As for the dry grade ferrous sulphate heptahydrate the availability period is about one year. The storage time for ferrous sulphate monohydrate cannot be over 2 year. If the storage time of ferrous sulphate is too long, the metamorphic would appeared. How is the metamorphic of ferrous sulphate created?
The reason of metamorphic is that ferrous sulphate has strong reducing capacity. It would happen hydrolysis, if placed in air too long. The iron (Fe2 +) in ferrous sulphate will be slow oxidized to iron (Fe3 + ). The color of ferrous sulphate heptahydrate would turn green to yellow. After the product oxidized, it will appear suspended materials when it dissolved in water and the solution is yellow too. Meanwhile, the ferrous sulphate would be caking if it is metamorphic.
How to prevent ferrous sulphate metamorphic?
The best way to prevent metamorphic is to keep ferrous sulphate storage in cool, dry, well ventilated area, removed from oxidising agents, sealed when not in use. The validity of drying grade ferrous sulphate is longer than normal grade.
Method: Add iron powder into ferrous sulphate solution. Ferrous sulfate oxidized becomes ferric sulfate. Ferric sulfate react with iron in solution get ferrous sulfate. Fe + 2Fe3 + = 3Fe2 +. This method is recommended with current use, dissolved after a long time is not recommended.