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Author: Subject: ammonium perrhenate reduction
watson.fawkes
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[*] posted on 1-4-2010 at 06:20


Quote: Originally posted by Fleaker  
I nearly burnt the hell out of myself upon removing it from the quartz tube when it took fire (hence my advice above to pull a vacuum and heat it up to remove hydrogen!!!)
What temperature was it (approximately) when you removed it?
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Fleaker
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[*] posted on 1-4-2010 at 12:29


Room temperature--it had cooled under a stream of H2.

The rule of never cooling a PGM in hydrogen is one that should be observed, especially if it is a fine powder (black). I don't think platinum black will actually "burn"-- I suspect it just promotes combustion. Palladium black does turn to PdO to some extent ( I know this because it does not all dissolve in aqua regia, and there is always a mass gain).




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watson.fawkes
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[*] posted on 1-4-2010 at 14:22


Quote: Originally posted by Fleaker  
Room temperature--it had cooled under a stream of H2.
That's both neat and dangerous. So my next question: While vacuum would certainly work to extract H2, wouldn't an inert sweep gas also work? Assuming Raoult's law holds at the metal surface, I'd say so, but I have a nagging doubt in my assurance that it does. I ask because, practically speaking, it's a little easier to set up a second gas under positive pressure than to exhaust an uncondensable, inflammable gas with a pump. As for a gas, at room temperature I'd imagine that Ar, N2, and CO2 would all work, just nothing too reactive.
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[*] posted on 1-4-2010 at 15:41


If you let in air slowly, will it catalytically oxidize the adsorbed hydrogen? Do it slowly so it doesn't sinter in the center or anything.

Tim




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Jor
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[*] posted on 2-4-2010 at 12:13


Ok, I just came home and rushed in the lab ASAP! :D

I put 100mg of ammonium perrhenate, together with about 450mg of 300 mesh zinc powder (I think it's pure but I'm not 100% sure) in a porcelain crucible, and heated for about 1 minute (maybe a little bit more) at red heat. I then left it to cool down. The mixture formed a cement like layer, wich however not that hard to remove. It was placed in a beaker and briefly boiled with some 6M HCl, until fizzing stopped. I was left with a small amount of black powder. The supernatant was decanted, the precitpate washed with water, and conc. HNO3 was added. The precitpate dissolved, with formation of NO2. the HNO3 was hot was the beaker was still hot. There was a formation of a orange color. This must be some NOCl (didn't rinse all HCl off), as perrhenate is produced when rhenium is oxidised by HNO3, wich is colorless.

I will just have to make sure that my zinc powder does completely dissolve in HCl, to rule out the possibility that it contains an impurity, but I think it is pretty pure.

So now the only question remains:
Is this rhenium metal or some oxide?

How can I test this? Do the oxides of rhenium dissolve in hot HCl? If they do, I certainly have Re powder here!
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[*] posted on 3-4-2010 at 03:52


Solid-solid reactions usually give impure products, so even if you have Re-metal expect it to contain quite some contamination (e.g. encapsulated NH4ReO4, maybe zinc, amalgated with the Re-metal).

In order to assess the purity of your material in terms of zinc-content, dissolve some in HNO3 and then slowly add dilute NaOH, drop by drop. If the solid material contains amalgated zinc, then you'll get a white precipitate of Zn(OH)2, while perrhenic acid does not give a precipitate. Do not add a lot of NaOH at once, because Zn(OH)2 redissolves in excess solution of NaOH.

I'm not sure whether you have Re-metal though. ReO2 is a black solid (like MnO2), but it is much less reactive and the heat might have made it somewhat inert. ReO2 certainly will not dissolve in conc. HCl giving Cl2. If it dissolves then I expect that some chloro-complex is formed, but I do not know about a specific complex.



[Edited on 3-4-10 by woelen]




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Jor
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[*] posted on 3-4-2010 at 07:53


I think this was not a solid solid reaction, as the zinc likely melted, but i'm not sure. I don't want to take the cover from the crucible, as i'm not sure if the zinc will ignite at those temperatures.
The only way I can think of now to see if I'm making the metal or oxide is by precise weighing, but my analytical balance broke, so I can't do this...
But don't you think that boiling with HCl simply removes all the zinc from the rhenium (oxides) ? Just like when you heat a Ni/Al alloy in NaOH, you remove the Al (leaving raney nickel).

[Edited on 3-4-2010 by Jor]
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[*] posted on 3-4-2010 at 12:52


It depends on how finely divided the materials are and whether particles of zinc can be encapsulated by a non-reactive layer of Re or Re-oxide. Encapsulation of solid materials by other solid materials is a common problem in this kind of reactions. If e.g. a slag is formed of Re and/or Re-oxide then this could lump up and form a protective layer around small globules of zinc and ammonium perrhenate. I'm not sure though about the melting points of all involved chemicals and about the temperature of your mix. These are just considerations, based on general knowledge.



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