The disodium salt of EDTA is preferred in hardness estimation mainly because it is more soluble in aqueous solutions which allows release of the EDTA ligand into solution to react.

Edetate disodium injection forms chelates with the cations of calcium and many divalent and trivalent metals. Because of its affinity for calcium, edetate disodium will produce a lowering of the serum calcium level during intravenous infusion. 

EDTA salts are more soluble in water as the pH increases: the more EDTA there is in the salt form, the higher the pH of a water solution, and therefore, the higher the room temperature solubility.

EDTA DISODIUM SALT N/50 SOLN