In order to distinguish between ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{NH}}_2$ and ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NH}}_2$ which of the following reagents is useful?

Hinsberg test is used distinguish between primary, secondary and tertiary amines. Both the given compound are primary amines. When Aniline $\left(C_6{H}_{5}N{H}_2\right)$ is treated with $HN{O}_2$ it undergoes diazotization and forms diazonium salt. It give azodye when treated with $\beta$ - Naphthol. 

When ethylamine reacts with $HN{O}_2$ , it does not undergo diazotisation but gives alcohol, which does not yield azodye on treating with $\beta$ - naphthol.