Oxalic acid is the simplest dicarboxylic acid and is a reducing agent. The conjugate base, oxalate, is a chelating agent for metal cations. Oxalic acid is found naturally in foods, but it can be harmful in larger amounts.
Oxalic acid is widely used for cleaning, bleaching, mordant, stain remover, and rust remover. It is effective for rust removal due to the formation of a stable water-soluble salt, ferrioxalate ion. Oxalic acid functions as a reducing agent for metal oxides, which is effective in removing tarnish, rust, and ink stains. Oxalic acid has applications as a mordant in the dyeing industry.