Rich_Insane
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What is Happening Here?
Hey everyone. Pardon me for my newbish setup and experimentation as well as question.
So one day I got kicked off my computer and got bored and added a little ferric ammonium carbonate to some Ph UP which is either NaOH or NaCO3 (as
said on their home page). To verify what it was, I just tapped a little NaCO3 (pure) into a beaker with 25 mls of water, and dropped it into another
tube. What happens from both of these is that it turns brick red while reacting (ferric carbonate most likely, as it is insoluble). It also bubbles.
This may be NH3 coming off from the mixture. But this means that there is an extra H running around (ammonium). Where could this have gone? Is this
really FeHCO3? Perhaps this is just some CO2 coming off.
So then I tried something else. I used my pure NaCO3 soln. and I got some 4% acetic acid (vinegar  ), added the CH3COOH first, watched it (no rxn), then added the NaCO3. This immediately reacted with the NaCO3 to form
NaCH3COO. Then I observed the solution reacting with the ferric ammonium sulfate, doing the same thing. Later, I observed that there was a
light-orange/brown layer and a deep scarlet layer. I assume that some iron (III) acetate formed, as well as the carbonate (this was highly dilute
acid). After a day, this setup in a test tube is still bubbling. There is a light layer on top, a scarlet one on the bottom, and a whitish/brown
precipitate.
I did some research and found that FeCO3 is red colored -- but generally insoluble in water. I also see now that the tubes that got just NaCO3 and
water (with ferric ammonium sulfate) formed a more brown precipitate.
Are my predictions correct? Can someone quench my curiosity?
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woelen
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Please use correct formulae. NaCO3? FeCO3? I think you mean Na2CO3 and Fe2(CO3)3. If you really want to understand chemistry then the first thing you
need to do is get things like oxidation states and simple formulae right.
Now going to you observations. All bubbles you obtained are carbon dioxide bubbles from carbonate ion. Ammonia is so incredibly soluble in water that
you never will see bubbles of this gas escaping from an aqueous solution. All red/brown precipitates you made are ferric hydroxide. You cannot get
ferric carbonate from aqueous solutions.
Carbonate ion, when brought in contact with ferric ions, will lead to hydrolysis and you get carbon dioxide and ferric hydroxide. Maybe when pH is
sufficiently high, you also might obtain a little basic ferric carbonate, but I even doubt that, I think that you only obtain ferric hydroxide.
With vinegar present, you get acetate ion, and this reacts with ferric ions in another way. At medium high pH the acetate ion forms a deep red/brown
complex with ferric ion. So, your scarlet layer must have been the acetato complex.
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Rich_Insane
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Sorry, sorry. I was condensing down the formula. My bad.
Hm, that would explain it. But why did the Fe(OH)3 appear red at first and turn a very dull brown overnight?
I wish I had pictures, but the moment I added the Na2CO3, it started bubbling off CO2, and the ferric ammonium sulfate turned a very bright, blood,
red. Of course, it was not soluble. I see an alleged picture of crystalline Fe(OH)3 here
It is bright red, but now the precipitate in the tube is very beige or dull brown.
If the CO2 is coming off from the Na2CO3, is the Na bonding with the sulfate to form Na2SO4 which is in solution?
Thanks woelen! That cleared things up.
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hodges
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Are you sure you didn't have a ferrous, instead of ferric, salt? I know some ferrous compounds are a bright brick red, and then
when exposed to air they then oxidize to ferric which are more brown in color.
Hodges
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12AX7
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Ferrous iron precipitates as gray to black, quickly oxidizing to black then brown.
Particle size has a large role in color. Electrolytically precipitated cuprous oxide (Cu2O) is bright yellow at first, turning to a rusty orange over
time. Chemically precipitated Cu2O (e.g., reduction of Fehling's solution) is typically orange to dull brick red, because the particles are larger.
Large crystals of Cu2O are dark red. The longer a precipitate has time to form, the larger crystals it grows.
Tim
[Edited on 9-4-2009 by 12AX7]
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Rich_Insane
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It is ferric. The bottle which is from an old chemistry set used to make Prussian Blue, says "Ferric Ammonium Sulfate" or iron alum. It probably is
crystal size, maybe not even any chemical reason, but just how much surface area there is. Obviously this is the hydrate, and is a little clumpy.
So that's probably why. Curious, but I really don't want to throw the stuff away. It's the only experiment i'll be doing in a while sadly  .
And what of the white precipitate that forms on the bottom of the test tube I added vinegar to?
[Edited on 7-9-2009 by Rich_Insane]
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