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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5$\begingroup$ 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". $\endgroup$– MauriceCommented Jan 31 at 20:19
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1$\begingroup$ @Maurice chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/… $\endgroup$– DavePhDCommented Feb 1 at 0:26
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1$\begingroup$ @DavePhD But there's 400 °C not 100. $\endgroup$– MithoronCommented Feb 1 at 2:21
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$\begingroup$ @Mithoron this 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. $\endgroup$– DavePhDCommented Feb 1 at 3:58
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$\begingroup$ @DavePhD But from hydroxide, not metal. $\endgroup$– MithoronCommented Feb 1 at 13:33
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