Formation Reactions of Metal-EDTA Chelates

We have already seen that since H4EDTA is a tetraacid, several reactions of formation of the same complex with the central metallic ion M + may occur. They are (see Chap. 25) 

According to the medium pH value, one of these reactions or even several ones occur to form the complex. This explains why the apparent stability constants of the metal-EDTA chelates may vary considerably with the pH value. 

The examination of the ehelate formation reaetions given above shows that the more acidic the medium is, the less the eomplex with EDTA tends to form. Inversely, the more stable the chelate is, the more aeidie the medium in which it will be formed may be. For example, according to the stability of the chelate, whether or not the titration of a metallic ion is possible depends on the solution s pH. Table 28.2 mentions the minimal pH range values at which chelates may still form, in particular the pH values at which titrations with EDTA may still be carried out. A precise pH value cannot be given since the chelate formation is also conditioned by factors other than the pH value, such as the initial concentrations of the different species entering in the formation reaction, for example. 

Roughly, complexes with the divalent ions are stable in slightly acidic or basic media, whereas complexes with tri- or tetravalent ions can be formed in more acidic media and, of course, in alkaline media. 

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