Ligand used in the estimation of hardness of water

The hardness of water is due to the presence of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions in it. The hexadentate ligand EDTA forms stable complexes with ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions and can be used for the estimation of hardness of water by simple titration method. Further, of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions, each can be selectively estimated by this method because of the difference in the stability constants of their EDTA complexes.