Complex Formation
Complexometric titrations
In complex formation, a ligand (L) binds a metal ion (M) to form a complex (ML). The formation constant Kf quantifies complex stability.
Kf = [ML] / ([M] × [L])
High Kf values indicate a strongly favored complex. In complexometry (e.g., EDTA titrations of Ca²⁺/Mg²⁺), Kf strongly affects endpoint sharpness and analytical precision.
- Water hardness analysis (Ca²⁺, Mg²⁺)
- Trace metal analysis with EDTA
- Dependence on pH and competing ligands
Formulas
Kf = [ML] / ([M] × [L])
[ML] = Kf × [M] × [L]
Complexation degree: α = [ML]/[M]total
Detailed examples
Example 1: Kf calculation
[ML]=0.0090; [M]=0.0010; [L]=0.0010
Kf=0.0090/(1e-6)=9000
[ML]=0.0090; [M]=0.0010; [L]=0.0010
Kf=0.0090/(1e-6)=9000
Example 2: EDTA-like case
Kf=10000; [M]=0.0010; [L]=0.0010
[ML]=0.0100 mol/L (strongly complexed)
Kf=10000; [M]=0.0010; [L]=0.0010
[ML]=0.0100 mol/L (strongly complexed)
Example 3: Complexation degree
[ML]=0.0090; [M]total=0.0100
α=0.90 (90% complexed)
[ML]=0.0090; [M]total=0.0100
α=0.90 (90% complexed)
Interpretation
Higher Kf leaves less free metal ion in solution, often producing steeper, easier-to-detect titration endpoints.
Higher Kf leaves less free metal ion in solution, often producing steeper, easier-to-detect titration endpoints.
Technical Background
Role of pH
Many ligands (e.g., EDTA) are protonation-dependent. Effective complex formation therefore requires buffered pH conditions for reproducible titration behavior.
Selectivity and interferences
Foreign ions may act as competing metals or ligands. In practical analysis, masking agents are often added to improve selectivity for the target ion.
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