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How does EDTA remove calcium and magnesium ions from hard water?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

EDTA (ethylenediaminetetraacetic acid) removes calcium and magnesium ions from hard water through a process known as chelation.1

As a polydentate ligand, EDTA acts as a chelating agent by attaching to a central metal ion at multiple points, forming a stable, ring-like coordination compound. Specifically, EDTA functions as a hexadentate ligand, meaning it can form six bonds with a single metal ion, such as calcium.2

When added to hard water, EDTA molecules bind to the dissolved calcium and magnesium ions, effectively "locking them up" within these stable molecular complexes. Once sequestered in this manner, the metal ions no longer exhibit their typical ionic properties, which prevents them from interfering with detergents or causing the buildup associated with hard water. This inactivation of the ions effectively softens the water.1