and its associated formation constant
A representative list of formation constants defined on this basis is given in Table 5.8.
Table 5.8 Some selected metal-EDTA formation constants
Cation KMY log 10 KMY Cation KMY log 10 KMYAg+ 2 × 107 7.3 Cu2+ 6.3 × 1018 18.80Mg2+ 4.9 × 108 8.69 Zn2+ 3.2 × 1016 16.50Ca2+ 5.0 × 1010 10.70 Cd2+ 2.9 × 1016 16.46Sr2+ 4.3 × 108 8.63 Hg2+ 6.3 × 1021 21.80Ba2+ 5.8 × 107 7.76 Pb2+ 1.1 × 1018 18.04Mn2+ 6.2 × 1013 13.79 Al3+ 1.3 × 1016 16.13Fe2+ 2.1 × 1014 14.33 Fe3+ 1 × 1025 25.1Co2+ 2.0 × 1016 16.31 V3+ 8 × 1025 25.9Ni2+ 4.2 × 1018 18.62 Th4+^2 ×^1023 23.2These data may be used for the practical comparison of formation constants by use of the conditional
constant principle (Chapter 3, p. 40), i.e. is given by
which has been obtained by the combination of equations (5.10) and (5.15).
From data in Tables 5.7 and 5.8, the conditional constant may be calculated, e.g. for Mg2+ – EDTA at
pH = 5
and at pH = 10
Hence, pH = 10 could be used for the titration of magnesium but not pH = 5 (stoichiometric reaction
requires ).