules.
Fe+3 + 6H2O Fe(H2O)63+
We should then get a colorless solution, just consider the possibility of a partial hydrolysis that might lead to a slightly colored solution (from yellow to red rust, according to the hydrolysis undergone). Notice that we are talking about a clear but colorful solution, no precipitates.
Fe(H2O)63+ [Fe(H2O)5(OH)]2+ + H+
- Confirmation with potassium ferrocyanide K4[Fe(CN)6]
Just add a few drops of potassium ferrocyanide to the solution suspected to contain Fe3+ . If present, ferric ferrocyanide is easily identified considering it gives a bright blue colored precipitate.
- Confirmation with potassium thiocyanate KCNS
This interesting test is based on the creation of a two-phase system by adding ether to our acqueous solution. We subsequently add a couple of spatula of potassium thiocyanate (KCNS). A complex forms and it has more affinity for the organic layer. Here is the reaction:
[Fe(H2O)5(OH)]2+ + NCS- [Fe(H2O)5NCS] 2+ + OH -
The compound on the right is called ferric thiocyanate, and it is a red precipitate.
This complex is not that strong and then it is recommendable to use a large excess of thiocyanate (KCNS) to shift the balance of complex formation to the right (don't forget the basis → le Chatelier principle). Just be carefull, because if we exceed to much we will get a new and different complex, red colored and soluble (that means it is not a precipitate) in the aqueous phase, specifically (K3Fe(CN)6).