MrHomeScientist
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Ammonium Bifluoride Reactions
Hey all,
The short version: How does the bifluoride ion participate in reactions? Does it act like an acid (i.e. by neutralizing hydroxides) or a base (i.e.
reacting with sulfates)?
The long version: I'm in the process of extracting neodymium metal from hard drive magnets, and I've managed to recover some neodymium sulfate. The
next step in the process is to convert this to neodymium fluoride, and to do so I've ordered some ammonium bifluoride. I've never worked with fluoride
compounds before, so I don't really know how they react. From what I've read, fluoride is in general quite different from the other halides. Despite
that, I am well aware of the dangers and will be taking maximum safety precautions (and use plastic labware). I'm thinking the reaction will go as
follows:
Nd<sub>2</sub>(SO<sub>4</sub> <sub>3</sub> +
3NH<sub>4</sub>HF<sub>2</sub> == 2NdF<sub>3</sub> + 3NH<sub>4</sub>HSO<sub>4</sub>
Is that correct? Or will I have to convert the neodymium sulfate into the hydroxide first? That would go:
Nd<sub>2</sub>(SO<sub>4</sub><sub>3</sub> +
6NaOH == 2Nd(OH)<sub>3</sub> + 3Na<sub>2</sub>SO<sub>4</sub>
2Nd(OH)<sub>3</sub> + 3NH<sub>4</sub>HF<sub>2</sub> == 2NdF<sub>3</sub> +
3NH<sub>3</sub>(g) + 6H<sub>2</sub>O (seems wrong)
Excuse me if all this is horribly incorrect - for some reason I'm having trouble thinking this through and I couldn't find much information through
google.
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woelen
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Your reactions seem plausible to me, assuming that NdF3 is only very sparingly soluble. Otherwise I do not see the upper reaction happen (Nd2(SO4)3 on
its own is only sparingly soluble). So, if that does not work, then indeed the route over Nd(OH)3 may be necessary.
The last reaction occurs, but probably you need excess NH4HF2 in order to have all of it reacted and then the main reaction product will contain
ammonium fluoride together with NdF3.
How about the impurities from the Nd-magnets? I'm not sure about these, but fluorides of other lanthanides probably have very similar properties and
may coprecipitate with NdF3.
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plante1999
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NH4HF2 react both as an acide an a base , but it act normaly as an acid (as HF) if you want to make some I could send you the synthesis, NH4HF2 is
somewath like H2O2. H2O2 act generaly as an oxidiser but it can act as an reductor.
I never asked for this.
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not_important
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The attached may be of interest:
Attachment: Study on Quick Precipitation of Neodymitun Fluoride.pdf (1.3MB)
This file has been downloaded 960 times
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MrHomeScientist
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Thanks for the responses! That was all very informative.
woelen:
The current plan is to get the neodymium sulfate to dissolve into a solution of ice cold water, then add the bifluoride solution to that. NdF3 is
supposedly quite insoluble. I probably can't do anything about other rare-earth impurities in the magnets. In the main thread I'm posting in (the
trouble with neodymium) it was suggested that Pr especially is present in these magnets because separating it is too much of a hassle as it doesn't
affect the magnet. The separation methods I've read about seem impractical to do in a home lab and with the small amount of salt I have. It's probably
something I'm just going to have to live with. Maybe I can just call it didymium and use it as two elements for my collection instead of just the one!
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MrHomeScientist
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I finally received my ammonium bifluoride, and since I got it off eBay I'd like to try and confirm that is actually what it claims to be. Right now
I've got an acidified (with H2SO4) solution of it sitting in a test tube to see if it etches the glass as it should, but it's taking quite a while to
do anything. I'll leave it overnight and see if it frosts the glass up.
Anyone know of any other tests I could do to confirm its identity? I looked up tests for the fluoride ion, but there really aren't any (you just have
to eliminate the other halogens, really).
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not_important
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A strong solution of ammonium bifluoride by itself will etch glass, try a small amount of the solid plus a drop of water on window glass or kitchen
glassware, which are more easily attacked than borosilicate.
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MrHomeScientist
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Excellent suggestion! I'll try that as soon as I get home.
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MrHomeScientist
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Alright I did some testing and I'm fairly satisfied that this is in fact ammonium bifluoride (or at least, a fluoride salt).
I did the silver nitrate test and no precipitate formed, ruling out the other halogens. It very slightly changed the color of the NH4HF2 solution to
what I'd describe as "fluorescent white," to just to make sure I wasn't seeing a very fine precipitate I made up another solution of ammonium
chloride. Adding this to the silver nitrate + ammonium bifluoride mixture instantly precipitated lots of silver chloride, as expected. That more or
less satisfies me that fluoride is present.
I also placed a small pile of NH4HF2 onto the bottom of an overturned glass jar, and added a few drops of water. Nothing much happened for a while, so
I added a few drops of sulfuric acid to it. Acidifying the bifluoride should make more HF available in solution to eat the glass. And indeed, not long
afterward the glass under the drop appears cloudy. Scraping this with a spatula shows that the cloudiness flakes away and later reforms. The flakes
feel reminiscent of orthosilicic acid that I've made before, so it seems to me that the fluoride salt is indeed reacting with the glass, converting it
to hexafluorosilicic acid.
Now all that's left is to try it on my Nd2(SO4)3 and hope for the best!
[Edited on 8-4-2011 by MrHomeScientist]
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MrHomeScientist
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I just did this reaction the other night, with some success:
Nd<sub>2</sub>(SO<sub>4</sub><sub>3</sub> +
3NH<sub>4</sub>HF<sub>2</sub> == 2NdF<sub>3</sub> (s) + 3NH<sub>4</sub>HSO<sub>4</sub>
I made solutions of ammonium bifluoride and neodymium sulfate, and added the bifluoride slowly dropwise to the sulfate solution. (I don't have exact
numbers on me right now, those will be posted in the 'trouble with neodymium' thread later) This quickly formed a milky white precipitate. This was
very fine and gelatinous, and took a while to settle out. I was surprised by this, as I had assumed it would be crystalline similar to other fluoride
salts like calcium fluoride.
Again I'm not positive that those are the correct products, but no ammonia smell was produced so that at least rules out one of the possibilities I
had considered. Anyone have any ideas on how to test the solution to determine if it contains ammonium bisulfate? I can do the standard tests for
sulfate and ammonium ions, but I'm not sure how to confirm it's the <u>bi</u>sulfate.
[Edited on 8-25-2011 by MrHomeScientist]
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franklyn
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There is no Bifluoride ion , the compound NH4.HF2 is actually NH4F • HF
obtained by heating NH4F to drive off ammonia. Both of the F ˉ, will
displace more soluble anions.
(NH4).HSO4 , how do I know thee , let me count the ways.
You already confirmed the absence of ammonia smell.
Just as heating NH4F drives off ammonia , heating (NH4)2 SO4
also drives off ammonia to form NH4.HSO4. Do a sniff test.
Heat some material in a tube held in boiling salt water.
NH4.HSO4 is strongly acid , it will burn your tongue.
(NH4)2 SO4 has a higher pH of around 5.5 , and will just taste bitter.
Use a pH meter to measure the value.
You can measure the index of refraction of a saturated solution.
(NH4)2 SO4 is nD 1.53 , NH4.HSO4 is nD 1.47
A Triammonium Bisulfate variation is hard to distinguish except by density.
You will need a large quantity to be accurate , in a graduated cylinder holding
charcoal lighter fluid , note the volume and weight. Add in the desiccated
material and shack vigorously to lose trapped air. Measure the increased
volume in the graduated cylinder and the added weight.
Density is weight divide by volume.
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