Oxalic acid. Molecular model of oxalic acid, a dicarboxylic acid, which is one of the strongest organic acids because of the joining of two carboxyl groups (COOH) at centre (grey). Oxalic acid is naturally found in some foods, such as cracked pepper, rhubarb stalks and spinach. However, it can combine with metal ions in the body to deposit crystals of the corresponding oxalates (for example calcium oxalate, better known as kidney stones) and therefore must be consumed in moderation. Atoms are colour-coded: carbon (grey), oxygen (red) and hydrogen (white).