The coordination reaction of EDTA
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:) :) :) In a Mg2+-containing solution with pH=10, EDTA is added for titration. Is the reaction equation Mg2+ + HY3- = MgY2- + H+ (1) or Mg2+ + Y4- = MgY2- (2)? The book states equation (1); why is that? Isn’t it only Y4- that can directly coordinate with metal ions?
This is related to the degree of dissociation of EDTA; you can calculate it using its Pka to determine what form EDTA exists in at a pH of 10. It does not necessarily need to be completely dissociated from the metal ion; the stability constant of the complex it forms with the metal is much higher than that of its acidic form, and the metal ion can take the place of hydrogen ions.