sulfates of divalent and trivalent iron, FeSO4 and Fe2(SO4)3.
The sulfate of divalent iron separates from aqueous solutions at 1.82°–56.8°C as pale green crystals of FeSO4.7H2O, known in industry as green vitriol. The solubility of anhydrous FeSO4 in 100 g of water is as follows: 26.6 g at 20°C and 54.4 g at 56°C. Green vitriol can be made by treating scrap iron or cuttings of roofing iron with dilute sulfuric acid. In industry it is a by-product of the pickling of iron sheets, wire, or other articles with dilute H2SO4 to remove scale. It is used in making ink, in dyeing (to dye wool black), and to preserve wood.
Ferric sulfate forms crystalline hydrates containing various amounts of water—for example, Fe2(SO4)3.9H2O (yellow). It is readily soluble in water. It forms iron alums with sulfates of alkali metals and (NH4)2SO4. It is made industrially by dissolving ferric oxide in 75–80 percent sulfuric acid. It is used in water purification as a coagulant and for making iron alum.