Oxalic acid: A familiar and dangerous acid "Oxalic acid" is an acid contained in a considerable variety of plants. It is abundant in Chenopodiaceae (Okahijiki, Akaza), Purslanes (Purslanes), Polygonaceae (Rumex japonicus, buckwheat, Japanese knotweed), Amaranthaceae (Byam, Amaranthus), Wood sorrels …