[HailoMan]

I have a 50mm diameter, 1m long quartz tube, on which the inside is covered with hematite (Pretty sure it's hematite, as I use ferrocene and heat the tube under instrument air after).
I've been trying to clean it up but with very little success.
Accessibility is of course an issue as I cannot easily get brush in there.
Also, the hematite doesn't seem to be on the surface of the tube because I can reach some areas with brush and even then it is still quite hard to remove.
Does anyone has any idea how I should go about cleaning it?

[Arkcon]

Have you tried a dilute nitric acid soak, then a through distilled water rinse?  That should work for iron compounds, won't hurt the quartz surface, and will be sufficiently "hands off" so you don't end up screwing up an optical surface, or other important quartz tube property.  It also saves you some clean-up time.  So long as your laboratory staff can handle this reagent being left out.

[Borek]

Hematite makes me think about HCl and H₃PO₄ - both should be better than nitric acid, as they not only dissolve, but also complex iron, don't they? Or am I missing something?

[HailoMan]

I am considering acid soak as well to leach the iron out. The issue is that the tube is 1m long and I would need a pretty big bucket for it. I am not sure the lab manager and my supervisor would approve I leave an open bucket of acid just sitting around the lab.
I tried just passing a 0.5M sulfuric acid, but it doesn't seem to work. I think it doesn't leach out fast enough.

[fledarmus]

Stopper one end of the tube, fill it, and let it soak. You only need as much acid as it takes to fill the tube.

[Arkcon]

The issue is that the tube is 1m long and I would need a pretty big bucket for it.

LOL.  Somebody forgot to pay attention to units.  That somebody was me.    Yes, my definition of "soak" now includes what fledarmus: said -- fill with dilute acid, and dump out.  Half molar nitric should attack iron oxides well, and leave minimal residue, you could just blow droplets out into the hood.  HCl is a good choice as well.  There are other chelating agents for iron that are weaker acids, off the top of my head, I believe citric acid is one, maybe also acetic acid.  But they may leave a residue that you have to rinse out well.  I tried Googling, but half the responses are New Age chelating therapy nonsense.  For more options, I may have to *gasp* *horror* look in a book.  I'll get back to you when I get a chance.

[Extracted]

Hematite makes me think about HCl and H₃PO₄ - both should be better than nitric acid, as they not only dissolve, but also complex iron, don't they? Or am I missing something?

I don't think you're missing anything but chelating is only really required in special cases.  6M nitric will produce a soluble iron salt so chelation isn't necessary.

I'm interested in why the poster believes he has hematite?  It's much more likely he has a funky iron carbide compound interspersed with iron oxide.  This was a pyrolysis tube, right?  Carbon-containing pyrolyzate and iron/iron oxides.  Sounds like hot nitric is just the ticket or maybe aqua regia or piranah etch.

[HailoMan]

I'm interested in why the poster believes he has hematite?  It's much more likely he has a funky iron carbide compound interspersed with iron oxide.  This was a pyrolysis tube, right?  Carbon-containing pyrolyzate and iron/iron oxides.  Sounds like hot nitric is just the ticket or maybe aqua regia or piranah etch.

I wasn't 100% sure that I have hematite. I assumed that it's mainly hematite because all the ferrocene that remains in the tube were subsequently oxidized in air at 800°C. This might be a dumb question, but would iron carbides be stable from oxidation at such high temperature?
Another reason why I assumed it was hematite was because of the reddish-orange coloration after the air oxidation step.

I don't think I can get away with soaking the tube with 1M nitric acid, let alone hot 6M nitric acid, aqua regia or piranha. Note that the tube is 5L in volume.

In any case, I had alumina blocks that I use in the work which also contaminated with the reddish iron containing compound. I decided to soak this in 2M nitric acid for a few days. It didn't seem to clean it up. Urghh... Hate stubborn stains.