# $NH_4CNS$ can be used to test one or more out of $Fe^{3+}, Co^{2+}, Cu^{2+}$

(a) $Fe^{3+}$ only
(b) $Co^{2+}, Cu^{2+}$
(c) $Fe^{3+}, Cu^{2+}$
(d) all

$CoCl_2 + 4NH_4CNS \overset{ether}{\longrightarrow} (NH_4)_2[Co(CNS)_4] \text{(Blue colour in ethereal layer)} + 2NH_4Cl$
$CuCl_2 + 4NH_4CNS\overset{ether}{\longrightarrow} (NH_4)_2[Cu(CNS)_4] \text{(Blue colour)} + 2NH_4Cl$
$FeCl_2 + 3NH_4CNS \overset{ether}{\longrightarrow} (NH_4)_2[Fe(CNS)_3] \text{(Blood red colour)} + 2NH_4Cl$
Thus, $NH_4CNS$ can be used to test all $Fe^{3+}, Co^{2+}, Cu^{2+}$ containing compounds.