nity produced by extensive hydrolysis in solutions of the tetrasodium salt. The four electron-rich acetate
groups together with the two nitrogen lone pairs constitute a sexadentate ligand which will form
complexes with octahedral geometry (Figure 5.4).
Figure 5.4
A proposed structure for a
metal-EDTA chelate showing its
octahedral geometry.Although it is probable that only four bonds are formed in many complexes, the cage-like structures
effectively prevent the formation of complexes other than those with 1:1 stoichiometry (Chapter 3).
This feature is of considerable analytical importance.
The Composition of Aqueous EDTA Solutions
Being a tetrabasic acid, EDTA dissociates in solution to give four different ionic species, H 3 Y–, H 2 Y^2 – ,
HY^3 – , Y^4 – , the relative amounts of which will depend upon the pH of the solution. The proportion of
any species present is represented by its α value, an idea introduced in Chapter 3.
where CL is the total amount of uncomplexed EDTA, given by
It is convenient to evaluate α 4 , which may be done by substituting into equation (5.11) from expressions
for the dissociation constants K 1 , K 2 , K 3 , K 4
whence (5.10) becomes