EDTA chelating agent, molecular model. EDTA (ethylenediaminetetraacetic acid) is used as a treatment for heavy metal poisoning, for instance by mercury or lead. Its structure wraps around and binds to (chelates with) the toxic metal ions, allowing them to be excreted by the body. It is also used for treating hypercalcaemia, excess calcium in the blood, as it removes calcium ions by the same method. As calcium ions are required for clotting, it is used in haematology as an anticoagulant, although it cannot be used in vivo for this purpose as it removes too much calcium, which can be fatal. Atoms are colour-coded: carbon (grey), hydrogen (pale blue), oxygen (red) and nitrogen (dark blue).