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Author: Subject: Very low solubility of erbium oxide in mineral acids
woelen
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sad.gif posted on 6-10-2012 at 01:33
Very low solubility of erbium oxide in mineral acids


I just received some erbium oxide (Er2O3), which I purchased from eBay. This is a fine compact pink powder.

To my bad surprise it is very hard (nearly impossible) to dissolve this material in mineral acids. I added some of the powder to 20% H2SO4 and boiled the solution for several minutes. Only a very small amount of the oxide dissolves and a very pale solution is obtained. I tried the same with 30% HCl and the result is similar. I just boiled off a lot of HCl, but the amount of Er2O3, which goes in solution is small, very small.

Are there other people over here who have experience with erbium oxide? How can this oxide be dissolved, such that a decently concentrated solution of Er(3+) ions can be obtained?

I also have Nd2O3, Pr6O11 and La2O3 and all of these dissolve easily in moderately concentrated mineral acids, especially when the solution is heated gently. I expected the Er2O3 to behave similarly, but this appears not to be so.




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Pok
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[*] posted on 6-10-2012 at 04:41


Rare earth sulfates are less soluble in hot water. This may be one reason? Are you sure, Er2O3 is left and not Erbium sulfate? But if the other RE dissolve well, this should not be the reason.

You should get the chloride by heating Er2O3 with NH4Cl:

http://de.wikipedia.org/wiki/Erbium%28III%29-chlorid

I also have some Er2O3 and always wanted to dissolve it in mineral acids. I will try it today with various acids and tell you about the success.

[Edited on 6-10-2012 by Pok]
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[*] posted on 6-10-2012 at 04:58


http://books.google.com/books?id=zNJDAAAAYAAJ&pg=PA140&a...

Manual of qualitative chemical analysis
By C. Remigius Fresenius

I bought Eu,Er,La,Nd,Sm,Pr and Tb oxides from an ebay seller and Erbium was the most stubborn so far. Praseodymium right flew out of the flask and quite near murdered me with the chlorine smelling fumes that escaped.

Here is what I did: 1g Erbium oxide to a minimum of 2mL conc.HCl. Stir at a temp just below boiling and replace any HCl that evaporates off.

You will have a lovely looking pink solution within an hour or so, probably sooner.
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[*] posted on 6-10-2012 at 04:58


Got it!

0,29 grams Er2O3 + 1ml HCl (30%) ---heating 2 minutes---> pure ErCl3 * xH2O


Erbium oxide (more pink in reality)


Cold crystallized Erbium chloride


After addition of some water. Some particles still remaining (much less than 5% of added oxide). Probably not heated long enough.

Edit: the HCl was boiled for 2 minutes, not only heated.

BTW: an interesting property of Er2O3 is that emits a greenish light, wenn heated directly and very strong with a bunsen burner flame. This has something to do with complementary colours (oxide red, glow green). The same is visible with thulium oxide (oxide slightly greenish, glow red). Almost impossible to get by a camera.

[Edited on 6-10-2012 by Pok]
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watson.fawkes
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[*] posted on 6-10-2012 at 06:05


Quote: Originally posted by Pok  
an interesting property of Er2O3 is that emits a greenish light, wenn heated directly and very strong with a bunsen burner flame. This has something to do with complementary colours (oxide red, glow green).
I knew erbium oxide has been used as a phosphor for decades. What I didn't know was what was reported in this paper, an up-conversion emission in the green band. This sounds a lot like what you're reporting.

Preparation and characterization of erbium oxalate and erbium oxide nanoparticles by microemulsion technique
Wenxiu Que, et al.
Materials Science and Engineering: C, Volume 16, Issues 1–2, 20 October 2001, Pages 51–54
http://dx.doi.org/10.1016/S0928-4931(01)00297-1

Here's the abstract:
Quote:
This paper reports the preparation and characterization of erbium oxalate and erbium oxide nanoparticles. The erbium oxalate and erbium oxide phosphor materials were synthesized through the microemulsion technique. Transmission electron microscopy (TEM) was used to characterize the phosphor materials and reveal a nanocrystal structure for the studied phosphors. A strong green up-conversion emission at 543 nm (4S3/24I15/2) and another weak emission at 528 nm (2H11/24I15/2) have been measured for the studied phosphors upon excitation at 993 nm (4I11/24I15/2). The up-conversion emission mechanism has been explained by means of an energy level diagram. The lifetimes of these emissions have been measured. The green up-conversion emission equally strong in both the phosphors and for these phosphors, color coordinates (X, Y) have been obtained in order to assess their color emitting efficiency.
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[*] posted on 6-10-2012 at 11:59


Is there any reason to believe that what holds for many metal oxides, that their solubility in mineral acids depends on temperature and duration of calcination, isn’t true for RE oxides? I don’t thinks there is.

