Does it protect against oxidation, if I solve it again in naptha after purification?

I suppose that will work, but you gotta think that you still gonna need to evap it when you want to use it.

There is no harm in storing the spice in a glass vial in the freezer, this will last for years with very little oxidation, or convert to fumarate wich should last for donkeys years.

Check through the FAQ, loads of info there on storage.