How to reduce Fe(III) impurity in Iron(II) sulfate?

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 $\ce{FeSO4}$?




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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I have purified $\pu{1 kg}$ of old iron(ii) sulfate, which was yellow to brown color, using this method:


At room temperature add $\pu{10 ml}$ of diluted $\ce{H2SO4}$ (about $10 \%$) to $\pu{40 ml}$ of distilled water, scale up to prepare enough quantity of such mixture (you can use $37 \%$ battery acid in first step, in second step I have used chemically pure $96 \%$ sulfuric acid, of course recalculate the amounts to obtain the same concentration).


Dissolve as much as possible impure iron(ii) sulfate in such acidified water (prepare saturated mixture), stir very well, let impurities settle down and after decantation filter using very fine filter paper (slow filtration).


Then make first crystallization. If it does not start, you have probably not saturated, but diluted mixture, you can promote crystallization with evaporating (do not boil!), ventilation or cooling (do not freeze!). Finally you should obtain small light-green crystals.


Then wash crystals with ethanol ($96 \%$).


If you are not doing second crystallization immediately, then store intermediate product in vacuum bag to avoid reaction with air (without any desiccant!).


For second crystallization (i.e. re-crystallization), prepare the same acidified water, but with high purity $\ce{H2SO4}$ (p. a. or chemically pure), heat to max. $\pu{55-60 °C}$, and dissolve intermediate product, prepare saturated mixture.


Filter until hot using fine filter paper to crystallization dish and put dish to thermally isolated box, leave it overnight. You should obtain very nice and pure crystals. If not, the solution was not saturated, you can help using cooling (do not freeze), or ventilation, or if you are not satisfied with the result, you can do another recrystallizations.


Gently dry the crystals and put them in vacuum bag, without desiccant.


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