Sciencemadness Discussion Board

Praseodymium Oxide

gdflp - 21-2-2014 at 06:28

I recently received about 50g of praseodymium oxide. However, instead of being pure Pr2O3 it is mixed valence Pr6O11. Is there any way to reduce all of the praseodymium into it's normal +3 valence state?

woelen - 21-2-2014 at 06:53

Why is this important to you? I also have Pr6O11 and this dissolves in acids quite easily (with production of oxygen or production of Cl2 if the acid is HCl), giving a pale green solution, which contains Pr(3+) ions. If you use it in ceramic processes, then Pr6O11 works equally well. All commercially available praseodymium oxide is Pr6O11, I have never seen pure Pr2O3. If you heat Pr2O3 in contact with air, then it absorbs oxygen.

gdflp - 21-2-2014 at 06:56

Won't using Pr6O11 result in mixed valence compounds when dissolved in acids?

blogfast25 - 21-2-2014 at 08:52

Quote: Originally posted by gdflp  
Won't using Pr6O11 result in mixed valence compounds when dissolved in acids?


No, as woelen explained: the higher oxidation state gets reduced to III, hence the Cl2 or O2 evolution.

[Edited on 21-2-2014 by blogfast25]

Mailinmypocket - 21-2-2014 at 09:01

I purchased some of this compound as well from a seller online. Should I assume this is Pr6O11 also or could it actually be the trioxide?


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blogfast25 - 22-2-2014 at 04:37

Quote: Originally posted by Mailinmypocket  
I purchased some of this compound as well from a seller online. Should I assume this is Pr6O11 also or could it actually be the trioxide?




Assume the mixed oxide. It's likely to have been calcined, so it won't be pure Pr(III) oxide. Assuming you actually want to use it then dissolving in acid should tell you for sure. But some of these oxides don't respond well to even the most concentrated of strong acids.

Try a small amount with strong HCl and observe for chlorine.

[Edited on 22-2-2014 by blogfast25]

aleskva - 5-5-2015 at 06:56

I have got a similar problem. I have got black powder of Pr6O11 and I want to change it to Pr2O3 and then dissolve it in H2SO4 and crystallize green Pr2SO4.8H2O crystals. I haven't got HCl, but I have got N2H4 or H2O2. Could I change Pr6O11 to Pr2O3 using one of these two compounds (which both should work well in my opinion)?

gdflp - 5-5-2015 at 07:26

As explained above to me(a year ago, I am now amazed at my lack of knowledge then), when the Pr6O11 is dissolved in any acid, sulfuric acid included, then the praseodymium will be reduced to the Pr<sup>3+</sup> oxidation state. No need to waste hydrazine on this. The relevant chemical equation is Pr6O11 + 9H2SO4 --> 3Pr2(SO4)3 + 9H2O + O2 I don't believe that Pr<sup>4+</sup> is stable under standard conditions in aqueous solution, hence if any was formed, it would immediately get hydrolyzed(reduced).

Edit:Added note from post by blogfast

[Edited on 5-5-2015 by gdflp]

aga - 5-5-2015 at 09:26

Quote: Originally posted by gdflp  
a year ago when I am now amazed at my lack of knowledge then

That's cheery and gives great hope !

I'm still amazed at my lack of knowledge, then and now.

blogfast25 - 5-5-2015 at 09:36

Quote: Originally posted by gdflp  
I don't believe that Pr<sup>4+</sup> is stable under standard conditions in aqueous solution, hence if any was formed, it would immediately get hydrolyzed.


'Reduced' (by water) is a better way of putting it. Hydrolysis is more associated with acid/base chemistry.

Did your oxide dissolve easily? If so, in what acid?

[Edited on 5-5-2015 by blogfast25]

gdflp - 5-5-2015 at 13:59

Sorry blogfast, I didn't see your question until just now. I didn't have any notes on my previous experiments, so I just tried it again on a test tube scale. It's insoluble in conc. H2SO4, that's not surprising though, I was expecting that. It did dissolve in dilute acids though rather readily. ~20% HNO3 had the fastest reaction, it began hissing and violently bubbling as soon as some Pr6O11 was added. ~30% H2SO4 dissolved it quite rapidly as well. I got surprising results with ~15% HCl however. It dissolved remarkably slowly(took about 5 minutes to accomplish what took the other two around 15 seconds) and no chlorine scent was noted. Any ideas?

