Sciencemadness Discussion Board

Iron Oxide + HCl ==> unknown toxic gas

ingrix - 17-12-2007 at 18:23

This is my first post, so I apologize if anything is unclear.

Earlier this evening I was trying to find a way to readily prepare Fe2O3, and I came upon the idea of mixing iron(II) oxide with HCl, then treating the Fe2+ solution with potassium permanganate to oxidize the Fe2+. The reactions I hypothesized being:

FeO + 2HCl -> FeCl2 + H2O
5Fe2+ + 8H+ + MnO4- ==> 5Fe3+ + Mn2+ + 4H2O

I tried a small amount in an open plastic dish, and it wen't well. This was the procedure (measuring is hard to do accurately because I am without volumetric or massing equipment at the moment):

*Run 10V DC current through a piece of steel wool (bought at a hardware store) to start the oxidation process.
* Put small amount of (< 2mL) ~11M HCl into small plastic reaction dish (solo cup)
* Drop oxidized steel wool into HCl
* Allow the solution to turn a bright yellow color
* Dilute the HCl with a decent amount of water (fill the cup half way up)
* Drop in small amount of KMnO4 (tip of a plastic spoon) into dilute HCl + FeCl2 solution
* Wait for dark red precipitate to form

As I said, this went well. I ended up with, I believe, Iron(III) hydroxide (dark red precipitate) in the bottom of the cup. As it should be, I tested it again with a larger amount to see if anything became more evident. This time I put ~2 grams iron(II) oxide into the bottom of a glass jar (also oxidized from steel wool) and used enough HCl to cover it. The reaction between the two proceeded as before, but this time I noticed that the reaction bubbled considerably. I wafted the gas towards me, only to discover an incredibly unpleasant smell. I ventilated everything properly and got out of there ASAP. I'm wondering what the **** this unknown gas is, because it doesn't smell like hydrogen chloride gas, it doesn't smell like chlorine, and I can't, for the life of me, figure out what else it may be. It reminds me of a hydrogen-sulfide like smell (I worry about impurities), and it almost smells like putrid butter. Does anyone know what I might have made here?

[Edited on 17-12-2007 by ingrix]

Fleaker - 17-12-2007 at 18:27

Probably a small amount of phosphine. Phosphide impurities are common in iron.

ingrix - 17-12-2007 at 18:28

Is there a way to test if it is phosphine with common chemicals?

The_Davster - 17-12-2007 at 18:39

It's phosphine, phosphides are common impurities in many metals.
Here we have these pyrotechnic devices for gassing moles(which is strange, as I have never seen a mole in my life) that combust producing phosphides and then releasing phosphine on reaction with water, and they smell the same.
Most technical grade metals I have dissolved(lanthanides are the worst it seems) produce this smell.

I do not know if the concentrations produced are detectable by any easy method?

12AX7 - 17-12-2007 at 18:42

Hmm, good question. It probably forms coordination compounds with some transition metals. Not very useful though. Your nose, properly calibrated, is one of the best tests, since as you can tell, mere parts per billion are sensibile.

Iron contains impurities of P and S, to the tune of 0.02% or less for most alloys.

You'll get much better yields of Fe(OH)3 (not Fe2O3) by adding a base. One option is to let the iron rust under solution; due to air or whatever oxidizer, Fe(II) oxidizes to Fe(III) in solution and precipitates some Fe(OH)3. The Fe(III) remaining gets reduced to more Fe(II) by the metal sitting around.

Tim

ingrix - 17-12-2007 at 19:58

Thanks for your replies guys. It does match up with the smell phosphine is described as having. So given that I could potentially make my own fumigant in my lab (which I don't want), would there be a better way to go about oxidizing iron into the 3+ state? I can get it to 2+ just fine with the steel wool and some electric current. It's getting it up the next ionization energy that I'm having trouble with.

not_important - 17-12-2007 at 20:43

Just precipitate the Fe(II) with aqueous ammonia, then bubble air through the suspension of the Fe(II) hydroxide for some time. You'll see the white/pale green-yellow Fe(II) turning rusty right away.

Alternative to bubbling air is just to shake it in a large bottle - lots of air - and then let the air exchange with the room for awhile, shake again, repeat.

woelen - 18-12-2007 at 04:42

Indeed, the best method of oxidizing it is making a precipiate of Fe(OH)2 (probably this becomes fairly dark green already, because your product most likely contains quite some iron(III) already). Let this precipiate be in contact with air. You'll be pleasantly amazed by how quick it turns brown.

Also keep in mind, that if you make FeCl2 (contaminated with FeCl3) from the steel wool and HCl, that there will be quite a lot of impurity in the steel wool, such as other metals and even carbon. If you dissolve it in HCl, you'll see that the solution does not become really clear, some dark grey gunk remains. Decant the clear solution and only use that, discard the turbid dark stuff.

The_Davster - 18-12-2007 at 11:58

When I had done something similar to produce Fe2O3, adding a small ammount of peroxide instantly oxidizes the suspension of hydroxide to a precipitate of Fe2O3 that settles and filters much easily than the hydroxide precipitate.

ingrix - 18-12-2007 at 17:46

Quote:
Originally posted by woelen
Also keep in mind, that if you make FeCl2 (contaminated with FeCl3) from the steel wool and HCl, that there will be quite a lot of impurity in the steel wool, such as other metals and even carbon. If you dissolve it in HCl, you'll see that the solution does not become really clear, some dark grey gunk remains. Decant the clear solution and only use that, discard the turbid dark stuff.


Done and done. Thanks for the advice!
Quote:
Originally posted by The_Davster
When I had done something similar to produce Fe2O3, adding a small ammount of peroxide instantly oxidizes the suspension of hydroxide to a precipitate of Fe2O3 that settles and filters much easily than the hydroxide precipitate.


That's a handy bit of information to have. Thanks.