I have been doing some synthesize, Prussian blue, in particular. I have done it previously by straight precipitation of solution FeCl3 and adding dropwise solution of K4Fe(CN)6, resulting into Fe4[Fe(CN)6]3, beautiful blue pigment appearing. But this time, I have decided to add some ammonia water - NH4OH, to FeCl3, before adding K4Fe(CN)6. From light yellow 0.02 molar FeCl3 it became very dark red-brown and then after adding dropwise the K4Fe(CN)6 solution, it changed into grey, and only the very top part of the solution became expected blue. Moreover, as I detched top blue part, the rest top part of the solution turned into blue again, like it oxidized.
So, the question is, what could the grey solution be?
Here are some pics:
Dear Olzhas Kaupbay ,
Both ferrocyanide and ferricyanide complexes are unstable at high pH, although the ferricyanide (Fe(III)) is relatively more stable. When you added ammonia you formed ferric hydroxide, which in a basic solution will not form Prussian blue.
The gray color you see is most likely a suspension of particles of mixed Fe(II) and Fe(III) hydroxide, precursor to (black) magnetite. You can try separation (centrifugation, filtration) and investigate further if you wish. But whatever you do, don't forget that your solution now contains free cyanide and should be handled and disposed of carefully.
As for the top of the solution turning blue: Yes it may be oxygen effect, generating some fresh Fe(III) out of the ferrocyanide-derived Fe(II). There may also be a pH gradient effect due to evaporation of the ammonia. The fresh Fe(III) then combines with cyanide and some unprecipitated Fe(II) to form the blue precipitate.
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