Equilibrium constants for the dissociation reactions H,L2-+ H+ + HL3and HL3-+ H' + L4-, where H,L represents di(2-aminoethoxy)-ethanetetra-acetic acid (EGTA) have been determined by a potentiometric method a t 5, 15, 25, and 35", and a t an ionic strength of 0 . 1 ~. Calorimetric measurements have been made of the heats of formation of 1 : 1 complexes of EGTA with the metal ions, MgZt, Ca2+, Sr2+, Ba2+, Zn2+, and Cd2+, and the data have been combined with known stability constants to give the corresponding entropy changes. The thermodynamic functions for the formation of the alkaline-earth complexes are discussed and compared with similar data for other aminocarboxylate complexes.T H E great interest in the aminocarboxylate ions lies in their ability t o form very stable complexes with a wide variety of metal ions. This has resulted in their frequent use as analytical reagents for the determination of metal ions in solution and, for successful application, a ligand should show a large difference in stability of its complexes with a series of closely similar cations. Much of the work, stimulated by the application of the ligandfield theory, has been done with the transition metal complexes, particularly with the EDTA molecule, whereas the alkaline-earth cations have received less attention. The latter form a particularly suitable series for study and, if the bonding were purely electrostatic, the order of stability would be expected to follow the inverse order of ionic radii, viz., K M g L > KCaL > KSrL > ITBaL. Although the last three members invariably conform to this sequence, the association constant for MgL is frequently considerably smaller than that predicted on the basis of simple ionic bonding. Thus, the stability constants of the 1 : 1 complexes of di-(2-aminoethoxy)-ethanetetra-acetic acid (EGTA) with Ca2+ (log K = 10.93), Sr2+ (log K = 8-45), and Ba2+ (log K = 8.32) ion are similar to those of the EDTA complexes, whereas the value for Mg2+ (log K = 5.29) is smaller by a factor of lo6 than that for Mg EDTA2-. I n order to be able to discuss the reasons for the observed orders of stability, it is desirable to know the heat and entropy as well as the free energy changes accompanying the association reactions. From a consideration of free energy data alone, the anomalous behaviour of magnesium chelates has often been attributed to the difficulty of a multidentate ligand to fold around the small cation sufficiently closely for all its donor atoms to be bound. This would result in fewerbonds being made to the magnesium ion, with the replacement of a smaller number of water molecules from its co-ordination shell. The relatively large and positive entropy changes for such reactions however, indicate that other factors may be important.In order to discuss in more detail the important factors involved in the association reactions, precise calorimetric heat changes have been obtained for the association of the EGTA ion with the alkaline-earth and some other divalent metal ions.
EXPERIMENTALThe CaZorivneter.--The...