aleskva - 6-5-2015 at 05:06

Thank you for your help. I want to change all Pr(4+) to Pr(3+) from that dark oxide Pr6O11 because I need to prepare as many crystals as possible for following analyse (and the oxide is not so cheap as well). And I want the crystals to be Pr(3+) (in Pr2(SO4)3.8H2O).

aleskva - 6-5-2015 at 05:26

Finally, I tried to dissolve black powder of Pr6O11 in calculated amount of 20% H2SO4. Not more than needed, because it couldn't crystallize product I wanted to make. The acid solution was transparent blackish with black powder stirred, not soluted. I tried to add distilled water, but nothing changed. The black color is typical for Pr(4+) or mixed Pr(3+,4+). I needed Pr(3+) so I tried to add 30% peroxide. At first just drops, finally tens of ml. And it was the right step. The black stirred powder was completely soluted and the transparent solution was finally bright greenish (between green-yellow), which indicates me only Pr(3+) are present. I am letting it crystalize freely. Hope it helps anybody in a future ;-)

gdflp - 6-5-2015 at 06:20

Interesting, those results differ from mine. Did you observe any release of gasses when you added the Pr6O11? My dilute acid was warm because I prepared it freshly by diluting conc. acid, so perhaps some heat is beneficial here. Glad you got it to work, but I bet that if you let it sit for a few mins, it would have dissolved if you had observed effervescence.

I just had another thought. Because the mixed oxide contains praseodymium(IV), could this higher oxidation state provide enough energy for a magnesium thermite to be favorable?

blogfast25 - 6-5-2015 at 07:16

Quote: Originally posted by aleskva  
Finally, I tried to dissolve black powder of Pr6O11 in calculated amount of 20% H2SO4. Not more than needed, because it couldn't crystallize product I wanted to make. The acid solution was transparent blackish with black powder stirred, not soluted. I tried to add distilled water, but nothing changed. The black color is typical for Pr(4+) or mixed Pr(3+,4+). I needed Pr(3+) so I tried to add 30% peroxide. At first just drops, finally tens of ml. And it was the right step. The black stirred powder was completely soluted and the transparent solution was finally bright greenish (between green-yellow), which indicates me only Pr(3+) are present. I am letting it crystalize freely. Hope it helps anybody in a future ;-)


That trick also works for Ce(+4) to Ce(+3).

Well done!

aleskva - 6-5-2015 at 13:10

Quote: Originally posted by gdflp  
Interesting, those results differ from mine. Did you observe any release of gasses when you added the Pr6O11? My dilute acid was warm because I prepared it freshly by diluting conc. acid, so perhaps some heat is beneficial here. Glad you got it to work, but I bet that if you let it sit for a few mins, it would have dissolved if you had observed effervescence.


Yes, I forgot, when I tried to dissolve Pr6O11 in sulfuric acid, it warmed up itself for a while and a poignant gas was being released. I am not really sure, what gas it was, but its releasing didn't help oxide to solute anyway. To dissolve it faster, I think you should cool the solution.
P.S.: I was stirring it during the whole process...

[Edited on 6. 5. 15 by aleskva]

aleskva - 6-5-2015 at 13:27

Quote: Originally posted by blogfast25  

That trick also works for Ce(+4) to Ce(+3).

Well done!


Yes, you are right, but my colleague also tried it today (he tried to prepare cerium(III) sulfate from cerium(IV) oxide) and unfortunately he was unsuccessfull somehow. He couldn't dissolve that oxide, no matter how many distilled water or 30% peroxide he added.

[Edited on 6. 5. 15 by aleskva]

aleskva - 6-5-2015 at 14:04

Quote: Originally posted by gdflp  
I just had another thought. Because the mixed oxide contains praseodymium(IV), could this higher oxidation state provide enough energy for a magnesium thermite to be favorable?


I don't know if I understand you correctly, but Pr(IV) oxide when heated in air (O2) over 200 °C falls part

gdflp - 13-8-2015 at 08:46

I just noticed something interesting. I went to clean up the test tubes from my dissolution tests several posts when I noticed something. The test tube which contained the conc. sulfuric acid and the praseodymium oxide has, over the course of the past several months, apparently reacted. The black oxide powder has disappeared and is replaced with light green crystals of praseodymium sulfate. I'm unsure if the reaction is just extremely slow between the conc. acid and the oxide, but I have a theory as to what happened. I'm guessing that over the course of the past few months, which have been very humid, the sulfuric acid, which was exposed to the open air, absorbed water from the atmosphere, reducing the concentration of the acid enough to allow it to react.