Woelen’s Er2O3 may have been calcined harder than Pok’s. My homemade Nd2O3 was also surprisingly resistant to 36 % HCl, even though it had been calcined ‘softly’. Only hot conc. H2SO4 converted it slowly to insoluble Nd sulphate (the ‘sandy’ version).




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[*] posted on 7-10-2012 at 07:14


Hi woelen,

I was able to dissolve 50mg of erbium oxide in 1.5ml of 10% HCl with some heating. Maybe you are trying to form too concentrated a solution?


[Edited on 7-10-2012 by strontiumred]
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[*] posted on 7-10-2012 at 09:22


Only 50 mg of erbium oxide in 1.5 ml of 10% HCl? I'll have to check out solubility of Er-chloride and Er-sulfate.

I now have been able to dissolve some of my Er2O3 in dilute H2SO4. But still, the amount is very small. The solution is very pale pink, nearly colorless and a lot of fine powder remains undissolved in the liquid. It now is standing here for more than 2 days and still hardly any material dissolved. The same is true for my brew in 10% HCl. The HCl-stuff is even more dilute.

I am somewhat disappointed about this behavior. I want reasonable concentrated solutions of Er(3+) salts, which I can use for further experimenting. Maybe I should make very dilute solutions and then boil them down.

I personally am inclined to think that blogfast25 is right with his remark about calcining of oxides. Apparently mine is calcined more strongly than Pok's.




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[*] posted on 7-10-2012 at 09:29


Woelen:

Try fusing with NaHSO4? That tends to digest even the hardest of oxides.

Then convert back to the hydroxide, the fresh hydroxide should dissolve much more easily.

Re. calcination. My homemade Nd2O3 (magnet Nd) was prepared simply by precipitating as hydated hydroxide with strong ammonia, very scrupulous washing and then drying on a full heat electical hot plate. It hardly constitutes 'calcining' in the narrow sense of the word. Yet the product proved almost completely impervious to hot 36 % HCl. And NdCl3 is of course highly soluble (I can't actually get it to crystallise!)




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[*] posted on 11-10-2012 at 12:39


I just allowed the material to stand for a long time. I added the Er2O3 to 10% HCl 5 days ago and it did not dissolve. Now, however, 5 days after adding it to the dilute HCl, all of the Er2O3 has dissolved and the solution is pale pink. Two days ago, the solution also was pale pink, but at that time, there still was some fine powder at the bottom of the test tube. I swirled it to mix the powder through the acid. So, my Er2O3 does dissolve in dilute HCl, but it takes a long time.

I also have some of the Er2O3 still standing in 20% H2SO4. This material still did not dissolve. However, its appearance has changed. It is not a fine powder anymore, but a thin crust, sticking to the glass. The color of this crust is more intense. The solution is pink. It seems as if the erbium oxide is converted to crystalline erbium sulfate, which does not dissolve further in the 20% H2SO4. Apparently Er2(SO4)3 is only sparingly soluble in water, especially in the presence of a high concentration of sulfate ions from another source.

But I am happy with these results. I now put a somewhat larger amount of Er2O3 in a little bottle, add some HCl and put it aside for a week or so. I then expect to have a decent amount, suitable for experimenting.

I did not try the suggested method of melting with NaHSO4. Now I have a method of dissolving it, although this method requires some patience and planning.

