In chelation chemistry, a stability constant (often written as K or log K) quantifies how tightly a ligand (the chelator) binds a metal ion at equilibrium — mathematically, it’s the ratio of bound complex to free metal and free ligand, so a higher log K means the complex is far more favored (for example, a log K of 20 reflects vastly tighter binding than a log K of 10 — each unit is a ten-fold difference).
Affinity is the practical expression of that binding strength: how strongly and selectively the chelator “prefers” one metal over others under real physiologic conditions (which are influenced by pH, competing minerals, redox state, and protein binding). In clinical or environmental detox contexts, a chelator with a high stability constant and appropriate selectivity will more effectively capture its target metal and hold onto it long enough for excretion — but excessively high affinity without selectivity can also strip essential minerals, which is why both the magnitude of the stability constant and the biological context matter.
Below is a clean ranking of EDTA stability constants (log Kf) ordered correctly highest → lowest, using commonly cited aqueous stability constants (approximate values; oxidation state matters).
EDTA Binding Affinity (log Kf)
Highest → Lowest
- Bi³⁺ (Bismuth) — ~27
- Hg²⁺ (Mercury) — ~21.8
- Ga³⁺ (Gallium) — ~21.0
- Ni²⁺ (Nickel) — ~18.6
- Pb²⁺ (Lead) — ~18.0
- Pt²⁺ (Platinum) — ~18.0
- Sn²⁺ (Tin) — ~18 (oxidation dependent)
- Gd³⁺ (Gadolinium) — ~17.3
- Cd²⁺ (Cadmium) — ~16.5
- Al³⁺ (Aluminum) — ~16.1
- UO₂²⁺ (Uranyl) — ~16–18 (depends on speciation)
- Sb³⁺ (Antimony) — ~20 (data variable; often lower under physiologic conditions)
- Ba²⁺ (Barium) — ~7.8
- Tl⁺ (Thallium) — ~6–7
- Cs⁺ (Cesium) — ~1–2
- Rb⁺ (Rubidium) — ~1–2
- As³⁺ / As⁵⁺ (Arsenic) — weak/variable with EDTA
- W⁶⁺ (Tungsten) — weak/limited reliable EDTA data
Important Technical Notes
Log Kf values depend heavily on oxidation state.
Trivalent and divalent heavy metals bind much more strongly than monovalent alkali metals.
Conditional stability constants at physiologic pH can differ from absolute formation constants.
