Ethylenediaminetetraacetic acid (EDTA) is a tetracorboxylic ligand that is widely used to determine metals in complexometric titration as it forms stable complexes with most metal ions. Complexometric can be defined as an analysis of a volume whereby the formation of coloured complex is used to indicate the end point of a titration. EDTA can be shown or represented as H4Y and when it is in alkaline conditions, it exists as Y4- ions. The Y4- form 1:1 complexes with metal ions like nickel (II) ions:
Y4- + Ni2+ NiY2-
A metal iron indicator which is an organic dye that changes colour when bound with metal iron, it shows the end of an EDTA complexometric titration. Nevertheless, for an ion metal indicator to be suitable in an EDTA titration, it must not bind strongly with metal ions as EDTA does. Murexide is therefore suitable.