5.9:
Complexometric EDTA Titration Curves
An EDTA titration curve plots the metal ion concentration as the p function against the EDTA volume.
Before the equivalence point, the free metal ion is in excess. After the equivalence point, EDTA is in excess.
At and after the equivalence point, the free metal ion concentration is calculated using the conditional formation constant of the metal–EDTA complex.
The shape of the titration curve is influenced by the favorability of forming the complex. Consider the EDTA titration curves of calcium and strontium at pH 10.
A larger conditional formation constant corresponds to a larger decrease in the metal ion concentration, making a sharper break at the equivalence point.
Since the conditional formation constant is pH-dependent, so is the shape of the curve. Consider the titration of calcium with EDTA at various pH levels.
At a lower pH, the formation of the calcium–EDTA complex is less favorable, producing a less sharp break.
A higher pH favors the formation of the calcium–EDTA complex, providing a sharper break at the equivalence point.
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 (Kf′) calculates the free metal ion concentration at and after the equivalence point, and the shape of the titration curve is affected by Kf′ of the complex. For example, the Ca–EDTA complex has a larger Kf′ than the Sr–EDTA complex. As a result, the Ca–EDTA titration curve has a larger break at the equivalence point.
The Kf′ 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 Kf′, and complex formation is more favorable. The curve has a large break at the equivalence point. At lower pH, the Kf′ of the complex is small, indicating less favorable complex formation. As a result, the curve has a small break at the equivalence point.