Unstable to air, light and acids. Impurities include formic acid, B-formylacrylic acid and furan-2-carboxylic acid. Distd over an oil bath from 7% (w/w) Na₂CO₃ (added to neuvalise acids, especially pyromucic acid). Redistd from 2% (w/w) Na₂CO₃, and then, finally fractionally distd under vacuum. It is stored in the dark.

Impurities resulting from storage can be removed by passage through chromatographic grade alumina. (CAS NO. ) can be separated from impurities other than carbonyl compounds by the bisulfite addition compound. The aldehyde is steam volatile.

It has been purified by distn (using a Claisen head) under reduced pressure. This is essential as is the use of an oil bath with temperatures of no more than 130 °C are highly recommended. When furfural is distd at atm press (in a stream of N₂), or under reduced pressure with a free flame (caution because the aldehyde is flammable) an almost colourless oil is obtained. After a few days and sometimes a few hours the oil gradually darkens and finally becomes black. This change is accelerated by light but occurs more slowly when kept in a brown bottle.

However, when the aldehyde is distd under vacuum and the bath temperature kept below 130°C during the distn, the oil develops only a slight colour when exposed to direct sunlight during several days. The distn of very impure material should NOT be attempted at atm pressure otherwise the product darkens rapidly. After one distn under vacuum a distn at atmospheric pressure can be carried out without too much decomposition and darkening. The liquid irritates mucous membranes. Store in dark containers under N₂.