Nerro - 22-6-2006 at 06:19
I did a bit of screwing around with FeSO4 yesterday, I made a solution of some ascorbic acid in hot water and added Na2CO3 to it untill it stopped
fizzing. I then added a small pile of FeSO4 (no measurements whatsoever) to the solution hoping to precipitate Fe powder. What happened in stead was a
lot of fizzing and bubbling which caused me to spill some of the solution on the carpet. (the stains were a bitch untill I discovered that HCl took
care of it pretty quickly. I wiped with some Na2CO3 soln. afterwards to neutralize the worst of it.) The solution was white below the surface and deep
purple on top. When I threw some on the ground it turned black within about a minute.
Does anyone have any idea what it was that I made?
Complexing iron salts
woelen - 22-6-2006 at 09:40
Ascorbate is very good at making complexes with transition metals, and often these complexes show internal oxidation/reduction. I noticed that
ascorbate makes complexes with many first row transition metals.
We already discussed the interesting properties of the copper/ascorbate complex and the formation of metallic copper. Iron will form similar
complexes, but I would not have expected any metallic iron to be formed in aqueous solution, iron simply is too reactive and even in slightly acidic
conditions the iron would be oxidized by the acid, forming iron (II) and hydrogen.
Complexing iron salts
chemoleo - 22-6-2006 at 15:59
I tried the same thing, with ascorbate, and FeSO4, both AS reagents. Adding ascorbic acid to FeSO4 aq doesn't do anything, only when I added NaOH,
then the NaOH droplets were immediatedly coated with a green translucent substance...first I thought these were green crystals, but no, they are just
coated green NaOH bubbles (it was 8 M, so quite concentrated). Upon mixing heavily, it turned violet as described, followed by a black fine
precipitate later, which remains black upon solution. No gas evolved by the way, none at all.
Hard to tell really what happened, but clearly it is the tautomeric ascorbate salt that does the job. Intersting also the violet colour, which
prettty much immediately disappears. I am wondering why though, after all even the ascorbate should be reducing (right?), so it can't be air
oxidation. Interesting.
unionised - 22-6-2006 at 23:48
Ascorbic acid is fairly weak so sodium ascorbate is quite alkaline. That means that the products of reaction of the acid and Na2CO3 are NaHCO3 and Na
ascorbate. The pH won't be low enough to decompose the bicarbonate (unless you boil it to get rid of the CO2)
When you add Fe++ you get FeCO3 (and/ or basic salts and hydroxidxes) and CO2 hence the mess.
My guess about the colour is that it's from traces of Fe+++. Quite a lot of Fe+++ complexes are violet.
That Fe+++ would be reduced by ascorbate so the colour would fade in the absence of air.
While the oxidation potentials indicate that Fe can't be precipitated in aqueous acid, it seems that the stuff hasn't read the books.
If you add Al foil to FeCl3 solution you get a fairly violent reaction and among other things some blackish powder. This is magnetic so I guess it's
iron. It might be the oxide but you can also get the same behaviour from Co++ and Ni++ in HCl. I don't think that NiO is magnetic and, anyway, it's
the wrong colour.