Complexometric titration

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Complexometric titration is a type of titration based on complex formation between the analyte and titrant. Complexometric titrations are particularly useful for determination of a mixture of different metal ions in solution. An indicator with a marked color change is usually used to detect the end-point of the titration.

Any complexation reaction can in theory be applied as a volumetric technique provided that:

1. the reaction reaches equilibrium rapidly following each addition of titrant.
2. interfering situations do not arise (such as stepwise formation of various complexes resulting in the presence of more than one complex in solution in significant concentration during the titration process).
3. a complexometric indicator capable of locating equivalence point with fair accuracy is available

In practice, the use of EDTA as a titrant is well established.

[edit] Complexometric titration with EDTA

EDTA, ethylenediamminetetraacetic acid, has four carboxyl groups and two amine groups that can act as electron pair donors, or Lewis bases. The ability of EDTA to potentially donate its six lone pairs of electrons for the formation of coordinate covalent bonds to metal cations makes EDTA a hexadentate ligand. However, in practice EDTA is usually only partially ionized, and thus forms fewer than six coordinate covalent bonds with metal cations. Disodium EDTA, commonly used in the standardization of aqueous solutions of transition metal cations, only forms four coordinate covalent bonds to metal cations at pH values less than or equal to 12 as in this range of pH values the amine groups remain protonated and thus unable to donate electrons to the formation of coordinate covalent bonds.

In analytical chemistry the shorthand "Na₂H₂Y" is typically used to designate disodium EDTA. This shorthand can be used to designate any species of EDTA. The "Y" stands for the EDTA molecule, and the "H_n" designates the number of acidic protons bonded to the EDTA molecule.

EDTA forms an octahedral complex with most 2+ metal cations, M²⁺, in aqueous solution. The main reason that EDTA is used so extensively in the standardization of metal cation solutions is that the formation constant for most metal cation-EDTA complexes is very high, meaning that the equilibrium for the reaction:

M²⁺ + H₄Y → MH₂Y + 2H⁺

lies far to the right. Carrying out the reaction in a basic buffer solution removes H⁺ as it is formed, which also drives the reaction to the right. For most purposes it can be considered that the formation of the metal cation-EDTA complex goes to completion, and this is chiefly why EDTA is used in titrations / standardizations of this type.

To carry out metal cation titrations using EDTA it is almost always necessary to use a complexometric indicator, usually an organic dye such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B or Murexide, to determine when the end point has been reached. These dyes bind to the metal cations in solution to form colored complexes. However, since EDTA binds to metal cations much more strongly than does the dye used as an indicator the EDTA will displace the dye from the metal cations as it is added to the solution of analyte. A color change in the solution being titrated indicates that all of the dye has been displaced from the metal cations in solution, and that the endpoint has been reached.

[edit] See also

• Titration