Chapter 5: Complexometric Titration, Precipitation Titration, and Gravimetry

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5.10:

Effects of EDTA on End-Point Detection Methods

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Analytical Chemistry

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JoVE Core Analytical Chemistry

Effects of EDTA on End-Point Detection Methods

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5.2: Complexometric Titration: Ligands

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5.11: Masking and Demasking Agents

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End-point detection in EDTA titration includes visual and instrumental methods. A visual metal-ion indicator, which forms a stable complex with a metal ion to signal the end point by a color change, is also called a metallochromic indicator. Metallochromic indicators are usually organic dyes and are useful if the indicator forms an intensely colored complex with the metal ion in a different color from the free indicator. This metal–indicator complex must be weaker than the metal–EDTA complex, but the reversible shift of the reaction must be rapid to see an abrupt color change. Since metallochromic indicators are usually acid–base indicators, the free indicator color changes with the pH of the solution. So, titration with a metallochromic indicator requires suitable pH ranges. If the color change at the end point is difficult to detect visually, spectrophotometric methods can monitor the absorbance to identify the equivalence point. Alternatively, with some metal ions indicator electrodes can be used to measure the change in potential signaling the end point.

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.