innervision
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Bi2O3 Preparation Stoich
I have made some Bi2O3 from Bi metal, but need some help figuring out the equations.
Wikipedia states two formulas for dissolving Bi in HNO3:
4HNO3 + Bi -> Bi(NO3)3 + 2H2O + NO
6HNO3 + Bi -> Bi(NO3)3 + 3H2O + 3NO2
There was a lot of NO2 generation, and the amount of acid required to dissolve the Bi was more than 4HNO3, but less than 6HNO3. What would the correct
equation be?
Also there is:
2Bi(NO3)3 + NaOH -> Bi2O3 + ???
What products do I end up with?
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woelen
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Both reactions occur at the same time, and not to some precise stoichiometric ratio. So, you could end up using anything between 4 and 6 HNO3
molecules of HNO3 per atom of Bi. Higher temperature and higher concentration of acid favour the second reaction with 6 HNO3.
In practice, you will even need more than 6 HNO3 per atom of Bi. The reaction with the acid is fast initially, but well before all acid is used up,
the reaction nearly comes to a halt. Heating mitigates this effect, but even with heating, you need quite a large excess amount of acid to get your
bismuth metal dissolved in a reasonable amount of time.
If you want to make Bi2O3 I would try heating the nitrate (first boil down, then heat the wet mass until all water and acid evaporates and then heat
to a red heat to decompose all remaining nitrate and leave behind Bi2O3). Using NaOH to precipitate Bi2O3 looks clean and nice on paper, but in
reality the result is not nice at all. You'll end up with a complicated mix of basic bismuth nitrate, bismuth oxide/hydroxide and trapped sodium ions.
Making pure Bi2O3 from that will be a pain in the ass. Heating that mix will leave you with impure bismuth oxide, which contains quite some sodium
ions as well, e.g. as NaBiO2 or NaBiO(OH)2
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chornedsnorkack
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Quote: Originally posted by woelen  | | Using NaOH to precipitate Bi2O3 looks clean and nice on paper, but in reality the result is not nice at all. You'll end up with a complicated mix of
basic bismuth nitrate, bismuth oxide/hydroxide and trapped sodium ions. Making pure Bi2O3 from that will be a pain in the ass. Heating that mix will
leave you with impure bismuth oxide, which contains quite some sodium ions as well, e.g. as NaBiO2 or NaBiO(OH)2 |
If bismuth nitrate were precipitated with NH3 rather than NaOH, would the precipitate trap a lot of ammonium ions? And would these ammonium
ions be stable on heating, or could these be volatilized?
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innervision
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In my experiment, I used 50g Bi and 60g HNO3. I diluted the acid quite a bit and added it in increments to the Bi pieces because reaction was very
strong. It didn't seem to take too long at all. In the end I had 7.22g Bi left in the beaker, so in my practice it used less than 6 HNO3.
As for using the NaOH, I am more or less following a paper on the synthesis of Bi2O3 nanoparticles. They used a quick addition of high levels of OH
with rapid stirring and heat. Possibly with these conditions the reaction favors Bi2O3 formation?
My product was a nice even yellow fine powder. I don't have a way to test its purity, so maybe it's a mix. I made it for dragon eggs and it works very
nicely.
I might give the red heat thing a try sometime though. It seems more economic.
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woelen
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For pyrotechnic purposes some impurity of trapped sodium ions and trapped nitrate ions does not matter. If you want to use it for more sensitive
chemical experiments, then of course such impurities are more problematic.
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innervision
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In the book: Concise Encyclopedia Chemistry, found on google books, it states that Bi2O3 turns brown upon heating. It doesn't say anything about
whether it reverts to yellow upon cooling or what the brown is. Do you know anything about this woelen? You seem to like to experiment with the many
colors of salts 
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Texium
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You can also make Bi2O3 by strongly heating powdered bismuth metal in air. I found this out accidentally a long time ago when
trying to melt my bismuth powder that I had gotten from Pepto Bismol. It'll glow red hot at first and then the powder begins turning yellow. You have
to break up any clumps that may form to ensure that as much surface is exposed to the air as possible. Eventually you end up with a uniform yellow
powder.
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MrHomeScientist
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I had the same thing happen to me! I played with the idea of heating the powder under inert material like mineral oil but that didn't really work out.
Another possibility is ZnCl<sub>2</sub> - its melting point is very close to Bi, so my hope would be that charging the crucible with Bi at
the bottom and the chloride on top would have the chloride melt and protect the metal (which would stay at the bottom due to density). I never tried
this, though. Inert atmosphere is also an option, of course, if you have that capability.
That's all about making a solid lump of metallic Bi, of course. If you're after the oxide, it's easy to make from the powdered metal!
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