Stability constant HEDTA

Figure 10.8 Effect of pH on the conditional stability constants at 25 °C of metal chelates of HEDTA [20]...

The charge on the R group also has a pronounced effect on the equilibrium constant of homolysis of complexes with metal-carbon -bonds. Comparing the stability constants of homolysis for (hedta)Fein-R for R= CH3 and CO, Table III (46), shows that the complexes for R = C02 are considerably more stable, than those formed with R= CH3 (46). This difference is attributed to the stronger electrostatic interaction between the central cation and the C02 ligand and probably due to some d- interaction (46). 

Table 36. Logarithms of stability constants of the mono-hydroxo rare earth HEDTA complexes [456]...

The stability constants for N/-(2-hydroxyethyl)ethylenediaminteri-acetic acid (HEDTA) [453], a closely related ligand to EDTA, and those of N, N -ethylenediaminediacetic acid (EDDA) [454] are compared in Table 34. A severe gadolinium break is observed in the case of both EDDA and HEDTA. 

The reaction of Pu with EDTA was investigated, and formation constants y5i=16.04, j3i=Ml were determined. Existence of mixed complexes Pu NTA EDTA of eomposition 1 1 1 was shown, with stability constant equal to logA=5.76 [49]. Thermodynamie eharacteristics for Am and Pu complexation with EDTA were ealeulated in the work [52]. The separation factor in the systems of Am or Cm, where 1 2 complexes with HEDTA are formed, is calculated to be equal to 7 [51]. Diethylentriaminepentaacetic acid (DTPA), which can serve as oetadentate ligand, forms more stable complexes. Values of stability eonstants for eomplexation of transplutonic elements Am-Fm are near the same and equal to 10 as was found by ion exchange technique [50]. The data on eomplexation of Am, Cm and Cf with different amine-N-polycarboxylie acids studied in the paper [46] are presented in Table 3. 

Equation 14 assumes that the fully ionized form of EDTA" is present in solution however, at low pH, other species will be present, that is, HEDTA, H2EDTA, and HaEDTA, and the undissociated H4EDTA. Thus, the stability constants become conditional upon pH. 

Conditional Stability Constants grading EDTA , HEDTA, HjEDTA, H3EDTA, H4EDTA... 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

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