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Author: Subject: Preparation of Red Selenium Gone Wrong
DysprosiumIon326
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[*] posted on 25-6-2020 at 13:43
Preparation of Red Selenium Gone Wrong


I had some of the typical gray selenium beads and wanted to make some red selenium powder, so I planned to go about doing so by first dissolving the beads, then diluting and reacting with acidic KI to precipitate solid iodine and red selenium.

I dissolved a gram of selenium in azeotropic nitric acid then separately dissolved a stoichiometric amount of KI in water and acidified with HCl. I mixed the two and the solution expectedly went very dark as iodine precipitated. I filtered some of it to check whether I had red selenium or not (and am currently in the process of filtering as I write this, sorry for the sloppiness). Then, I washed it with 91% isopropanol and dilute NaOH to clear away the iodine.

The problem is: what I'm left with seems to still be gray selenium! I'll update if I have any of the red allotrope once I'm done filtering but it doesn't seem like it right now. Any idea why this might be the case?

Rushing to clean up the product has also of course left me with some iodoform in the filtering flask, making the whole place smell like a hospital. Fun...
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DysprosiumIon326
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[*] posted on 25-6-2020 at 14:06


Interestingly, it seems like the selenium formed a black puck and settled out. I have no idea why this is the case, as its melting point is 220C. I will probably try redissolving the precipitate in HNO3 and then diluting even more and bubbling in SO2.
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[*] posted on 25-6-2020 at 22:58


Using iodide as reductor is a bad idea. It is an expensive reductor and your result is highly contaminated with iodine. You get a sticky mass, consisting of Se and I2.

Use sodium sulfite (or metabisulfite, both do the job) and add that to a solution of your selenic acid (prepared from HNO3 and Se), to which you also added some HCl. You'll see a nice red precipitate, forming in a few minutes.

The red selenium can be isolated. I have done that once. You get a nice brick red solid, but unfortunately the stuff is not stable. It quickly turns grey. Very soon, the red color becomes less saturated, it is as if you slowly turn down the saturation level of your color TV and the material becomes more and more grey.

You can keep it in red form for a longer time, if you keep it udner water, with excess SO2. The SO2 somehow stabilizes the red selenium.




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DysprosiumIon326
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[*] posted on 26-6-2020 at 11:09


Got it, thanks a lot! I'm trying to prepare some triphenylphosphine selenide so I'll be sure to wash the selenium quickly while it's still nice and red and react it immediately with the Ph3P. I'll also try keeping it in an ice bath to get more SO2 in solution and see if the temperature affects the rate of conversion back to gray.
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[*] posted on 21-10-2020 at 01:51


Hi guys, how did you get the grey Se beads to dissolve in HNO3/aqua regia? Mine seem to stubbornly stay there.



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[*] posted on 21-10-2020 at 02:59


You need to heat the liquid. Be careful with little droplets coming from the liquid if it is boiling. The solution contains selenate and selenium dioxide. The HNO3 and NO2 in de droplets quickly evaporates and disappears, the selenium lingers and can be inhaled as fine dust, also much later. Cover the beaker with a paper tissue, when you heat the liquid. Gases can go through, droplets are absorbed.



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[*] posted on 21-10-2020 at 21:42


Hi, it worked more or less once I refluxed it in aqua regia (with appropriate precautions, worry not). I got the red Se without any issues afterwards, with my main problem now limited to figuring out how to get residual Se off the glass frit.



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[*] posted on 21-10-2020 at 23:27


Pour some bleach through the glass frit. Even better is to use bleach, to which a little HCl is added, such that it just starts producing chlorine. I do this, by pouring bleach on the frit, and then immediately after, I add a few drops of HCl. The selenium fairly easily dissolves in that liquid, giving H2SeO3 and/or H2SeO4, which remains dissolved and can be rinsed away.



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ChemichaelRXN
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[*] posted on 22-10-2020 at 00:55


A little off topic, but have you noticed that selenous acid in ppm in the air makes you nauseous? I had a little sublimate before from the closed container and it caused to me be ill. Anyway, I have some selenium beads to process one day, so thanks for the info.



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MidLifeChemist
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[*] posted on 22-10-2020 at 06:14


Quote: Originally posted by ChemichaelRXN  
A little off topic, but have you noticed that selenous acid in ppm in the air makes you nauseous? I had a little sublimate before from the closed container and it caused to me be ill. Anyway, I have some selenium beads to process one day, so thanks for the info.


"Like many selenium compounds, selenous acid is highly toxic in excessive quantities, and ingestion of any significant quantity of selenous acid is usually fatal, however it is an approved dietary source in proper amounts. Symptoms of selenium poisoning can occur several hours after exposure, and may include stupor, nausea, severe hypotension and death."
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ChemichaelRXN
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[*] posted on 22-10-2020 at 07:31


Thanks for the info. I notice just a slight sublimated amount in the air makes me nauseous. I must have some sensitivity.




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