Page 67 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

The stability of metal-EDTA complexes may be altered (a) by
variation in pH and (b) by the presence of other complexing agents.

(a) pH effect
Since the actual complexing species is Y4-, complexes will form
more efficiently and be more stable in alkaline solution. If however,
the solubility product of the metal is low, it may be precipitated if
the hydroxyl ion concentration is increased too much. On the other
hand, at lower pH values when the concentration of Y4- is lower, the
stability constant of the complexes will not be so high. So, the
stability of a given chelate increases as pH increases.

Table 3: Conditional formation (stability) constant for CdY2-

pH Conditional
formation constant

2 1.1×103

5 1.0×1010

7 1.4×1013
9 1.5×1015

10 1.0×1016

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.

Table 4: Stability with respect to pH of some metal-EDTA complexes

Minimum pH at Selected metals
which complexes
66
exist