[o.k.]

Hi!

I've bought FeSO4×7H2O for gardening use (lowering the pH). Sadly it was stored on ar and now it has turned to brown. I think it has oxidated, and Fe2(SO4)3 + Fe2O3 formed. Can You recommend a cheap method to reduce the Fe2(SO4)3 content to FeSO4?
Thanks for reply.

[Borek]

I am not aware of any reasonably simple method that would work. Probably buying another batch of FeSO₄·7H₂O is your best (and cheapest) option.

[Enthalpy]

Is Fe₂(SO₄)₃ + Fe₂O₃ unusable for gardening?

Or Fe₂(SO₄)₃ alone, which is soluble in water while Fe₂O₃ is not? Is that any feasible?

[o.k.]

i dont know if fe2so43 also good for lowering ph. I was told to use the green feso4. By the way can i use electrolysis to reduce fe3+ to fe2+?

[Borek]

By the way can i use electrolysis to reduce fe3+ to fe2+?

Won't be easy. Fe³⁺ is practically insoluble in water, as it precipitates as Fe(OH)₃. To keep it in the solution you need very low pH, which means you need to add a lot of acid - so it is no longer just a solution of Fe³⁺ salt. Unfortunately, in highly acidic solutions H⁺ is reduced much earlier than Fe³⁺,so you will be electrolyzing water, not converting Fe³⁺ to Fe²⁺. Perhaps playing with electrode material and complexing agents it can be possible to design a system in which Fe³⁺ gets reduced first, but then you will need some way of purifying the salt afterwards. Doesn't make sense.

[Arkcon]

As I understand the chemistry, you have some iron sulfate as a pH buffer, but its oxidized to rust.  You want to make iron sulfate from rust -- chemically or electrochemically.  That's not possible, for a novice.

Enthalpy: said its not necessary.  Iron sulfate is water soluble, and rust isn't.  You can dissolve in water, strain out, and you will simply have less iron sulfate by the weight of the remaining rust.  Or simply use it -- eventually bacteria will reduce the rust to plant usable form.