(a) A freshly prepared ferrous sulphate solution is used, because on exposure to the atmosphere, it is oxidised to ferric sulphate, which will not give the brown ring test.
(b) The brown ring of nitroso ferrous sulphate is formed at the junction of the two liquids because the conc. sulphuric acid being heavier settles down and the ferrous sulphate layer remains above it resulting in the formation of a brown ring at the junction.