Distilling some fresh, absolute (water and oxygen free) tetrahydrofuran from sodium and benzophenone.
Traces of water/oxygen/other dissolved gases and impurities are most commonly removed from etheral solvents by refluxing them with a little benzophenone and metallic sodium or potassium. From benzophenone (diphenyl ketone) and alkali metals benzophenone ketyl radical forms what has an intense blue color in etheral solvents and it’s quite reactive.
What happens here? Ph2CO + Na -> Na[Ph2CO]
And what is that ketyl thing? This:
R and R’ is a phenyl in this case and next to the negative charge somewhere there is a sodium ion.
When is the THF pure? When it’s deep blue, so a lot benzophenone ketyl radical is in the solution.