EDTA (Ethylenediaminetetraacetic acid) is a hexadentate chelating agent.

• Application: Specifically used to measure the Total Hardness of water, which is caused by the presence of $C{a}^{2+}$ and $M{g}^{2+}$ ions.
• Indicator: Eriochrome Black T (EBT) is used. In the presence of metal ions, EBT forms a wine-red complex. As EDTA is added, it 'steals' the metal ions from the EBT. Once all metal ions are complexed by EDTA, the indicator returns to its free blue color, signaling the end point.