100 g of phthalic acid is poured into 250 mL beaker, start heating close to 200°C on a hot plate with some mechanical stirring until the mixture turns completely clear indicating of water loss and conversion to the anhydride, place an ice cold RBF on top of the beaker, it collects all of the sublimed phthalic anhydride. Allow it to cool with the RBF on, otherwise, the anhydride will sublime out. Recrystallize in warm acetone.
Start to heat, some water will evaporate
RBF was added to the top of the beaker, the reaction is complete because the acid has melted.
Some of the sublimed crystals