Oxalic acid is the chemical compound with the formula C2O2(OH)2 orHOOCCOOH. This colourless solid is a relatively strong carboxylic acid, being about 3,000 timesstronger than acetic acid. The dianion, known as oxalate, is a reducing agent as well as a ligand formetal cations. Many metal ions form insoluble precipitates with oxalate, a prominent examplebeing calcium oxalate, the primary constituent of the most common kind of kidney stones.Typically oxalic acid is obtained as the dihydrate.