How is $\left({\mathrm{H}}_2\mathrm{S}\right)$ used as an analytical reagent?

Analytical reagent:

• It is a substance or compounds added to a system to cause a chemical reaction, or test if one occurs.

Hydrogen sulphide $\left({\mathrm{H}}_2\mathrm{S}\right)$ is used to determine basic radicals:

• Basic radicals such as elements of $\mathrm{Pb},\mathrm{Cu},\mathrm{Hg},\mathrm{Cd},\mathrm{Bi}$.
• If these radicals react with reagents like ${\mathrm{H}}_2\mathrm{S},\mathrm{HCl},{\mathrm{NH}}_4\mathrm{OH},{\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3$ they produce corresponding Sulphides, Chlorides, Hydroxides, and carbonates.
• Example:- $\left({\mathrm{H}}_2\mathrm{S}\right)$ react with given basic radicals and produces corresponding precipitates (ppt) like
• $\mathrm{PbS}$-black ppt
• $\mathrm{CuS}$-black ppt
• $\mathrm{HgS}$-black ppt
• $\mathrm{CdS}$-Yellow ppt
• ${\mathrm{Bi}}_2{\mathrm{S}}_3$-black ppt
• For example, the formation of lead(II) sulphide by using the basic radical and $\left({\mathrm{H}}_2\mathrm{S}\right)$.

$\underset{\mathrm{Lead}\left(\mathrm{II}\right)\mathrm{nitrate}}{\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}\mathrm{sulphide}}{{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)}\to \underset{\underset{(\mathrm{Black}\mathrm{ppt})}{\mathrm{Lead}\left(\mathrm{II}\right)\mathrm{sulphide}}}{\mathrm{PbS}\left(\mathrm{s}\right)}+\underset{\mathrm{Nitric}\mathrm{acid}}{2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}$

Therefore, hydrogen sulphide is mainly used for the detection of metal cations (basic radicals) present in metal salts.