Sodium Hydroxide + Iron Oxide: Green Tint Explained?

[BWV]

Was spraying oven cleaner on a somewhat rusted grill and my son, who is taking AP chemistry, asked about the green color that appeared. googled sodium hydroxide + iron oxide and got this

FeO + 4NaOH = Na₄FeO₃+ 2H₂O

but can't find any info on Na₄FeO₃ - would this have a green tint, and is there an explanation?

 

[Vanadium 50]

It looks like that has Fe++, which tends to be greenish.

 

[Keith_McClary]

Also: Fe2O3 + 2 NaOH → 2 NaFeO2 + H2O

 

[Borek]

Fe(II) compounds in the presence of atmospheric oxygen are in most cases unstable and get oxidized to Fe(III) which is not green. My bet would go on copper or chromium - was any part of the grill plated?

 

[DrStupid]

Fe(II) compounds in the presence of atmospheric oxygen are in most cases unstable and get oxidized to Fe(III) which is not green.

But there are stable mixtures of both oxidation states (e.g. Fe₃O₄). There is even a green rust mineral: https://en.wikipedia.org/wiki/Fougèrite.

 

[Borek]

But there are stable mixtures of both oxidation states (e.g. Fe₃O₄).

Black.

There is even a green rust mineral: https://en.wikipedia.org/wiki/Fougèrite.

Somehow I don't expect it on the rusted grill

It is not like there are no green Fe(II) compounds, sure they exist. But - apart from some specific cases, like Mohr's salt - they are not stable in the presence of oxygen, and chances of finding them on a random, rusted iron surface are next to zero.

 

[Vanadium 50]

they are not stable in the presence of oxygen, and chances of finding them on a random, rusted iron surface are next to zero.

Immediately after gook has been sprayed on them? It's not like they have had years of exposure to oxygen. More like minutes.

 

[BWV]

Thanks for the responses - the material looks to be just plain flat-rolled steel. Wonder if there may be a reaction with the carbon?

There are other chemicals beside NaOH in Easy Off

https://www.wired.com/2008/05/st-easyoff/

 

[DrStupid]

Somehow I don't expect it on the rusted grill

It was just an example for a green stable Fe(II/III) compound.

they are not stable in the presence of oxygen

But they usually don't instantly turn into Fe(III) compounds when exposed to air.

 

[TeethWhitener]

How green are we talking here?

FeO + 4NaOH = Na₄FeO₃+ 2H₂O

Rust isn't FeO. It's a mix of almost exclusively iron(III) compounds.

Also: Fe2O3 + 2 NaOH → 2 NaFeO2 + H2O

NaOH is not really a reducing agent, and I seriously doubt this reaction is occurring in any appreciable amount. Also, the author of that site references no sources and looks like he might be 15 years old.

I think @Borek is on the right track if you have a vivid green material. However, if it's more of a dull greenish beige, it could easily be some sort of hydrated rust compound (still probably iron(III), though):
Picture of hydrated iron(III) oxide

 

[BWV]

How green are we talking here?Rust isn't FeO. It's a mix of almost exclusively iron(III) compounds.NaOH is not really a reducing agent, and I seriously doubt this reaction is occurring in any appreciable amount. Also, the author of that site references no sources and looks like he might be 15 years old.

I think @Borek is on the right track if you have a vivid green material. However, if it's more of a dull greenish beige, it could easily be some sort of hydrated rust compound (still probably iron(III), though):
Picture of hydrated iron(III) oxide

The shade was a little darker than a copper patina, looked almost like mold - dull greenish beige would be a good description

 

[Borek]

Immediately after gook has been sprayed on them? It's not like they have had years of exposure to oxygen. More like minutes.

Yes, immediately. In my experience iron that corrodes on air gets directly to red, I have never seen it getting through a green phase (nor heard about such a situation).

But they usually don't instantly turn into Fe(III) compounds when exposed to air.

Yes, dry salts (especially if stored in controlled conditions) are a bit more stable, but we are taking about a highly alkaline solution, not a dry solid. Pourbaix diagram for such systems says that around 0.4 V (which is more or less what I would expect here) and at high pH the only stable form is Fe₂O₃/Fe(OH)₃.

In general Fe(II) salts are unstable and can't be used as Fe(II) source in analytical chemistry. Only Mohr's salt is stable enough to be considered a primary substance, in all other salts enough Fe(II) is oxidized to Fe(III) to make them unreliable.

 

[Vanadium 50]

Yes, immediately. In my experience iron that corrodes on air

I don't think that's what it is. (Or rather, that's not what the son who is taking AP chemistry thinks it is, nor what the title says it is) The title chemical has Fe(2+) in it. That is, it's not corroded iron, but it's an iron reaction product.

 

[DrStupid]

In my experience iron that corrodes on air gets directly to red, I have never seen it getting through a green phase (nor heard about such a situation).

Yes rust is red, but no it is not only Fe(III).

Pourbaix diagram for such systems says that around 0.4 V (which is more or less what I would expect here) and at high pH the only stable form is Fe₂O₃/Fe(OH)₃.

That doesn't mean Fe(II) turns into Fe(III) immediately. It just means that the conversion starts spontaneously.

 

[chemisttree]

But if it were a nickel coated steel grate... and most are, it could easily be Ni+2.

 

[James Demers]

Nickel could be one explanation - EasyOff contains ethanolamines that could produce complexed nickel species, and those tend to be intensely blue or green.
The grill presumably featured quite a bit of carbon, as well as rust. (That's why the O.P. was using a char remover, not a rust remover.) Pyrolysis of Fe(III) oxides with carbon will reduce the iron - that's how we make steel - so it's not unreasonable to expect some Fe(II) in the mix.