In the brown ring test, the brown colour of the ring is due to

• A

  ferrous nitrate
• B

  ferric nitrate
• C

  a mixture of $\mathrm{NO}$ and $\mathrm{NO}_{2}$
• Correct answer
  D

  nitrosoferrous sulphate

Solution

The brown ring is due to the formation of a brown complex between nitric oxide and $\mathrm{Fe}^{2+}$ ions. $\mathrm{NO}_{3}^{-}+3 \mathrm{Fe}^{2+}+4 \mathrm{H}^{+} \longrightarrow 3 \mathrm{Fe}^{3+}+\mathrm{NO}+2 \mathrm{H}_{2} \mathrm{O}$ or

Hence, the correct answer is option: 'D'.