Dear Blanka,

I would think that the addition of petroleum ether is to ensure a clear separation between the aqueous and organic phases. Sometimes it is difficult to separate chloroform from water because of the presence of emulsifying agents or chemical entities that can interact with both phases; to solve there are two possibilities: to add a brine solution or  an anti-solvent such as petroleum ether.

Hoping this will be helpful,

Rafik

Dear Rafik,

Thank you for your answer, it was helpful!

Blanka