Two weeks ago I tried to prepare praseodymium acetate. For this Pr(OH)3 was precipitated from a solution of Pr2(SO4)3 with a 4% ammonia solution. This
yielded a voluminous precipitate, that was filtered with suction on a fine fritte (to my surprise it ran though a coarse fritte without any residue
remaining in the fritte). The filtered precipitate was put in a beaker and mixed with about 10 times its volume of demiwater under strong stirring, so
that the lumps formed during filtration were turned into fine particles. This was again filtered with suction. Shortly before all fluid had ran
though, some demiwater was poured on the residue, after which it was filtered until the lumps came apart from the fritte.
Next, some of the lumps of Pr(OH)3 (some carbonate probably has formed in the process by absorbing CO2 from the ambient) were put in a beaker and an
excess of 80% HOAc was added. This slowly dissolved part of the precipitate, but not all of it. Very minor bubbling indeed was noticed as it partly
went into solution. The part which did not dissolve was more whitish (less green) than the lumps from the fritte. The content was stirred for about
half an hour and was then left to settle. The liquid decanted and fresh 80% HOAc was added to the precipitate. To my amazement, no visible amount of
the precipitate dissolved in the HOAc.
To some of the other lumps of Pr(OH)3 from the fritte, also an excess op HOAc was added, to which about twice the volume of demiwater was added. This
gave the same result as with 80% HOAc, leaving the more whitish precipitate behind and turning the solution green.
I found one article (Inorg. Chem. 199130, 2273-2277), where it is also mentioned, that not all of the Pr(OH)3 would
dissolve in HOAc. In the article, glacial acetic acid is used, see the first page attached.
So, the question I'm left with is: what is the precipitate that does not dissolve and why does it not dissolve?
nezza - 17-6-2014 at 00:09
Purely a guess, but using sulphates as a starting point is probably not ideal for lanthanides. I suspect that on precipitation sulphate complexes of
low solubility are formed. I would be inclined to try using nitrates or chlorides of the lanthanide if they are available. I have had a lot of trouble
myself with various neodymium sulphate/sulphate complexes. kmno4 - 17-6-2014 at 03:52
Perhaps it was mentioned in the rest of the paper?
I think nezza is correct.Brain&Force - 17-6-2014 at 09:42
I doubt this is much of a help, but praseodymium exhibits multiple oxidation states. Some of it may have oxidized.
If you can, try dissolving some of the white stuff in hydrochloric acid and report what you get. ESPECIALLY if you get the smell of chlorine.Bezaleel - 17-6-2014 at 13:18
Thank you all for your answers and suggestions! Nezza's answer pointed me to the possible existence of a basic sulphate. In order to
test this, I mixed a spatula full of the precipitate in a test tube half filled with demiwater and added close to 1 ml of 36% HCl solution. The liquid
became opalescent and turned completely clear after some shaking. The addition of a few drops of BaCl2 solution immediately gave a very fine
precipitate.
The greenish precipitate could therefore be Pr(OH)SO4. Since Ba(OAc)2 is well soluble, BaSO4 is the only insoluble substance that could be formed
here.
About the article: kmno4 is right, proving I shouldn't be doing any analysis at midnight.... (It says they re-precipitated the acetate by adding 25%
ammonia solution, and filtering again - probably in order to remove as much as possible of the original anion.)
@Brain&Force: I doubt the existance of Pr(IV) in watery solution. Also in acidic environments, Pr6O11 (formally containing 1 atom of Pr in the +IV
oxidation state) splits off oxygen.Brain&Force - 17-6-2014 at 16:10
I had already known about basic sulfates of neodymium as well...I don't know how I missed that. Higher oxides of praseodymium tend to be dark and
nonstoichiometric.
When I was filtering out terbium as the sulfate a lot of it seeped through the Büchner funnel I was using. Adding excess oxalate to the filtrate
caused a massive precipitation of terbium oxalate to occur. I did convert the terbium sulfate to the hydroxide without any issues and it redissolved
without difficulty (it did absorb a large amount of carbon dioxide, though). My source of hydroxide was household ammonia, but I'm not sure what the
concentration was.blogfast25 - 18-6-2014 at 12:00
Nice way of demonstrating the presence of basic sulphates, Bezaleel.
