Maleic acid

Maleic acid (molecular formula: C4 H4 O4), also called cis-butenedioic acid or toxillic acid, is the simplest unsaturated dicarboxylic acid. Its salts are called Male(in)ate. There is a cis-trans isomerism to fumaric acid. In contrast to fumaric acid, maleic acid does not occur in nature.

Maleic acid forms colorless crystals at room temperature. The carboxylic acid is highly soluble in water and in ethyl alcohol. The acid dissolves less well in acetone and diethyl ether. After UV irradiation, the maleic acid isomerises to form the much more stable fumaric acid.

Maleic acid and its salts have a nephrotoxic effect on oral intake. In case of local skin application they may have sensitizing effects. In the technical field maleic acid is used for the production of polymers and synthetic resins. In combination with citric acid the acid also serves as a decalcifying agent.

Cosmetics: Maleic acid is used as buffer substance in cosmetic formulations under the INCI designation "acid maleic".

Furthermore, maleic anhydride (MA) is widely used in cosmetics.