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.
