[science2000]

What's the best way to store ferrous salts in solution so they don't "spoil" and turn ferric?

[Alberto_Kravina]

You should store it in absence of water and oxygen!

H₂O + 0.5 O₂ + 2e^--------> 2 OH^-

This electrons can oxidize Fe²⁺ to Fe³⁺

[AWK]

Store solutions under nitrogen in the presence of iron filings

[woelen]

Storing them in acidic environments already helps a LOT. So, add a little amount of acid and the solutions won't oxidize that fast. But in the long run, your solutions will oxidize.

I have a similar problem with solutions of hydroquinone and sodium sulfite. What I do is buy such dust-off cans (e.g. 3M dust-remover) with pressurized gas. This gas is some hydrocarbon or a non-toxic fluorohydrocarbon, which is insoluble in water. Before I close a bottle with the solution, I blow some of that gas in the bottle and immediately cap it. This also helps a lot. Each time I have opened up the bottle, I repeat this.

Combining the two measures I mentioned should allow you to keep your ferrous salt solutions at acceptable quality for a reasonable amount of time.

[science2000]

I may want to crystallize a ferrous salt, would letting it evaporate in a dark undisturbed place be okay?

[woelen]

Probably the crystals will be highly contaminated with ferric ions and there is a big chance that you get some basic ferric salt as strong impurity. I have some very pure ferrous sulfate, but in the course of a year or so, it is covered by a brown crust. The salt ammonium ferrous sulfate (Mohr's salt is stable though).

If you want to give it a try, do this at a dry, undisturbed place with as much light as possible. The best place to store ferrous salts is in transparent glass bottles in bright light (best is sunlight). Ther reason for this is that ferric ions are somewhat light sensitive and in strong light they are easily converted to ferrous ions, when a suitable reductor is present.

[AgG]

store them in oil! (har har!)

[pantone159]

The best place to store ferrous salts is in transparent glass bottles in bright light (best is sunlight). Ther reason for this is that ferric ions are somewhat light sensitive and in strong light they are easily converted to ferrous ions, when a suitable reductor is present.

Does that go for ferrous salts in solution, as well?

[woelen]

Does that go for ferrous salts in solution, as well?

Yes, this also is true for ferrous salts in solution.

[constant thinker]

Why not just seal it in a vial filled with Argon or some other inert gas. I don'to know if you have the ability to this but it's a nice thought.

[science2000]

If ferrous ions are so unstable, why do many ferrous minerals exist in nature? That doesn't really make sense.

Another thing, do all ferric salts dissolve iron to form ferrous salts? Is that a rule?

[woelen]

If ferrous ions are so unstable, why do many ferrous minerals exist in nature? That doesn't really make sense.

Another thing, do all ferric salts dissolve iron to form ferrous salts? Is that a rule?

Not all ferrous salts are unstable. It is a matter of combination of anion and cation and even more likely, a matter of ligands.
For many metals, there is not a single preferred oxidation state. The preferred oxidation state strongly depends on ligands. E.g. cobalt with aqua ligands is stable in the +2 oxidation state, in the +3 oxidation state it is very strongly oxidizing, even water is oxidized by that stuff.

However, with ammonia ligands or cyanide ligands, cobalt in the +2 oxidation state is strongly reducing and in fact, it is impossible to keep a cobalt salt in the +2 oxidation state pure when these ligands are present. It simply sucks oxygen from the air, very eager to go to the +3 oxidation state.

With iron I expect the same holds.

Another reason, why many minerals of iron are in the +2 oxidation state is the absence of oxygen at the place where the minerals were formed. E.g. sulphur and iron give rise to pyrite (FeS2), with iron in the +2 oxidation state and S2(2-) being the disulfide ion. Many of these +2 oxidation state minerals, however, are oxidized to the +3 state, if exposed to air, especially, when combined with moisture.