[Edited on 11-10-12 by woelen]




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[*] posted on 11-10-2012 at 19:41


It's a non-standard usage, but has anybody tried dissolving these calcined rare earths with a Soxhlet apparatus and azeotropic HCl? If the problem is that there are low-solubility phases on the exterior, washing with fresh solvent should help. I'll freely acknowledge this might not be the problem. A fritted thimble might be required for solvent compatibility. Possibly no thimble is needed, if decomposition to powder get you rapidly enough to the end goal.
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[*] posted on 13-10-2012 at 18:26


Quote: Originally posted by woelen  
I just received some erbium oxide (Er2O3), which I purchased from eBay. This is a fine compact pink powder.


I recently bought some, too. I hope we weren't bidding against each other. All I've done with mine so far is admire its pretty color.

BTW, I like your work on Faraday's law, I may try to build something following your ideas. My daughter is studying electricity in school.




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[*] posted on 13-10-2012 at 20:22


I hope this is not a naive comment. A while back I tried dissolving some magnetite, Fe3O4, in HCl. I obtained the magnetite by magnetic separation out of common sand. I found that some would dissolve in the HCl, coloring the solution as I would expect for a mixture of FeCl2 and FeCl3. But even after waiting weeks the magnetite was only partially dissolved. I tried grinding the magnetite into a fine powder before applying the acid, with the same result. I did not try boiling it, nor did I try H2SO4.



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[*] posted on 13-8-2015 at 22:49


I recently purchased a lot of Nd2O3 again, because of its very low price (only EUR 53 per kilo!!). The material is a light grey very fine powder with a bluish hue under fluorescent light, it has a pinkish hue in daylight.

This Nd2O3 is surprisingly hard to dissolve. The other Nd2O3 I have (from 2012 or so) dissolves easily when added to dilute mineral acid. My new batch does not dissolve at all, not even in boiling hot acid. It does not dissolve at all, not even the faintest pink/lavender color can be observed after prolonged boiling in 10% HNO3.

The other older sample of Nd2O3 gives a faint hissing noise when added to the acid, leads to slight warming of the liquid and quickly dissolves. Getting the last little amount of solid particles in solution only requires gentle heating for a few minutes.

So, different samples of the same oxide can have very different behavior.




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[*] posted on 14-8-2015 at 07:56


Here is something that may be worth a try based on this paper ("SOLUBILITIES OF SOME CALCIUM MINERALS AND PREPARED CALCIUM COMPOUNDS IN EDTA ... ") noting for some calcium calcined salts EDTA, via complexing, was able to improve solubility.

Link: https://www.google.com/url?sa=t&source=web&rct=j&...

With respect to acids, try Oxalic acid which is noted for forming complexes as well.

[Edited on 14-8-2015 by AJKOER]
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[*] posted on 14-8-2015 at 09:24


Can anyone tell me what calcining does to the material (its crystal structure or whatever) to reduce the effectiveness of the reaction with acid? What goes on at the atomic level?



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[*] posted on 14-8-2015 at 13:45


Quote: Originally posted by annaandherdad  
Can anyone tell me what calcining does to the material (its crystal structure or whatever) to reduce the effectiveness of the reaction with acid? What goes on at the atomic level?


I don't know exactly either and it probably depends also on material, temperature and duration of the calcination.

One effect of drying/calcining is reduction of water content to the point of complete elimination and water is obviously a vector for water soluble acids.

After that, high temperature calcining must somehow 'close' the crystalline structure of the material, making it less and less 'accessible' to acids (or H<sub>3</sub>O<sup>+</sup> ions).

It's important to note that the solubility limit of the material in acid is probably not affected, but the dissolution rate is (very negatively).

[Edited on 14-8-2015 by blogfast25]




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[*] posted on 14-8-2015 at 14:39


Quote: Originally posted by blogfast25  


I don't know exactly either and it probably depends also on material, temperature and duration of the calcination.


Thanks, my Fe3O4 was probably calcined very well in the molten magma out of which it crystallized. I get a slight solution in HCl after several days, but nowhere near complete. I may try fusing it with NaHSO4 to see what happens.




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[*] posted on 14-8-2015 at 14:45


Sodium bisulphate is great for that kind of thing. Needless to say, the finer you grind the target substance, the higher the dissolution rate is likely to be.

I'd appreciate it if you would post any experimental results with that particular oxide here or in the 'Rock molester's club' thread. Thanks.

[Edited on 15-8-2015 by blogfast25]




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