Chapter 5: Complexometric Titration, Precipitation Titration, and Gravimetry

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5.7:

EDTA: Direct, Back-, and Displacement Titration

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Analytical Chemistry

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JoVE Core Analytical Chemistry

EDTA: Direct, Back-, and Displacement Titration

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5.2: Complexometric Titration: Ligands

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5.9: Complexometric EDTA Titration Curves

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EDTA titration includes direct, back, displacement, indirect, and alkalimetric titration. In a direct titration, a metal ion solution is directly titrated with EDTA—for example, a lead(II) ion solution is buffered at pH 10, with tartrate already present to prevent lead(II) hydroxide precipitation. This solution is directly titrated with standard EDTA until the end point is reached. However, some metal ions react too slowly or block the indicators, and some precipitate without EDTA. So, a known excess of EDTA solution is added and back-titrated with a second metal ion. For example, after mixing an aluminum ion solution with excess EDTA, the remaining EDTA is back-titrated with a visual indicator and a standard zinc ion solution to reach the end point. For metal ions that don't have a good indicator, displacement titration can be used. For instance, titration of calcium ions with an excess magnesium–EDTA solution displaces magnesium to form a stable calcium–EDTA complex. The displaced magnesium ions are titrated with standard EDTA to reach the end point.

5.7:

EDTA: Direct, Back-, and Displacement Titration

The EDTA titration types for metal ion analysis include direct titration, back-titration, and replacement titration.

Direct titration involves buffering the metal ion solution to the desired pH and directly titrating with standard EDTA until the endpoint. The optimum pH ensures a large conditional formation constant of metal−EDTA and visibility of the free indicator color in the solution. In addition, auxiliary complexing reagents are used to prevent the precipitation of metal hydroxides and maintain the concentration of free metal ions in the solution.

Back titrations are useful when the metal ions block the indicators, react slowly, precipitate, or form inert complexes. This titration method adds a known excess of EDTA solution to the metal ion, and the solution is buffered to a desired pH. The excess EDTA is then back-titrated using a suitable indicator with a standard solution of a second metal ion to reach the endpoint.

Displacement or substitution titration is used when metal ions do not react adequately with the indicator. Since solochrome black is a poor indicator for directly titrating calcium ions, the calcium ions are first titrated with less stable excess [Mg(EDTA)]²⁻solution. The calcium ions displace all the magnesium ions to form a more stable [Ca(EDTA)]²⁻complex. The free magnesium ions released into the solution are equivalent to the calcium ions previously present. Finally, the free magnesium ion is titrated with a standard EDTA solution to obtain the endpoint.