(nitrilotrimethanediyl)tris(phosphonic acid), {ethane1,2diylbis[nitrilobis(methylene)]}tetrakis(phosphonic acid), [(bis{2[bis(phosphonomethyl)amino]ethyl}amino)methyl]phosphonic acid, {hexane1,6diylbis[nitrilobis(methylene)]}tetrakis(phosphonic acid), {[(2hydroxyethyl)imino]bis(methylene)}bis(phosphonic acid), and [(bis{6[bis(phosphonomethyl)amino]hexyl}amino)methyl]phosphonic acid; salts thereof