HEDP·Na₂ (disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid) acts upon a broad range of metal ions, with a particularly strong affinity for calcium.
Primary Target Ions
Calcium (Ca²⁺): This is the ion HEDP·Na₂ most notably targets. It forms stable six-membered ring chelates with calcium ions in water systems, which is the basis for its excellent scale inhibition properties.
Iron (Fe), Copper (Cu), and Zinc (Zn): HEDP·Na₂ can form stable complexes with these common metal ions and is also capable of dissolving metal surface oxides.
Other Ions It Can Chelate
Research indicates HEDP (the parent acid) and its salts can also interact with:
Trivalent Cations: Such as Al(III), Cr(III), and Fe(III).
Divalent Cations: Including Mg²⁺, Fe²⁺, and Zn²⁺ (with chelation constants generally lower than for trivalent ions).
Other Metals: It may also chelate with Co, Ni, Pb, Sn, and Sb.
Important Note on Selectivity
While HEDP·Na₂ binds many ions, its behavior in complex water systems can be selective. One study notes that in mineralized waters, HEDP primarily complexes calcium and may not significantly interfere with the speciation of heavy metals like Cu or Zn (compared to stronger ligands like ATMP).