Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Bill.D.Katt. on 22/03/2011 05:23:31

Title: How to precipitate ferric chloride?
Post by: Bill.D.Katt. on 22/03/2011 05:23:31

I have a solution of ferric chloride that is fairly concentrated, but contains a fair amount of HCl. FeCl2 is far more soluble in room temp solutions that in cold solutions so I dropped the temp to a little below 0 (C). I got a few crystals at the bottom, but after extracting them, drying them off, and placing them in a container, it turned back into a liquid. Any ideas why? Is the compound hygroscopic? Are there any other ways I can force the FeCl2 out of solution? I tried the common ion effect but for some reason it didn't work.

Title: How to precipitate ferric chloride?
Post by: Bored chemist on 22/03/2011 06:51:53

Do you mean FeCl2 or FeCl3?

Title: How to precipitate ferric chloride?
Post by: lightarrow on 22/03/2011 12:48:27

If it's ferric chloride, that is FeCl₃ (not FeCl₂) the eplanation is simple: the compound is highly hygroscopic and also partially hydrolizes in water forming HCl and brown ferric hydroxide (upon dilution).

Title: How to precipitate ferric chloride?
Post by: Bill.D.Katt. on 23/03/2011 04:56:17

I think it is a mix of FeCl2 and FeCl3, considering the excess HCl I'm guessing it's predominantly FeCl3. So should I just wait until the water evaporates? I can't boil it right now (broken Pyrex beaker).

Title: How to precipitate ferric chloride?
Post by: Bored chemist on 23/03/2011 06:54:12

HCl will not turn FeCl3 into FeCl2.

Title: How to precipitate ferric chloride?
Post by: lightarrow on 23/03/2011 10:42:58

Quote from: lightarrow on 22/03/2011 12:48:27

If it's ferric chloride, that is FeCl₃ (not FeCl₂) the eplanation is simple: the compound is highly hygroscopic and also partially hydrolizes in water forming HCl and brown ferric hydroxide (upon dilution).

If you have also FeCl₂ and you want FeCl₃ only, you have to oxidize the solution: for example adding an excess of H₂O₂.

Title: How to precipitate ferric chloride?
Post by: Bill.D.Katt. on 25/03/2011 04:32:27

I used a poor-mans aqua regia mix (HCl and 20% H2O2) to make the solution. I just said it was probably mostly FeCl3 because the electron config of Fe would seem a little more stable with all the s orbitals gone and a 1/2 d shell, twas just a guess. Anyway which one it is isn't the most trying part to me. I'm mostly trying to get it to precipitate out, and the methods I know for precipitation of solutes would be about the same for the two of them (however, both have failed). Although I noticed that when I added more H2O2 to the solution it bubbled quite vigorously; is this the H2O2 decomposing?

Title: How to precipitate ferric chloride?
Post by: Bored chemist on 25/03/2011 06:58:49

I think it would help if you started again.
What were you trying to achieve?
What did you do?
What did you observe?

Title: How to precipitate ferric chloride?
Post by: Bill.D.Katt. on 26/03/2011 03:30:04

Here is the exact procedure that I did today: I have around 100 old BB's from which I have removed the Cu coating so only the steel is left. I add HCl (~34%, I'm guessing by volume) at this point the solution becomes significantly more yellow; wait an hour or so and add 20% H2O2 and the solution bubbles vigorously and becomes a deep murky brown (FeCl3 in solution?). This solution is highly acidic. I am attempting to force the FeCl3 from solution to obtain the (hexa)hydrated solid.

Title: How to precipitate ferric chloride?
Post by: lightarrow on 26/03/2011 12:56:01

To make crystals of Fe³⁺ salts is extremely difficult, because of the hydrolysis:

Fe³⁺ + nH₂O ↔ (FeOH)^(2-n)+ + nH⁺, 1≤n≤3.

With FeCl₃, when you try to dry it, even "naturally" at the air, you are left with a mix of FeCl₃, Fe(OH)Cl₂, Fe(OH)₂Cl, Fe(OH)₃ and HCl gone away with vapour.

Title: How to precipitate ferric chloride?
Post by: Bill.D.Katt. on 26/03/2011 17:35:56

So how is it done? It looks like I need to maintain a high [H3O] while the solution is drying, is that right?

Title: How to precipitate ferric chloride?
Post by: lightarrow on 26/03/2011 21:52:47

Quote from: Bill.D.Katt. on 26/03/2011 17:35:56

So how is it done?

Good question [:)]
Maybe they remove water vapour while keeping enough HCl vapour pressure in the container.