Simultaneous reactions thermodynamic stability conditions

If we consider how easily the [Fe (EDTA)] complex forms as a function of the experimental conditions, in particular and overall as a function of pH, we realize that an acidic medium favors its formation in accordance with the last reaction. This is not necessarily true, because, simultaneously, the acidic medium induces the protonation of EDTA, as relations (25.12)-(25.15) indicate. Therefore, it is difficult to predict which of the two effects is more important. Only the stability constant knowledge of all the superimposing equilibria allows this prediction, at least on thermodynamic grounds. To reinforce these considerations, weTl mention the fact that the [Fe(EDTA)] complex is also a diacid since it may accept two hydroxide ions (it is a Werner diacid) ... 

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