= Solution
\b[Not as a general aqueous separation reaction for the other <lanthanides>.] Most strongly favour the trivalent state in ordinary water chemistry. Ce(IV) is unusually accessible because it has the empty $4f$ shell and is strongly stabilized by oxide formation. Some other <lanthanides> have accessible alternative <oxidation states>, such as Eu(II), and higher oxides of Pr or Tb can exist under suitable conditions; this does not imply that they undergo the same Ce(III)/Ce(IV) pH-controlled aqueous oxidation.
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