An organic compound contains C, H, N, S and Cl. For the detection of chlorine, the sodium extract of the compound is first heated with a few drops of concentrated HNO₃ and then AgNO₃ is added to get a white ppt of AgCl. The digestion with HNO₃ before the addition of AgNO₃ is
 

$\mathrm{Na} + \mathrm{C} + \mathrm{N}\stackrel{∆}{\to }\mathrm{NaCN}$
$2\mathrm{Na}+\mathrm{S}\stackrel{∆}{\to }{\mathrm{Na}}_2\mathrm{S}$
Both NaCN and Na₂S will react with AgNO₃ to form AgCN (white ppt.) and Ag₂S (black ppt.)
They must be removed before performing AgNO₃ test for halogens
Thus, on adding HNO₃, NaCN and Na₂S

$$\begin{gathered} \mathrm{NaCN} + {\mathrm{HNO}}_3 \to {\mathrm{NaNO}}_3 + \mathrm{HCN} \uparrow \\ {\mathrm{Na}}_2\mathrm{S} + 2{\mathrm{HNO}}_3 \to 2{\mathrm{NaNO}}_3 + {\mathrm{H}}_2\mathrm{S} \uparrow \end{gathered}$$
Hence, CN^– and S^2– are removed as HCN and H₂S by adding HNO₃.