At the junction of the layers the acid reacts with nitrateVsalts to form NitricVacidHNO3 NitricVacidHNO3 is reduced to Nitrogen IIoxide by the IronIIsulphateVI salt to form the complex compound NitrosoironIIsulphateVIFeSO4NO  NitrosoironIIsulphateVI is brown in colourIt forms a thin layer at the junction between concentrated sulphuric VIacid and the miscible mixture of freshly prepared IronII sulphateVI and the nitrateVsalts as a brown ring