In a qualitative analysis when ${\mathrm{H}}_2\mathrm{S}$ is passed through the solution of a salt acidified with $\mathrm{HCl}$, a black precipitate is obtained. On boiling the precipitate with dil. ${\mathrm{HNO}}_3$, it forms a solution of blue colour. Addition of an excess of an aqueous solution of ammonia to this solution will give ________.