nitrilotriacetate(3-) tricarboxylic acid dianion nitrilotriacetate(1-) NTA nitrilotriacetate(2-) hydrogen 2,2',2''-nitrilotriacetate CHEBI:39056 189.123 C6H7NO6 Hnta(2-) 189.02734 0 chebi_ontology