oxalic acid Oxalic acid is an organic substance with the chemical formula of C It is a metabolite of organisms. It is a binary weak acid, which is widely distributed in plants, animals and fungi and plays different functions in different organisms. It is found that more than 100 kinds of plants are rich in oxalic acid, especially spinach, amaranth, sugar beet, purslane, taro, sweet potato and rhubarb. Because oxalic acid can reduce the bioavailability of mineral elements, it is easy to form calcium oxalate with calcium ions in human body, resulting in kidney stones, oxalic acid is often considered as an antagonist for the absorption and utilization of mineral elements. Its anhydride is carbon trioxide.