LnEDTA complexes

The important difference between the dissociation mechanism of LnMEDTA complexes and the acid catalyzed dissociation of LnEDTA is the formation of mixed LnMEDTA-Ac" complex. The stability constants of such mixed complexes are small (1 to 10) and this may explain why such mixed complexes do not show up in the kinetics of dissociation or exchange of LnEDTA complexes. Further, in the present case [LnMEDTA]0 association with an acetate anion should be more favourable than [LnEDTA]- with an acetate anion based on electrostatic theory. Reference to Table 7.14 shows that Km values increase with increasing atomic number (decreasing radii) for heavy lanthanides but are independent for La-Nd series. 

Figure 2. Log K, the formation constant, of LnEDTA complexation vs Atomic Number. A smooth curve has been drawn through the more accurate data potentiometric values for the lighter lanthanides and polaragraphic/KNOs values for the heavier lanthanides. The dotted line represents polaragraphic/KCl data. Adapted from ref. 19.

When the oxide is in excess and the ligand concentration is small, a hydroxylated form of the complex is formed. Species of the type Ln2(OH)2EDTA or Ln5(OH)7(EDTA)2 have been obtained by the addition of NaOH to LnEDTA solutions [39]. Reversed-phase chromatography may be used in identification of various species in solution [40]. 

Mixed complex formation kinetics between LnEDTA and L, where L stands for 5-sulfosalicylate [74], 8-hydroxy quinoline-5 sulfonate and picolinate [75] were studied and two pathways namely acid-dependent and acid-independent pathways have been identified. The overall reaction stoichiometry may be written as... 

---