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I dissolved a zirconia ceramic containing rhodium with sulfuric acid, i tought this process will dissolve only zirconia ZrO2 at 100 °C and lower but it turned out that rhodium in form of powder can dissolve also.
I Proceeded in 3 way's the last one turned brown (no water were added) compared to the first and second one's (red; note here i diluted with water from the begining).

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    Are you sure rhodium is not present as a fine suspension in the solution ? Have you tried to filtrate it? After Greenwood and Earnshaw, Chemistry of the Elements,p. 1292, Rhodium is not soluble in aqua regia. The only way of dissolving it is to fuse it with $\ce{NaHSO4}$. The behavior of rhodium in presence of acids is qualified as "extreme inertness". – Maurice Jan 31 '24 at 20:19
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    @Maurice https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejic.201601247 – DavePhD Feb 01 '24 at 00:26
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    @DavePhD But there's 400 °C not 100. – Mithoron Feb 01 '24 at 02:21
  • @Mithoron this https://pubs.acs.org/doi/10.1021/ja01624a009 (cited in the above article) says 0 °C and dilute sulfuric acid to form a yellow sulfate from the trihydroxide and heating that to 100 °C yields a red sulfate. – DavePhD Feb 01 '24 at 03:58
  • @DavePhD But from hydroxide, not metal. – Mithoron Feb 01 '24 at 13:33
  • @Tintin. The video shows that rhodium is attacked by concentrated sulfuric acid if the reaction is maintained $80$ hours (more than $3$ days) at $435$°C. This is more or less equivalent to dissolve it in fused $\ce{NaHSO4}$, as I stated it $20$ hours ago. . – Maurice Feb 01 '24 at 16:40
  • @ªTintin. According to the reference, rhodium is dissolved in sulfuric acid at 435°C in a sealed ampule. The inside pressure is not given. If sulfuric acid gets decomposed à 320°C at atmospheric pressure, its decomposition is probably prevented at higher pressure.Now metallic rhodium may be separated by filtration. – Maurice Feb 01 '24 at 17:30
  • Please explain how you have obtained the seven figures. Fig. 1 looks an emulsion of a black stuff. How was it obtained ? Fig.2 is a colorless solution with a "mirror" on the bottom of the flask. Fig. 3 is an orange-brown solution. How was it obtained ? Fig .4 shows two plastic bottles with colorless but poorly defined content. Fig. 5 is a colorless precipitate in a colorless solution. How was it obtained ? Fig. 6 is a dark precipitate. Fig. 7 is a colorless precipitate in a pale yellow solution. How were all these solutions and precipitates obtained? Explain ! – Maurice Feb 03 '24 at 17:00
  • @Tintin. You aren't clear. You state that Fig. $1$ is "where you did not dilute" the acid. What is this "where" ? Is it the result of the action of sulfuric acid on zirconia ? It should be mentioned. You state that Fig. 7 is the remain slurry of it". So the remain slurry is white !! How come if the liquid is black ? You state that Fig.2 is the same solution. Well. Which same solution ? Is it the black liquid from Fig.1 ? What is this mirror ? All the other figures are equally unclear. I will not describe my trouble for all. It would take too much time. – Maurice Feb 07 '24 at 12:45

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