Fuming Sulphuric acid

This is prepared by the dry distillation of sulphate of iron.
Copperas (FeSO • 7 H20) is gently heated in the air until the water is all expelled and part of the ferrous sulphate is oxidized to the ferric state. The mixture of sulphates is then put into small iron retorts, which are heated in a fumace. Decomposition, according to the following equation, ensues:-

Fe2 (SO.)3 . 2 FeSO. = 2 Fe2O3+4 SO3+ SO2.

The vapors of sulphur dioxide and sulphuric anhydride are condensed in receivers containing oil of vitriol, which dissolves them.
The Nordhausen acid thus formed is a brown oily liquid, fuming in the air, owing to the escape of the dissolved sulphur oxides. By cooling to 0° C., crystals of pyrosulphuric acid (H2S2O7) separate.
Nordhausen acid is now chiefly produced in Bohemia, where a pyritiferous shale is found, which, when weathered, forms a basic sulphate of iron, that is dried and distilled.