Iron alum (5%)
10 g ammonium ferric sulfate (FeNH4[SO4]2•12H2O)
200 mL H2O
Iron alum (ammonium ferric sulfate) should consist of violet crystals that dissolve completely in H2O to make a light orange-brown solution. If the crystals are large, it may be necessary to leave the mixture overnight on a magnetic stirrer. There are many bad batches of this compound, which contain insoluble white or light brown material. Even with clean iron alum, a precipitate forms after several months, and the solution should then be discarded.
