# In a combination of $NO_3^-, Br^-$ and $I^-$ present in a mixture, $Br^-$ and $I^-$ interfere in the ring test for $NO_3^-$. These are removed by adding a solution of

(a) $AgNO_3$
(b) $Ag_2SO_4$
(c) $Ag_2CO_3$
(d) None of these

Answer: $Ag_2SO_4$

$2Br^- + Ag_2SO_4 \longrightarrow 2AgBr \downarrow + SO_4^{2-}$
$2I^- + Ag_2SO_4 \longrightarrow 2AgI \downarrow + SO_4^{2-}$
Adding $AgNO_3$ will introduce $NO_3^-$ ions in the solution. $Ag_2CO_3$ is insoluble in water. Thus, $AgSO_4$ solution must be added in order to remove $Br^-$ and $I^-$ ions.