Ethylenediaminetetra-acetic acid chelate complexes

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

The complexing ability of ethylenediaminetetra-acetic acid (EDTA, H4Y) has been exploited in the coulometric titration of metal ions. The method depends on the reduction of the mercury(II) or cadmium chelate of EDTA and on the titration of the metal ion (for example, magnesium) to be determined by the anion of EDTA that is released. 

The number of monodentate groups such as hydroxide, halide, water molecule, etc., bonded to a central metal ion, usually equates to its coordination number. However, a remarkable feature occurs if two or more of these ligand donor groups are united, possibly by a short chain of hydrocarbons there will be a markedly enhanced preference for forming such bidentate complexes with the transition metal ion, and indeed in the ultimate, it is possible to go through tri- to tetra- to penta-to hexadentate chelate ligands such as EDTA (ethylenediaminetetra-acetic acid) which is widely described in this book. 

Complex formation is used extensively in water analysis. The determination of hardness, Ca and Mg concentration, employs the com-plexation of these metals with the chelating agent ethylenediaminetetra-acetic acid (EDTA) the titrimetric finish of the chemical oxygen demand (COD) test uses the complexing agent 1,10-phenanthroline to detect the presence of Fe and so indicate the endpoint chloride analysis by. the mercurimetric method depends on the formation of the complex HgCl taq) between Hg " and the chloride ion. 

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