5.9 : Complexometric EDTA Titration Curves

EDTA titration curves determine the free metal ion concentration. The titration curve represents the change in concentration of free metal ions (p function) as a function of the volume of EDTA added. This curve consists of three regions: before, at, and after equivalence points. Excess free metal ions are present before the equivalence point. Equal concentrations of metal ions and EDTA are present at the equivalence point. After the equivalence point, excess EDTA exists. This means slight dissociation can be observed at and after the equivalence point.

The complex's conditional formation constant (K_f′) calculates the free metal ion concentration at and after the equivalence point, and the shape of the titration curve is affected by K_f′ of the complex. For example, the Ca–EDTA complex has a larger K_f′ than the Sr–EDTA complex. As a result, the Ca–EDTA titration curve has a larger break at the equivalence point.

The K_f′ of the complex depends on the pH of the solution. For instance, Ca–EDTA exhibits various shapes at different pH. At higher pH, Ca–EDTA has a larger K_f′, and complex formation is more favorable. The curve has a large break at the equivalence point. At lower pH, the K_f′ of the complex is small, indicating less favorable complex formation. As a result, the curve has a small break at the equivalence point.