5.10: Effects of EDTA on End-Point Detection Methods
Different methods, such as visual observance of metal-ion indicators, spectroscopic techniques, and potentiometric methods, can determine the endpoint of an EDTA titration.
In the visual method, metal-ion indicators (metallochromic dyes), which have distinct colors in their free and complex forms, are added to the mixture to signal the titration's end point. They form stable complexes with metal ions, but these complexes are weaker than the corresponding metal–EDTA complexes. As a result, EDTA will displace the indicator to form a more stable metal–EDTA complex as the endpoint is reached.
Since the color of the free indicators also depends on pH, titration must be performed in the appropriate pH range.
If the titration solution is already intensely colored or the endpoint color change is subtle, the visual method might not be precise enough. In such cases, the spectroscopic method is employed. Here, the endpoint is detected by monitoring the absorbance of the mixture at a particular wavelength.
Another method is potentiometric titration, which detects the endpoint by measuring the change in the potential of the metal ion during the titration using an appropriate electrode for the metal.
