Heat 3 mL of water and 10 mL of conc. sulfuric acid to 60°C, then add 4 g of terbium and mechanically stir few times. When all of the terbium has reacted, cool down and place the solution into a freezer for 5h to crystallize out the terbium sulfate. Filter and recrystallize in hot water.
2 Tb + 3 H2SO4 = Tb2(SO4)3 + 3 H2
The reaction taking place