Oxalic Acid is the chemical compound formula H2C2O4. It is a strong organic acid. Oxalic acid exhibits many of the reactions characteristic of other carboxylic acids. Oxalic acid solutions are used to lighten and brighten dingy gray wood. This is a highly corrosive and toxic substance. Oxalic acid is widely used as an acid rinse in laundries. Oxalic acid is a poisonous & colorless substance.