Qualitative Analysis of Ferric (Iron III) Ion
To the original solution and add little concentrated HNO3, and boil to oxidise Fe2+ to Fe3+ and then add excess of solid NH4Cl followed by excess of ammonium hydroxide. White or reddish brown precipitate is obtained.
Reddish brown precipitate indicates the presence of Fe3+ ions.
Addition of large excess of NH4OH may start dissolving group III precipitates. Hence, it should be avoided.
Confirmatory Test
Reddish brown precipitate of Fe(OH)3 on dissolving in dilute HCl, produces a yellow solution.
Fe(OH)3 + 3HCl → FeCl3 + 3H2O
On adding K4[Fe(CN)6] to yellow solution, a prussian blue precipitate is obtained.
4Fe3+ + 3K4[Fe(CN)6] → Fe4[Fe(CN)6]3 + 12K+
On adding KCNS to yellow solution, a red colouration is obtained.
Fe3+ + 3SCN- → [Fe(SCN)3] (Ferric thiocyanate, red colour)