The formation of complex ions is widely used in chemical analysis, for example in complexometric titrations.

The anion of ethylenediaminetetraacetic acid (edta for short) forms very stable complexes with metal ions such as Ca²⁺.

[INSERT LEE_DU_01_ima]

edta can form six dative bonds to the calcium ion with the formation of a cage-like chelate structure. 

[INSERT LEE_DU_02_ima]

At the beginning of an edta titration, an indicator such as Eriochrome Black T is added to the metal ion solution in a conical flask. The indicator forms a red complex with the metal ions. As edta solution is added from a burette a ligand exchange reaction occurs in which the indicator ligand in the metal ion complex is replaced by the edta ligand. At the end point of the titration, all the indicator has been exchanged and the blue colour of the uncomplexed indicator can be seen:

[metal(indicator)] + edta _∏ [metal(edta)] + indicator

                                                (red)                                                    (blue)

Titrations are also covered in the Analytical chemistry course and titration calculations are covered in the Quantitative chemistry course, both of which you can subscribe to.