Browse Questions

# Potassium cyanide is used for separating

(a) $Co^{2+}$ and $Ni^{2+}$
(b) $Mn^{2+}$ and $Zn^{2+}$
(c) $Ba^{2+}$ and $Ca^{2+}$
(d) $Ca^{2+}$ and $Cd^{2+}$

Answer: $Co^{2+}$ and $Ni^{2+}$

When heated with KCN solution $Ni^{2+}$ forms potassium nickelcyanide $K_2[Ni(CN)_4]$ and $Co^{2+}$ forms potassium cobaltocyanide, $K_2[Co(CN)_4]$ on treatment with NaOH and $Br_2$ water, $K_2[Co(CN)_4]$ is oxidised to very stable potassium cobalticyanide $K[Co(CN)_4]$ and does not yield $Co^{3+}$ ions while potassium nickelocyanide being unstable, decomposes to black nickel oxide giving rise to Ni ions.
$2K_2[Ni(CN)_4] + 4NaOH + [O] \longrightarrow Ni_2O_3 + 4NaCN + 2H_2O + KCN$