Air-free sublimation

Apparatus

a = Rubber cone (typically used to form a vacuum seal in a Büchner flask filtration) which is selected so as to fit snugly around the neck of the Schlenk tube

b = Schlenk tube

c = Gas/vacuum inlet

d = Teflon tap (or stopcock)

e = Syringe

Method steps

1 = Impure solid to sublime (brown) is placed in a Schlenk tube, avoiding contaminating the sides of the tube (e.g. by careful evaporation from a solution containing the brown solid).

2 = Rubber cone (black) is pushed near top of the Schlenk tube (forming a tight seal around flask) and filled with a coolant such as dry ice/acetone (blue/white) to cool the neck of the Schlenk tube. The bottom of the Schlenk tube is heated (red shading) under vacuum (blue arrow), so that the impure solid (brown) sublimes as a pure solid (purple) at the cooled neck area (blue shading).

3 = After sublimation, the cooling-cone (black) is removed, leaving the concentrated impurity as a residue (dark brown) in the bottom of the flask, and the purified sublimed solid (purple) at the neck of the Schlenk tube.

4 = Solvent (blue) is inserted via syringe to dissolve the residue (green/brown), taking care to avoid washing off the sublimed solid (purple).

5 = Residue solution is then removed by syringe (green/brown) (Steps 4 and 5 can be repeated as required).

6 = Purified sublimed solid (purple) is washed off the neck of the flask with fresh solvent (blue) via a syringe.

(In steps 3 to 6 the flask is held under an atmosphere of an inert gas via Schlenk tube side arm).

7 = Solvent is removed under vacuum to give the purified sublimed solid (dark purple).