Iron `(+II)` is one of the most important oxidation states, and salts are called ferrous salts. Most of `Fe(+II)` salts are pale green and contain `[Fe(H_(2)O)_(6)]^(2+)` ion. `Fe(+II)` compounds are easily oxidised by air and so are difficult to obtain pure `Fe^(2+)` form many complexes like `K_(4)[Fe(CN)_(6)]`
`FeSO_(4)` is used in brown ring test for nitrates and nitrites. In this test , a freshly prepared `FeSO_(4)` solution is mixed with solution containing `NO_(2)^(-)` or `NO_(2)^(-)` and the conc. `H_(2)SO_(4)` is run down the side of the test tube. If the mixture gets hot or is shaken.
`I)` the brown colour disappear
`II)` No is evolved
`III)` a yellow solution of `Fe_(2)(SO_(4))_(3)` is formed