blogfast25
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Mini double mystery with Ferric Ammonium Alum...
Having ran out of NH4Fe(SO4)2.12H2O I decided to make about a 1/2 mol myself via:
FeSO4 + 1/2 H2SO4 + 1/3 HNO3 --> 1/2 Fe2(SO4)3 + 1/3 NO + 2/3 H2O
and
1/2 Fe2(SO4)3 + 1/2 (NH4)2SO4 ---> NH4Fe(SO4)2
So I set out mixing the FeSO4.7H2O with H2SO4 (94.6 %) (stochio amounts) and some water and heated it to dissolve the FeSO4 and then added the HNO3
carefully and gradually. Some NO2 later the solution had gone a dark brown, typical of concentrated Fe3+, and I added the ammonium sulphate, simmered
for a bit and let it cool down, adding a few seed crystals. Sure enough, in a few hours some crystals of the right shape and colour formed and
fridgeing overnight I got a yield of about 87%, not too bad for a first attempt. Nearly all of the container was crystallites, quite hard to break up.
For some reason I then tried to make another 1/4 mol with a slightly different procedure. The FeSO4 and H2SO4 were combined with some water, then
heated to dissolve, then cooled down to RT. The nitric was then added to the cool mix. It darkened immediately but on heating and boiling no NO2
evolved and after simmering and cooling no crystals were obtained. Why not?
Now FAA appears notorious for crystallising slowly but I wanted to recrystallise the first crop, so I decanted off the 50 ml of supernatant liquid and
replaced it with clean water, then remelted the FAA (it has a very low MP, less than 100) into the water and its own crystal water. Allowed to cool
with seed crystal, no crystals formed, not on fridgeing either.
Sensing the solution was perhaps too concentrated and perhaps too clean, I then added another 50 ml of clean water and bingo! about the same
nice crop of crystals appeared in a few hours.
Another test with a 1/4 mol and it seemed I had reduced the volume of the solution a bit too much and no crystals formed but on adding 50 ml of water
they did within hours.
The crystal are a nice light amethyst, not distinguishable from the commercial product...
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woelen
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This is a nice way of making ferric ammonium alum. The deep brown color, however, is not due to dissolved Fe(3+). Hydrated Fe(3+) actually is nearly
colorless. The well-known brown color is due to hydrolysis and the well-known yellow color is due to coordination of chloride. Apparently none of
these occur in your solution.
The deep brown color you observe is due to formation of another complex of iron(II) and NO. These react with each other, forming a NO(+) ligand, and
the iron goes to oxidation state +1.
http://woelen.homescience.net/science/chem/solutions/fe.html
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blogfast25
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The nitrosyl complex was my first thought too at the time. My (very old) Holleman describes it as (Fe.NO)SO4 but doesn't mention any changes in
oxidation state of the Fe or NO. He considers them homologues of the carbonyl compounds.
Also, the Fe.NO compound (whatever its actual structure) is described as very labile and should, on heating dissociate easily. If that didn't happen,
oxidation of Fe +II to Fe +III with nitric acid couldn't happen, while clearly it does...
Yes, hydrolysis, that is what I meant of course. Even quite dilute solutions of FAA tend to be brownish and more brown at higher temperatures...
When I poured the nitric into a bit of FeSO4 solution without stirring I could se dark swirls of what I believe to be the nitrosyl compounds. But on
heating the solution turns yellow, if dilute enough...
I'll get some K3Fe(CN)6, a good indicator of Fe2+...
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