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I have put some old nails and screws in the 30% sulfuric acid to prepare an electrolyte for plating. After some bubbling and horribly irritating smell (probably $\ce{SO2}$), the reaction slowed down. Overnight, beautiful green/blue crystals formed:

iron in sulfuric acid

I just love them. After another day, the reaction almost stopped. I suppose the acid was all bound to iron. To continue the process, I poured some concentrated sulfuric acid into the solution. That was a big mistake - the crystals reacted with concentrated acid and formed ugly white dust:

iron in concentrated sulfuric acid

What did I create? What are the crystals and what's that dust? How to reverse the reaction? Does the dust dissolve to be electrolysed?

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    $\begingroup$ These are some lovely pictures. $\endgroup$
    – LordStryker
    Commented Oct 1, 2014 at 19:50
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    $\begingroup$ I had to support my camera with phone flash-light to take them :) $\endgroup$
    – Tomáš Zato
    Commented Oct 1, 2014 at 20:09
  • $\begingroup$ how much time did it take after adding concentrated H2SO4 again to form that dust $\endgroup$
    – dsinghvi
    Commented Oct 2, 2014 at 17:43
  • $\begingroup$ Less than a minute. $\endgroup$
    – Tomáš Zato
    Commented Oct 3, 2014 at 7:08

3 Answers 3

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I realize this question is over two years old, but I have to add the correct answer here: the iron (II) sulfate you had at the beginning was in the heptahydrate form. SEVEN molecules of water for every iron (II) sulfate ion pair. Now, what do you think would happen to that crystal structure when you add one of the most powerful dehydrating agents in existence? That's right, it sucks all the water out of your crystals, and you're left with a powdery, noncrystalline form. You could have just added less concentrated acid, that was diluted enough to not be such a strong dehydrating agent.

Incidentally, iron is a surprisingly strong reducer of sulfates. I did this reaction recently and noticed gold flecks floating around in the solution. I was shocked to see gold, since I obviously never added any, until I realized that the iron had reduced the sulfate all the way down to sulfide, and what I was seeing was iron pyrite -- fool's gold. Adding more acid eliminated that almost immediately.

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  • $\begingroup$ I seriously doubt, that concentrated sulfuric acid has any dehydrating effect after diluting it with an aqueous solution, as the OP stated. $\endgroup$
    – Gerhard
    Commented Jun 4, 2016 at 18:20
  • $\begingroup$ OP claimed to have "poured" "concentrated sulfuric acid" into the solution. Remember, crystals need a 7:1 ratio of H2O to Fe (II) SO4 to form. There's certainly going to be some Fe (II) SO4 in solution still; possibly even an excess of what's necessary to form the heptahydrate salt. Adding concentrated sulfuric acid would have pulled even more water out of the solution, and H2SO4 being heavier than water, would have sunk to the bottom and dehydrated the heptahydrate crystals preferentially. $\endgroup$
    – virnovus
    Commented Jun 6, 2016 at 20:27
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A quick look at the Wiki suggests that you first formed iron(II) sulfate:

$\ce{Fe + H2SO4 → FeSO4 + H2}$

which is blue-green. It is then possible that you obtained iron(III) sulfate, which is pale. But you need to oxidize the iron(II) to iron(III), which usually requires oxygen, and sulfuric acid is not an oxidizing acid. Mere exposure to oxygen works:

$\ce{12 FeSO4 + 3 O2 → 4 Fe2(SO4)3 + 2 Fe2O3}$

but I would assume not that quickly. Still, this looks like the best explanation, and perhaps some impurities managed to produce the oxidation. Just a guess.

As to how to get back the iron(II), you would need to reduce the iron(III), and given that the oxidation direction is basically spontaneous in air, this might take some work. So why not just start again with more iron and dilute acid?

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  • $\begingroup$ Yeah, I'll probably diluthe the solution, flush it and start over. But it took a day to form, so I'm a little sad. $\endgroup$
    – Tomáš Zato
    Commented Oct 2, 2014 at 6:10
  • $\begingroup$ Concentrated sulfuric acid is an oxidising acid but if he was just using 30% then that's not concentrated- 98%+ is oxidising. $\endgroup$
    – user2617804
    Commented Oct 13, 2014 at 22:40
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I think you merely grew some Iron Sulfate crystals and then dissolved them by adding more solvent.

Depending on how much sulfuric acid you added the second time you could add a proportional amount of the same mixture of nails and screws to reform the crystals except larger. Alternately you could evaporate the acid you added by putting it on a hot plate or something. or as trb said you could just start over.

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