Why does the complex [Fe(H2O)5NO]SO4 form a ring in between two layers in the brown ring test for nitrates

Question

for anyone who would be able to help me with an answer by knowing the procedure, here is the procedure for you:

we add the nitrate salt solution, freshly prepared ferrous sulphate solution and dil.H2SO4 in a test tube and shake well . then conc.H2SO4 is added slowly along the innersides of the test tube. after this the BROWN RING ( [Fe(H2O)5NO]SO4 ) APPEARS.

PS : this can also be done by using Na2CO3 extract of the nitrate salt.

Answer 1

the brown color forms where Fe+2  meets NO molecules

this hapens only at the interface between the more dense concentrated H2SO4 @ the bottom,   & the less dense Fe+2 / NO3)-1  water solution floating on top of the con H2SO4, (we do not mix the 2 solutions)

for only at the interface is NO produced in any noticable quatity,  because at that place H+ from the con H2SO4  reacts with NO3)-1 & Fe+2 to make the NO molecules:

Fe+2 & H+ & NO3)-1 --> Fe+3  & NO  & H2O

then the NO molecules forming @ the interface combine with some of the excess Fe+2  in the same vicinity to make the brown complex ion

Answer 2

iron

some ferrous or ferric salts, like rust (ferric oxide), are reddish brown colored

Answer 3

Conc H2SO4 has a density almost twice that of water. Hence forms a separate layer at the bottom. Unless we mix the two solutions they donot mix with each other.

NO is formed at the point at which the two solutions meet. This is because Conc H2SO4 can react with the nitrate present in the solution in contact with it only to give NO. This will in turn combine with FeSO4 present in solution side to give the brown colour. Because it is formed at the junction only, a ring is formed otherwise, if you shake the solution, you donot get the ring but the entire solution turns brown.