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Author: Subject: Iron (III) oxide is not soluable in acids?
hodges
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[*] posted on 6-10-2019 at 17:56
Iron (III) oxide is not soluable in acids?


I had some iron (III) hydroxide that I was going to make iron sulfate from. I inadvertently let it dry. Presumably, it dehydrated to iron oxide - it looked like rust.

I tried adding fairly strong (3M) H2SO4 to it, but it does not dissolve - it just stays suspended like it would in water. I even tried heating the solution to boiling, with no effect.

So I take it that once iron hydroxides dehydrate the the oxide, they are no longer reactive with acids. I was surprised at this.
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[*] posted on 6-10-2019 at 18:15


A lot of metal oxides are really, really unreactive towards acids. You might find hydrochloric works faster.



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Aqua-regia
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[*] posted on 7-10-2019 at 02:06


The solubility of Fe2O3 is depending on ignition/decomposition teperature of the hydroxide.
The more you ignited, the more is insoluble.


[Edited on 7-10-2019 by Aqua-regia]
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[*] posted on 7-10-2019 at 06:24


many metal oxides becomes very inert, when they are heated for prolonged time at high temperatures. This is because they are converted to compact crystalline form, instead of the more fluffy amorphous form, which is obtained on formation of many oxides (or hydroxides). The following are known to be really inert:
- Al2O3
- Fe2O3
- Fe3O4
- Cr2O3
- TiO2
- SnO2
- Co3O4
- CeO2

I personally had similar issues with Nd2O3, Pr2O3, and Er2O3. Useless, totally unreactive powders. Even sitting under conc. H2SO4 for weeks did not dissolve any of it. Nor boiling in conc. HCl, conc and dilute H2SO4, or conc. HNO3 does dissolve any of this. However, with these you can have easy to dissolve oxides as well. I received a little Er2O3 from our member Bezaleel and had Nd2O3 from another seller, and these dissolved in acid fairly easily (required some mild heating).

Moderately reactive, requiring mild heating of the acid, are:
- Ni2O3
- V2O4
- V2O5 (dissolves well in alkalies)
- MnO2
- SnO
- Tl2O3 (does not dissolve in dilute to moderately conc. HNO3, but does dissolve fairly easily in moderately conc. HCl).

Easy to dissolve in acids are:
- CuO
- ZnO
- CdO
- CaO




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teodor
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[*] posted on 7-10-2019 at 08:00


Calcined SnO2 is not so inert, it easily reacts with molted sulfur if Na2CO3 is present giving water soluble Na2SnS3. I was surprised with that. The temperature of this reaction is in the same range as with boiling conentrated H2SO4.


[Edited on 7-10-2019 by teodor]
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12thealchemist
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[*] posted on 7-10-2019 at 09:17


Quote: Originally posted by hodges  
I had some iron (III) hydroxide that I was going to make iron sulfate from.


Are you after iron(II) or iron(III) sulphate? If the former, then a reducing agent like oxalic acid will aid the dissolution of the oxide immensely. A sub-stoichiometric amount would prevent contamination of the product.

[Edited on 7-10-2019 by 12thealchemist]




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fusso
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[*] posted on 7-10-2019 at 09:56


But oxalic acid will coordinate to Fe(III) to form ferrioxalate, no redox rxn occur...



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[*] posted on 7-10-2019 at 23:14


Quote: Originally posted by teodor  
Calcined SnO2 is not so inert, it easily reacts with molted sulfur if Na2CO3 is present giving water soluble Na2SnS3. I was surprised with that. The temperature of this reaction is in the same range as with boiling conentrated H2SO4.


[Edited on 7-10-2019 by teodor]

This is much harsher than what I refer to in my post. I refer to aqueous solutions with temperatures of at most around 100 C. What you describe is at 250 C or even hotter. I can imagine that the strongly alkaline melt also (slowly) attacks glass, so that it cannot be done in glass without etching it.




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[*] posted on 8-10-2019 at 01:16


I agree, this is a different kind of reactions. But you mentioned boiling in concentrated H2SO4. Well, you definitely can use the same setup for H2SO4 boiling as boiling in any other acid.
I periodically try to do something with such inert oxides. I think I was lucky once with dissolving a bit of FeCr2O4 (chromite) in hot H2SO4 in presence of MnO2 but not finished this experiment yet.
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