THERMOMETRIC TITRATION STUDIES OF DIVALENT METAL-NITRILOTRIACETATE COMPLEXATION REACTIONS

The complexation reactions of nitrilotriacetic acid (NTA) and divalent metal ions were studied using the thermometric titration method. NTA was titrated with 1M solutions of Cu(ll), Zn(ll), Ni(ll), and Co(ll), and the complexation reactions were studied as a function of pH. Titration curves obtained in acid media were endothermic; however, as the NTA solution became more basic the curve showed first an exothermic portion followed by an endothermic portion. The two end point nature of the titration curve indicated that sequential reactions occurred. Because only HNTA2- and NTA3- species exist in the pH region 8 to 12, this suggested the exothermic portion represented the metal(ll)-NTA3- reaction. The endothermic portion then represented the metal(ll)-HNTA2- reaction and the observed heat was endothermic because the heat of deprotonation was greater than the heat of complexation. Further indication that the NTA ion species was responsible for the unusual results was obtained using a substitution reaction. This was accomplished by first forming the MgNTA- complex before a subsequent replacement titration with zinc(ll). This gave a solution with a single reactive species, MgNTA-, and the resulting titration curve was exothermic with a single end point. © 1969, American Chemical Society. All rights reserved.