194. Discuss briefly the role of coordination compounds in analytical chemistry.

Analytical chemistry:

(i) Multidentate ligand, EDTA (ethylene diamine tetra acetic acid) forms highly stable complexes with metal ions like Ca²⁺ and Mg²⁺. This fact is used to estimate the hardness of water by a simple titration method using EDTA solution.

(ii) A confirmatory test for the detection of copper(II) involves the formation of a deep-blue coloured complex, [Cu(NH₃)₄]²⁺, on addition of ammonia solution to a solution of copper(II) salt.

${\mathrm{Cu}}^{2+}+4{{\mathrm{NH}}_3}^{+} \to \underset{\underset{(\mathrm{Deep} \mathrm{blue})}{\mathrm{Tetra} \mathrm{amine} \mathrm{copper} \left(\mathrm{II}\right) \mathrm{ion}}}{ {\left[\mathrm{Cu}({\mathrm{NH}}_3{)}_4\right]}^{2+}}$

(iii) The separation of group I precipitate of AgCl, Hg₂Cl₂ and PbCl₂ involves in addition of aqueous ammonia solution to the precipitate, when only silver chloride dissolves, due to the formation of the complex ion, [Ag(NH₃)₂]⁺

AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl^–

Hg₂Cl₂ and PbCl₂ do not form complex ions with NH₃ and hence, do not dissolve.

(iv) A confirmatory test for nickel consists in adding a solution of dimethyl glyoxime, when a scarlet-red coloured precipitate is formed, due to the formation of a chelate complex. .