Phthalic anhydride purification process

A process for purifying crude phthalic anhydride obtained by the oxidation of naphthalene with oxygen containing gases such as air comprises heating the crude phthalic anhydride in the presence of a phthalate of potassium and in absence of any substantial amount of oxygen at a temperature below 250 DEG C. and below the boiling point of phthalic anhydride and thereafter distilling the phthalic anhydride. The potassium phthalate may be formed by the addition of potassium hydroxide or another potassium compound e.g. potassium carbonate which will react with phthalic anhydride or phthalic acid to the crude phthalic anhydride employed as starting material. The examples relate to the purification in a nitrogen atmosphere of crude phthalic anhydride obtained by the catalytic vapour phase air oxidation of naphthalene over a vanadium catalyst.