Complexometric titration, also known as chelatometry, is a kind of titration (volumetric technique) that employs a colored compound for the determination of a chemical reaction endpoint. It is the most effective technique that can be used in the pharmaceutical sector and other fields to identify the amount of a certain type of metal ion that is present in a sample solution.
For titrations of metal cations with EDTA, it is necessary the use a complexometric indicator like eriochrome black T, calcein, murexide, fast sulphon black, etc. to detect when the equivalence point/endpoint has been achieved.
The endpoint has been obtained when there is a color change, which indicates that the indicator has been displaced from the metal cations in the solution (often by EDTA). Thus, the free indicator (rather than the metal complex) serves as the endpoint indicator. In complexometric titrations, organic dyes are the most commonly used indicators.
Requirement for metal ion indicator:
A successful EDTA titration depends on an accurate estimation of the endpoint at which the reaction will stop. The following characteristics must be present in a metal ion indicator to be suitable for use in the visual detection of endpoints:
- The color reaction should occur before the endpoint, which occurs when almost all of the metal ions have complexed with EDTA.
- The indicator needs to have a color response that is specific or selective.
- The color must have an intensity that is high enough to allow the use of a minimum indicator.
- The indicator must be stable in the titration medium, and on storage.
- It should be easily available in the market with a sufficiently high purity level.
- The color change of the indicator should occur at a near equivalence point, and it should be particularly sensitive to metal ions.
- The color contrast between the indicator and metal-indicator complex should be easily observable.
- The Metal-indicator complex should be stable enough to make a clear color change, but less stable than the Metal-EDTA complex.
- The change in equilibrium from the metal-indicator complex to the metal-EDTA complex must be rapid.
- In the pH range where the proposed titration is to be done, all of the above conditions must be met.
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