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EDTA is the commonly used abbreviation for (Ethylenedinitrilo) tetraacetic Acid (also called Ethylenediaminetetraacetic Acid or Edetic Acid).
The Disodium salt of EDTA is the most commonly used form of this versatile organic complexing agent, which is used for the determination of many metal ions either by direct titration or by back-titration. Adjustment of reaction conditions, either by controlling the pH or by use of masking agents, and selection of the appropriate indicator permits a wide variety of metal ions to be determined using EDTA.
Molarity is best used in expressing EDTA concentration, because the reaction involves the complexation of one metal ion by one EDTA molecule or ion, whether the free acid form of EDTA or one of its more soluble Sodium salts is used. The general reaction involving the Disodium salt is:
EDTA2- + Mn+ → [EDTA-M]+n-2
where M = Ca, Mg, Cd, Mn, Ni, Al, Zn, Bi, Co, Cu, or other metal ion
An indicator is added that forms a colored complex with some of the metal ion being determined. When the endpoint is reached, all of the free metal ion has been complexed by the EDTA. The last addition of EDTA then removes the metal ion from its complex with the indicator, producing a color change. The relative strength of the EDTA-metal ion complex compared to that of the indicator-metal ion complex determines whether a direct titration or a back-titration is applicable.