Complexometric titration also known as chelatometry is a type of volumetric analysis that uses the formation of a colored complex to identify the titration's endpoint. Complexometric titrations are very effective for determining the metal ions by use of complex-forming reactions.
The endpoint of the titration is commonly detected using an indicator such as Eriochrome Black T (EBT) capable of producing a noticeable color change. Direct titration, back titration, and displacement titration are the types of complexometric titration.
Why EDTA is used in complexometric titration?
Ethylenediaminetetraacetic acid (EDTA) has four carboxyl groups and two amine groups, all of which can behave as electron pair donors or Lewis bases. EDTA is a hexadentate ligand as it can donate its six lone pairs of electrons to form coordinate covalent connections with metal cations. In practice, however, EDTA is only partially ionized, forming less than six coordination covalent connections with metal cations. In an aqueous solution, EDTA forms an octahedral complex with the majority of 2+ metal cations, M2+.
The major reason why EDTA is widely used in the standardization of metal cation solutions is that most metal cation-EDTA complexes have a relatively high formation constant, which means that the equilibrium for the reaction M2+ + H4Y MH2Y + 2H is far to the right.
Reacting in a basic buffer solution at pH 10.00 eliminates H+ as it allows the EDTA-metal cation complex to form the reaction product more easily. For most applications, the formation of metal cation EDTA complexes is considered complete, and this is what EDTA is mainly used for.
Why do we use only EDTA in complexometric titration and no other standard solutions what is the reason behind this?
EDTA is a hexadentate ligand that can easily form complexes/ chelates with practically any metal, and the stability is dependent on the pH of the solution so that titration can be performed preferentially in solution with one metal ion in the presence of the other.
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