Lion850
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Cerium oxide and orthophosphoric acid
I need help to understand what products I may have gotten!
I tried different ways to dissolve cerium oxide CeO2. Very hot concentrated sulphuric acid works, but I was hoping to find something at lower temp.
While searching online I saw a comment from a member of the Russian Academy of Science that orthophosphoric acid can be used to activate CeO2.
I decided to react 20g of cerium oxide with 20g of 85% orthophosphoric acid. The acid was poured into a small beaker and 10g cerium oxide added, and
stirred while being slowly heated. The mixture bubbled and formed a pale yellow putty. It was not possible to add the other 10g of oxide, it was too
solid.
I added 150ml water to see if it would dissolve. Boiling resulted in a pale yellow-green solution with a large amount of pale yellow ppt.
The water was decanted off, the ppt washed and then dried in air. It was still quite wet 12 hours later and it was then placed on a steam bath for
some 4 hours. After this it seemed very dry, a pale yellow mass of hard pieces. See photo showing the product with some of the original oxide for
comparison of the color. The final weight of the product was 16.5g.
I am not sure what this is. The below equation gives the closest weight to what I got, having started with 10g CeO2:
3CeO2 + 4H3PO4 = 1Ce3(PO4)4 + 6H2O
However I cannot find a lot of info online about cerium phosphates. I did some tests on the product with acids:
1g was placed in a test tube with 6ml 50% sulphuric acid. There was immediately a strong yellow solution but it seems a lot of the cerium phosphate
was unreacted. The was not heated.
1g was placed in a test tube with 35% hydrochloric acid. A brown-red solution quickly formed. I suspect this could have been CeCl4 that as far as I
know is only known to fleetingly exist in solutions. There was also a faint smell of chlorine. I placed the test tube in a beaker of boiling water,
there was a reaction with bubbles and the solution first went yellow and then clear, with a fine white ppt. I assume the clear solution could be
CeCl3, but what is the white ppt?
1g was placed in a test tube with 4ml of 70% HNO3. There was a yellow solution, and when the test tube was placed in boiling water nitrous oxide brown
gas was released and the solution became much paler yellow and again with a fine white ppt.
In the above reactions, what happened to the phosphorus originally present in the acid?
I then reacted 1.8g of cerium oxide with 3.5g orthophosphoric acid, but I left it on the hotplate at full heat for more then 30 minutes. A darker
yellow product formed, which even after 30 minutes of heating at more then 200C was still a bit sticky (when hot) and still occasionally popping of a
bubble. There is no smell. See photo of the beaker with the darker yellow stuff. I'll try to check the weight in the morning.
Any ideas will be much appreciated.
  
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SWIM
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I can at least say something about the last reaction.
You added excess phosphoric acid and the bubbling when you tried to dry it was probably orthophosphoric acid giving up water to become meta or poly or
some other dehydrated phosphoric acid derivative.
A mixture of phosphoric acids will give up water when heated strongly and get thicker and thicker until it's solid even at high temperatures.
It can't be distilled or evaporated.
Hot phosphoric acid also etches just about everything it doesn't outright dissolve, so dehydrating phosphoric acid is very hard on your glassware.
As for the other tests, I probably know much less than you about cerium salts.
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Bedlasky
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Phosphates, oxalates, hydroxides, oxides and fluorides of lanthanoids and actinoids are insoluble (tetravalent are less soluble then trivalent). So I
think that you made cerium(IV) phosphate which is maybe still mixed with CeO2, which explains solid residue on the bottom of test tubes.
Ce(IV) forms with hydrochloric acid [CeCl6]2- complex. Ce(IV) forms complexes with almost anything. With HNO3 [Ce(NO3)6]2- is formed (nitrate is
bidentate ligand), with H2SO4 forms [Ce2(SO4)8]8-. Complex formation strongly affects oxidizing properties of Ce(IV). In HClO4 E° = 1,7V, in HNO3
1,61 V, in H2SO4 1,44V and in HCl 1,28V.
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Lion850
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Gents thank you for the help.
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