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Edta (pronounced E. D. T. A.) is short for ethylenediaminetetra-acetic acid. The displayed formula shows that there are six available lone pairs for dative bond formation with a central metal ion. The edta molecule surrounds the central metal ion in a sort of pincer movement - hence the namechelating ligand (from the Greek for crab's claw).
The complexes of edta are particularly stable and the solution is used in complexometric titrations. These can be used to determine the quantity of calcium and magnesium ions in hard water. The stability of the edta complex is a result of its hexadentate nature. When a ligand exchange reaction occurs only one edta molecule is needed, and it often displaces up to six other ligands. This results in a large increase in entropy, and it is this which is the driving force behind the reaction:
|
Co(H2O)62+(aq) +
edta(aq) |
In the above example two species turn into seven, resulting in a large increase in DSsystem.
Co(edta)2+(aq) + 6H2O(l)