Sciencemadness Discussion Board

Iron Oxalate

Hexavalent - 3-1-2012 at 14:04

In my efforts to create pyrophoric iron by decomposing iron oxalate dust to iron micro-particles and carbon dioxide, I found that I needed a source of this salt.

I firstly prepared a solution of iron (II) chloride by reacting steel wool with HCl and filtering, and then made up a stoichiometric solution of oxalic acid. I dumped the oxalic solution into the filtered iron chloride solution, expecting iron (II) oxalate to precipitate out due to its extreme insolubility in water.

But nothing happened.

The iron salt solution was pale green and the oxalic acid solution colourless. When mixed, they formed a urine-coloured mixture which was clear.

My first thought was that there may have been a small amount of residual HCl in my iron chloride solution and somehow it was preventing precipitation (complexing possibly?) of the iron (II) oxalate, so I stuck the mixture on a hotplate and heated to drive it off until no more white fumes were given off.

Still no precipitate.

Any ideas?

nezza - 3-1-2012 at 14:31

I would suspect that the mixture is too acidic for precipitation. Try Sodium or ammonium oxalate as a source of oxalate ion.

neptunium - 3-1-2012 at 15:14

oxalic acid is a weak acid HCl is a strong one...nothing should happen untill ALL the HCl is out of the solution i would rinse thoroughly the iron solution or maybe neutralise it with sodium carbonate ( never mind the sodium they stay in solution)

Polverone - 3-1-2012 at 15:53

If you want to salvage your existing solution, I second the suggestions that you raise the pH.

In the future you can use a less expensive, more massive form of iron (steel) like nails. If you like, first strip any galvanization with a preliminary brief soak in dilute hydrochloric acid. Immerse the pieces of metal in warm oxalic acid solution in a bottle. Add a small amount of sodium chloride or (better) hydrochloric acid to act as a catalyst. The iron (II) oxalate will form as a crust over the metal and needs to be removed by vigorous shaking from time to time. The dislodged masses of iron oxalate are later easily ground to fine powder.

In summer the solution can be kept warm just by placing it in direct sun; in winter you may want to use a heating pad in a cardboard or foam box to keep its temperature up. The process works in the cold too but much more slowly.

There are two small advantages to this approach. One is economy of reagents, using only a catalytic amount of hydrochloric acid and a less expensive form of steel. Since the reagents are cheap this is only a minor benefit. The greater advantage is that a little hydrochloric acid in an oxalic acid solution will not fume hydrogen chloride the way the neat acid does. You can keep the mixture indoors without dooming every ferrous object in the room to visible corrosion.

AJKOER - 3-1-2012 at 16:56

Aqueous Iron Chloride is an acid salts which is clearly evident from its hydrolysis:

FeCl2 + 2 H2O <==> Fe(OH)2 + 2 HCl

Boiling is a tedious way to impact the pH by gaseous evaporation of the HCl.

Note, HCl is a much stronger acid than Oxalic acid, and I would doubt the creation of iron (II) oxalate until the solution is neutralized.

You also may wish to see the thread on Iron oxide, as I discuss a way to create a Iron salt without the use of HCl.

[Edited on 4-1-2012 by AJKOER]

Endimion17 - 3-1-2012 at 19:30

Too low pH, obviously.
Just buy some green vitriol and dump its fresh, filtered solution (use degassed, distilled water) to a solution of oxalic acid. Green vitriol is available as a source of iron for plants and is quite cheap.

woelen - 4-1-2012 at 00:20

Too low pH is one problem, but you also should be careful not to add too much oxalate. Iron forms very stable complexes with oxalate ion. Iron(II) forms a very stable brown/yellow complex and iron(III) forms a very stable green complex. The funny thing is that the oxalate complexes have colors, which are almost the reverse of all other iron(II) and iron(III) compounds (usually the first are green and the latter are yellow/brown, but with the oxalato complex it is the other way around).

So, in another experiment, try to make a solution of potassium oxalate (not sodium oxalate, as that is only sparingly soluble) from slight excess of oxalic acid and potassium carbonate and dissolve as much as possible iron in HCl. After filtering off the remaining iron, neutralize this solution with potassium carbonate, such that the solution only remains marginally acidic. Do not add too much carbonate, otherwise iron is oxidized to iron(III) by oxygen from the air. After this is done, you can mix both slightly acidic solutions. Use excess amount of the iron-containing solution.

[Edited on 4-1-12 by woelen]

Chemistry Alchemist - 4-1-2012 at 06:36

Would Iron(III) Chloride work better?

Hexavalent - 4-1-2012 at 09:07

No, as I want the iron (II) oxalate salt and not the iron (III) oxalate . . .so you'd only have to reduce it back again anyway to what you started with.

Endimion17 - 4-1-2012 at 12:27

Why are you sticking to the chlorides so hard? Is your goal to obtain iron powder, or to see how viable it is to take the chloride route?

Hexavalent - 5-1-2012 at 08:42

I'm not sticking so hard to the chlorides, its just that I have a very large amount of steel wool and large amounts of HCl available.

I could also use iron (II) sulfate I guess, but my only source of concentrated sulfuric acid is unfortunately impure and I want high grade iron oxalate (and no, I am not going to distill concentrated H2SO4). I do have some iron sulfate and nitrate powders available, but they are very pure, very expensive and don't want to waste them on a synthesis like this - rather save such purity for analyses.

Last night I heated my mixture gently on a hotplate again for three hours until no more white fumes were driven off . . .very shortly later a massive clump of the compound precipitated out in the beaker. Yay!

After a filtration and washing and drying, I had approx 15.6g of my compound, or 94% of the theoretical. Most of it was lost mechanically as it extremely insoluble in water (I believe 0.0008g in 100ml of water at 20C).

I didn't want to neutralise the original mixture directly using a base because I feared that iron oxides and hydroxides might form and contaminate the product.

If you're interested, I'm going to use the salt for making pyrophoric iron - heat a small quantity of the crystals in a test tube until they decompose and turn black . . .at which point you invert the test tube and watch the sparks fly!