[Pico]

What is the periodic trend in Ln stability, and how can the Ln3+ ions be separated from each other?

[kriggy]

Basicaly all of them are stable in +III oxidation state. +IV and +II are also quite common. Do you have any specific compounds in mind?
Separation is difficult because they have very similar chemical properties. Fraction crystalization was used in a past (I heard that one guy did over 1500 crystalizations to get the pure element). Today extraction or chromatography is used. Look at wikipedia its all there - unless you want specifics, then I suggest some peer-rewieved journals or other literature (maybe chemistry of elements?)