[Yeti12]

Hello everyone,

first i have to apologize for my bad english, if there's a german part of this forum please send me there. 

Now to my questions, i have a seperation process for Cations (first to fourth main group cations).

I have proved that i have Pb2+ , Na+ and Al3+ in my sample. The Assitant of this internship said that i have 4 cations in this sample and that the last is very difficult because it's difficult to solve it (get the cation in solution).
So my tests gave me some answers and that this sample can't be solved by acids like Sulfuric acid (H2SO4).
Then i got a tipp that this thing is as difficult as MgO and it could be SnO2 or an alkaline earth sulfate (for example something like BaSO4).
If i use aqua regia and if i boil it then the stuff can be solved, but whem it cools  i have the solid stuff again.
It's a white solid.

My question is if you have any idea what it could be.