and α 4 is seen to depend on the Ka values and the pH only.
The overall variations of solution composition are summarized in Figure 5.5 and Table 5.7, from which
it will be seen that the major species present in the operating pH range of 2–10 are H 2 Y^2 – and HY^3 –.
Figure 5.5
The variation of α values with pH for EDTA species.The Formation and Stability of Metal-EDTA Chelates
EDTA forms stable complexes with a wide range of metal ions. The exceptional stability is conferred
by the large number of donor groups and the subsequent isolation of the metal ion in the cage-like
structure. It is clear from the foregoing section that complex formation will be pH dependent and may
be represented by different equations depending on the pH and the ionic form of the EDTA. However,
for purposes of comparison it is best to use the equilibrium
Table 5.7 Values of α 4 for EDTA as a function of pH
pH α 4 pH α 42.0 3.7 × 10 – (^14) 7.0 4.8 × 10 –^4
3.0 2.5 × 10 – 11 8.0 5.4 × 10 –^3
4.0 3.6 × 10 – (^9) 9.0 5.2 × 10 – 2
5.0 3.5 × 10 – 7 10.0 3.5 × 10 –^1
6.0 2.2 × 10 – (^5) 11.0 8.5 × 10 –^1
12.0 9.8 × 10 –^1