The better way to dissolve CeO2 is using HCl. The dissolving behavior very much depends on the history of the CeO2. CeO2 annealed at high temperatures dissolves really very slowly. Irradiated CeO2 (in a nuclear reactor) also may dissolve much more slowly. The other lanthanide oxides (Ln2O3) are usually dissolved significant faster.    

The dissolution behavior of CeO2 in sulfuric acid was examined using a batch reactor with various acid concentrations (8–14 M) at different temperatures (75–130°C). The dissolution of CeO2 was slow and it took more than 3 days to dissolve 4 mmol of a CeO2 powder completely in 100 mL of 12 M sulfuric acid at 90°C. The dissolution rate increased with increases in the temperature and the sulfuric acid concentration.

http://link.springer.com/chapter/10.1007/978-4-431-53910-0_22

The better way to dissolve CeO2 is using HCl. The dissolving behavior very much depends on the history of the CeO2. CeO2 annealed at high temperatures dissolves really very slowly. Irradiated CeO2 (in a nuclear reactor) also may dissolve much more slowly. The other lanthanide oxides (Ln2O3) are usually dissolved significant faster.    

Read: http://link.springer.com/chapter/10.1007/978-4-431-53910-0_22#page-1

Hi Dénes,

In fact, several works reports the CeO2 dissolution in different acids solutions such as HCl, H2SO4, HNO3 etc. However, it seem to be so hard to  completely dissolve CeO2 when we aim using room temperature without pressure controlling. Then, for entire dissolution of  CeO2 in acid solution, you can easily to that by using aqua fortis (H2O2 + HNO3).  If you use a mixture of H2O2 and HNO3 (1:1 ratio by volume), definitively you will get CeO2 completely dissolved at ambient conditions (If you are not an expert in lab, please take a time to learn how you may proceed to hand dangerous chemical).

Good luck!

As stated above, the "history" of  cerium oxide play a very important role. Especially, the preparatives annealed at high temperatures are very difficult to dissolve. The best way is to use a mixture of strong mineral acid with some reducing agent - this is also the case of nitric acid with hydrogen peroxide (acting here as reducing agent). Maybe surprisingly, a very good dissolving agent for cerium dioxide is hydrobromic acid. A mixture of HCl with KI works in a similar way. We have published some papers on this topic in the mod of 1980s, unfortunately hardly available today.

Actually, you can dissolve ceria in concentrated phosphoric acid. The rate of dissolution depends strongly on ceria particle size.

As discussed above, dissolution of CeO2 is tricky and to a large extent depends on the production history of the chemical. However, our observation too has been that dissolution can be achieved using a combination of HNO3 + H2O2. The details are recorded in the following research article

https://www.researchgate.net/publication/239271088_Prospects_and_problems_in_the_production_of_143_Pr_for_radionuclide_therapy_applications

Article Prospects and problems in the production of 143 Pr for radio...

Could you please elaborate on the procedure of dissolving CeO2 with a mixture of HNO3 and H2O2. I tried dissolving CeO2 with a mixture of HNO3 and H2O2 as stated above but my attempt remained unsuccessful. Thank you in advance.