$\mathrm{CoS}$ (black) obtained in group IV of salt analysis is dissolved m aqua regia and Is treated wit excess of ${\mathrm{NaHCO}}_3$ and then ${\mathrm{Br}}_2$ water. An apple green colored stable complex is formed. It is :

$\mathrm{CoS}$ reacts with aqua regia o form cobalt chloride.

$3\mathrm{CoS}+6\mathrm{HCl}+2{\mathrm{HNO}}_3\to 3{\mathrm{CoCl}}_2+2\mathrm{NO}+3\mathrm{S}+4{\mathrm{H}}_2\mathrm{O}$

Cobalt chloride reacts with excess of sodium bicarbonate to form

${\mathrm{Na}}_2\mathrm{Co}{\left({\mathrm{HCO}}_3\right)}_4$.

${\mathrm{CoCl}}_2+4{\mathrm{NaHCO}}_3\to 2\mathrm{NaCl}+{\mathrm{Na}}_2\mathrm{Co}{\left({\mathrm{HCO}}_3\right)}_4$

reacts with bromine water to form sodium cobalticarbonate.

${\mathrm{Na}}_2\mathrm{Co}{\left({\mathrm{HCO}}_3\right)}_4\stackrel{\text{ bromine water }}{⟶}\mathrm{NaCo}{\left({\mathrm{CO}}_3\right)}_2+{\mathrm{NaHCO}}_3+1.5{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$