Exercise 8. Ferrous Sulfate

Ferrous sulfate may be taken as an example of a salt which is not very easily oxidized, so that elaborate precautions are not necessary. Its ease of oxidation is 

Pour 50 cc. of concentrated sulfuric acid slowly into about 1 liter of water in an evaporating dish and add 50 g. of iron nails or turnings. When the action becomes slow heat the dish until the acid is practically all neutralized, as indicated by the fact that evolution of hydrogen ceases. Filter from the undissolved iron, carbon, silicon, and other residues, using a folded filter, and evaporate to crystallization. If the solution oxidizes appreciably in the operation additional iron and acid must be added to effect reduction. Oxidation will be indicated by a change in color from bottle green to a yellowish shade of green, or by the formation of a rusty precipitate. 

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As a preliminary, ferric sulfate is made by the oxidation of ferrous sulfate. Dissolve 100 g. of ferrous sulfate in 100 cc. of boiling water, to which has been added before heating 10 cc. of sulfuric acid. Add concentrated nitric acid portionwise to the hot solution, until a diluted sample gives a reddish-brown (not black) precipitate with ammonia. This will require about 25 cc. Boil the solution down to a viscous liquid to get rid of excess nitric acid, dilute to about 400 cc., and add the calculated weight of ammonium sulfate. The crystallization is conducted as in the former exercise, preferably under 20°. By the addition of potassium sulfate, the corresponding potassium iron alum may be secured. In this case, it is necessary to concentrate the solution until there is about four parts of water to one of the hydrated alum and cool to about zero to secure crystallization. Both of these alums are amethyst in color, the potassium salt being much less stable and having a rather low transition point. 

The toxicity of heavy metals can be ehminated completely by precipitation as sulfides. Ferrous sulfate is the favored source of sulfide. Sodium sulfide can be used, but care must be exercised to avoid sulfide toxicity. 

EXERCISE 54. FERROUS AMMONIUM SULFATE (MOHR S SALT), (NHihSOcFeSOvBHaO... 

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