錯数測定による EDTA 滴定曲線

An EDTA titration curve plots the free metal ion concentration (p function) against the volume of EDTA added. The curve displays three distinct regions: before the equivalence point where excess metal ions remain, at the equivalence point where metal and EDTA concentrations are equal, and after the equivalence point where excess EDTA exists. This visualization helps determine metal ion concentration changes during complexometric titration.

The conditional formation constant (Kf′) directly determines the sharpness of the break at the equivalence point. A larger Kf′ produces a sharper, more pronounced break because the metal ion concentration decreases more dramatically. For example, the Ca–EDTA complex has a larger Kf′ than the Sr–EDTA complex, resulting in a steeper curve break. The conditional formation constant is pH-dependent, so curve shape varies with solution pH.

pH affects the conditional formation constant of the metal–EDTA complex, which controls curve shape. At higher pH, the Ca–EDTA complex formation is more favorable, producing a larger Kf′ and a sharper break at the equivalence point. At lower pH, complex formation is less favorable, resulting in a smaller Kf′ and a less pronounced break. This pH dependence means the same metal ion produces different titration curves at different pH levels.

After the equivalence point, free metal ion concentration is calculated using the conditional formation constant of the metal–EDTA complex. At this stage, EDTA is in excess and slight dissociation of the complex occurs. The Kf′ value allows chemists to determine how much metal ion remains free in solution despite the presence of excess EDTA, which is essential for accurate titration analysis.

Calcium and strontium form EDTA complexes with different conditional formation constants at the same pH. The Ca–EDTA complex has a larger Kf′ than the Sr–EDTA complex, making the calcium titration curve exhibit a sharper, more dramatic break at the equivalence point. This difference in complex stability directly reflects the favorability of complex formation between each metal ion and EDTA.

Before the equivalence point, excess free metal ions are present in solution because not all metal has been complexed by EDTA yet. The free metal ion concentration remains relatively high and decreases gradually as more EDTA is added. This region of the titration curve reflects the ongoing complexation process before stoichiometric equivalence is reached.

The favorability of metal–EDTA complex formation directly determines titration curve steepness at the equivalence point. When complex formation is highly favorable (large Kf′), the metal ion concentration drops sharply, creating a steep curve break. When complex formation is less favorable (small Kf′), the metal ion concentration decreases more gradually, producing a gentler curve. This relationship allows analysts to predict curve behavior from complex stability data.