Usual impurities of (CAS NO.) are traces of and other oxidisable substances and water. (Glacial acetic acid is very hygroscopic. The presence of 0.1% water lowers its m by 0.2°C.) Pwificd by adding some acetic anhydride to react with water present, heating for 1h to just below boiling in the presence of 2g CrO₃ per 100 mL and then fractionally distilling. Instead of CrO₃, 2-5% (w/w) of KMnO₄, with boiling under reflux for 2-6h, has been used.

Traces of water have been removed by refluxing with tetraacetyl diborate (prepared by warming 1 part of boric acid with 5 parts (w/w) of acetic anhydride at 60°C, cooling, and filtering off), followed by distn.

Refluxing with acetic anhydride in the presence of 0.2g % of 2-naphthalenesulfonic acid as catalyst has also been used. Other suitable drying agents include CuSO₄ and chromium triacetate: P₂O₅ converts some acetic acid to the anhydride. Azeotropic removal of water by distn with thiophene-free benzene or with butyl acetate has been used. An alternative purification uses fractional freezing.

Rapid procedure: Add 5% acetic anhydride, and 2% of CrO₃. Reflux and fractionally distil.