The principal of disodium EDTA (ethylenediaminetetraacetic acid) is its ability to chelate metal ions, forming stable complexes that prevent them from participating in chemical reactions. This chelating property is due to the presence of six donor atoms (two nitrogen atoms and four oxygen atoms) in the EDTA molecule, which can coordinate with metal ions to form octahedral complexes.

The stability of the metal-EDTA complexes depends on the charge and size of the metal ion, as well as the pH of the solution. In general, EDTA forms stronger complexes with metal ions that have a higher charge and smaller ionic radii. The pH also affects the chelating ability of EDTA, as the protonation of the EDTA molecule can reduce its affinity for metal ions.

Disodium EDTA is commonly used as a chelating agent in a variety of applications, including:

* Water softening: EDTA is used to remove calcium and magnesium ions from hard water, preventing the formation of scale in pipes and appliances.

* Metal ion analysis: EDTA is used to determine the concentration of metal ions in solutions by titrating the solution with a known concentration of EDTA until the metal ions are completely chelated.

* Pharmaceutical formulations: EDTA is used as a chelating agent in some pharmaceutical formulations to improve the solubility and stability of metal-containing drugs.

* Food preservation: EDTA is used to chelate metal ions that can catalyze the oxidation of fats and oils, thereby extending the shelf life of food products.

The chelating properties of disodium EDTA make it a valuable tool in a wide range of applications, from water treatment to pharmaceutical formulations.