Sciencemadness Discussion Board

Peculiar Iron Compound

ShadowWarrior4444 - 11-7-2008 at 13:22

I was busily making some sodium metal via electrolysis of NaOH when I decided to attempt using a steel (non-stainless) anode. The result was the formation of a green mineral-like compound that appears to be deliquescent. Discerning the identity of the compound is likely quite simple, however since it is a pretty compound, I thought I'd post the pictures of it here.





12AX7 - 11-7-2008 at 16:50

Ferrite?

ShadowWarrior4444 - 11-7-2008 at 17:34

Quote:
Originally posted by 12AX7
Ferrite?


As in transformer-core ferrite? Those are ostensibly iron-oxide ceramaics, usually not green or soluble.

I suspect some sort of sodium-iron compound, perhaps somewhat analogous to Prussian blue. A sodium-iron mineral of sorts? It dissolves into a green solution, though. The green color indicates Fe2+, the rest is perhaps some mixture of Na, O, and OH. Due to its hygroscopic nature, I suspect OH groups to be present.

I should also note that the black on the bottom of the green is most likely an accumulation of carbon, or other various metals that were in the iron alloy. (MnO2?)

An update:

After a short while in a petri dish--




And a tryst on some cast iron:


Interestingly, any of the compound left on the cast-iron seems to turn from green to yellow/off-white, perhaps due to some catalytic oxidation via Fe2O3.

It also seems that the substance (revised term) is only partially deliquescent; the green (iron component) seems to dissolve into solution, leaving behind a mineral-like white substance. Unless this is simply an illusion created by the atmospheric oxidation of the outer layers.

All pieces are also completely non-magnetic, as tested with N48 magnets-of-doom.

[Edited on 7-11-2008 by ShadowWarrior4444]

JohnWW - 11-7-2008 at 21:33

That lime-green color is characteristic of the hydrated Fe(II) cation, Fe(H2O)6++, which however is free from hydrolysis only in acid solution. It is unclear what the associated anion could be, if that is what it is. Because of the strongly alkaline environment from which it came, molten NaOH, it is more likely to be instead a sodium ferrite(II), possibly as [Fe(OH)4]-- or [Fe(OH)6]4-. Sucjh a species would be very liable to oxidation to Fe(III) by atmospheric oxygen.

ShadowWarrior4444 - 11-7-2008 at 22:06

Quote:
Originally posted by JohnWW
That lime-green color is characteristic of the hydrated Fe(II) cation, Fe(H2O)6++, which however is free from hydrolysis only in acid solution. It is unclear what the associated anion could be, if that is what it is. Because of the strongly alkaline environment from which it came, molten NaOH, it is more likely to be instead a sodium ferrite(II), possibly as [Fe(OH)4]-- or [Fe(OH)6]4-. Sucjh a species would be very liable to oxidation to Fe(III) by atmospheric oxygen.


Incidentally, what is the most commonly accepted deffinition of a ferrite--"An unstable compound of a strong base and ferric oxide which exists in alkaline solution, such as NaFeO2," an iron-containing material with a body centered cubic structure, or "a class of chemical compounds with the formula AB2O4, where A and B represent various metal cations, usually including iron."

I should mention that during the electrolysis, the steel anode did bubble vigorously. I also found that at times a closed circuit was formed, requiring the anode to be raised slightly in order to continue the electrolysis. In addition, when molten this substance was tan/brown, it was only upon cooling that it turned green.