Page 86 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 ANDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

For other cations, the reactions may be expressed as:

One mole of the complex-forming H2Y2- reacts in al1 cases with one mole of
the metal ion and in each case, also, two moles of hydrogen ion are formed. It
is apparent from equation (O) that the dissociation of the complex will be
governed by the pH of the solution; lowering the pH will decrease the stability
of the metal-EDTA complex. The more stable the complex, the lower the pH
at which an EDTA titration of the metal ion in question may be carried out.
Table 2.3 indicates minimum pH values for the existence of EDTA complexes
of some selected metals.

Table 2.3 Stability with respect to pH of some metal-EDTA complexes
Minimum pH at which Selected metals
complexes exist
1-3 Zr4+; Hf4+; Th4+; Bi3+; Fe3+
4-6 PbZ+; CuZ+; znZ+; CoZ+; NiZ+; MnZ+; FeZ+; Al3+; CdZ+; SnZ+
8-10 CaZ+; SrZ+; Ba2+. M 2+
, g

It is thus seen that, in general, EDTA complexes with metal ions of the charge
number 2 are stable in alkaline or slightly acidic solution, whilst complexes with
ions of charge numbers 3 or 4 may exist in solutions of much higher acidity.

2.27 STABlLlTY CONSTANTS OF EDTA COMPLEXES
The stability of a complex is characterised by the stability constant (or formation
constant) K:

Some values for the stability constants (expressed as log K) of metal-EDTA
complexes are collected in Table 2.4: these apply to a medium of ionic strength
I=0.1 at 20°C.

Table 2.4 Stability constants (as log K) of metal-EDTA
complexes