When concentrated HCI is added to an aqueous solution of ${\mathrm{CoCl}}_2$ its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

${\left[{\mathrm{CoCl}}_4\right]}^{2-}$ gives blue colour when concentrated hydrochloric acid is added to the aqueous solution of cobalt chloride.

Cobalt chloride, sometimes known as ${\mathrm{CoCl}}_2$, is a powder with a reddish pink colour and a strong odour. Cobalt chloride in an aqueous solution turns from reddish pink to deep blue when sufficient hydrochloric acid is added.

We have a cobalt chloride aqueous solution. Cobalt ion hydration takes place in aqueous. The chloride ions from the concentrated hydrochloric acid subsequently react with this hydrated cobalt ion.

The reaction is as follows

${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}+4{\mathrm{Cl}}^{-}\stackrel{\text{ Heat }}{⟶}{\left[{\mathrm{CoCl}}_4\right]}^{2-}+6{\mathrm{H}}_2\mathrm{O}$