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I have bought 50 kg $\ce{FeSO4}$, but it was stored on air, and it has turned to brown. Is there a cheap method to recover some $\ce{FeSO4}$?

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  • $\begingroup$ You speak of Iron(III) sulfate in your title, yet your question's body only contains Iron(II) sulfate. Care to clarify? $\endgroup$ – M.A.R. Sep 22 '15 at 5:29
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    $\begingroup$ Quite possibly, most of your $\ce{FeSO4}$ is still good. Whether or not some admixture of $\ce{Fe}^{3+}$ will do any harm, depends on your application. $\endgroup$ – Ivan Neretin Sep 22 '15 at 6:03
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    $\begingroup$ @inɒzɘmɒЯ.A.M I was confused by the title at first but the question itself is clear enough, some of the iron(II) was oxidised by air. $\endgroup$ – orthocresol Sep 22 '15 at 10:06
  • $\begingroup$ FeSO4×7 H2O is green/blue, but when I opened the bag I saw a brown/yellow stuff, with some green parts. So I think it has oxidated. It was not good idea to store outside. I've read that when FeSO4 oxidates Fe2(SO4)3 + Fe2O3 is the product. I need ironsulphate for gardening (lowering the pH in soil). Do you think it is still usable? Thanks for reply. $\endgroup$ – user21150 Sep 22 '15 at 10:56
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    $\begingroup$ @user21150 Please add the information (that you want to use it for gardening) to your question, because people don’t want to read the comments to be able to answer. There is an edit link just below the tags. $\endgroup$ – Jan Sep 22 '15 at 13:30
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I recommend you dissolve your iron sulfate in hot water, dissolving as much sulfate as possible in as little hot water as possible. While keeping it hot, dip some coiled iron wire in it as well as some steel wool, and stir gently. The metallic iron should reduce your iron III back to iron II.

Once the solution is clear green, remove the iron wool, but keep the iron wire in it, and let it cool down as slowly as possible, while protecting it from air the best you can. Finish the cooling by putting it in a fridge

You should be left with beautiful large, green crystals of highly pure iron(II) sulfate heptahydrate, and a clear solution of relatively dilute iron(II) sulfate.

Since you have 50 kg of it, you will likely have to proceed in several batches, so I recommend you keep that solution for the next batch to minimize product losses.

Once you are done recrystallizing your iron sulfate, you can heat up that solution to a boil, until some crystals start forming on its surface. Then, immediately stop the heating and cool down that solution again. You'll have some lower grade iron sulfate, but that should still be perfectly usable for gardening.

And if you want to keep some of the prettiest crystals you made in the process, I recommend you keep them in a sealed container with a small piece of moist cloth. Otherwise, they may tend to slowly dehydrate and turn into powder.

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  • $\begingroup$ For gardening application, do you think it need any processing at all? From the description it seems most of the material is iron(II), and I don't think gardening would need high purity material. Also, in this scale it sounds like an awful slow reduction reaction. $\endgroup$ – Greg Nov 10 '15 at 2:24
  • $\begingroup$ Hi! Thanks for the advice. I'll try it spring. I've spreaded 25 kg yet on 1000 sqm. I made solution of pH 4.5. It was 1 month ago, but it seems that the soil pH is still the same (~7.8). I'll try the recrystallizing, and write the results. $\endgroup$ – user21150 Nov 11 '15 at 8:11

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