836 PRINCIPLES OF CHEMISTRY 



is obtained ; but on exposure to the air, even under water, it turns 

 green, becomes grey, and finally turns brown, which is due to the 

 oxidation that it undergoes. Ferrous hydroxide is very sparingly 

 soluble in water ; the solution has, however, a distinct alkaline reaction, 

 which is due to its being a fairly energetic basic oxide. In any case, 

 ferrous oxide is far more energetic than ferric oxide, so that if ammonia 

 be added to a solution containing a mixture of a ferrous and ferric 

 salt, at first ferric hydroxide only will be precipitated. If barium 

 carbonate, BaC0 3 , be shaken up in the cold with ferrous salts, it 

 does not precipitate them that is, does not change them into ferrous 

 carbonate ; but it completely separates all the iron from the ferric 

 salts in the cold, According to the equation Fe 2 Cl 6 -}- 3BaCO 3 + 3H 2 O 

 == Fe 2 3 ,3H 2 + 3BaCl 2 + 3CO 2 . If ferrous hydroxide be boiled with, 

 a solution of potash, the water is decomposed, hydrogen is evolved, and 

 the ferrous hydroxide is oxidised. The ferrous salts are in all. respects 

 similar to the salts of magnesium and zinc ; they are isomorphous 

 with them, but differ from them in that the ferrous hydroxide is not 

 soluble either in aqueous potash or ammonia. In the presence of an 

 excess of ammonium salts, however, a certain proportion of the iron 



also by the action of moisture and air on iron pyrites, especially when previously roasted 

 (FeS 2 + O 3 = FeS + SO 2 ), and in this condition it easily absorbs the oxygen of damp air 

 (FeS + O 4 = FeS0 4 ). Green vitriol is obtained in many processes as a bye-product. 

 Ferrous sulphate, like all the ferrous salts, has a pale greenish, colour hardly perceptible 

 in solution. If it be desired to preserve it without change that is, so as not to contain 

 ferric compounds it is necessary to keep it hermetically sealed. This is best done by. 

 expelling "the air by means of sulphurous anhydride or ether , sulphurous anhydride, 

 SO 2 , removes oxygen from ferric compounds, which might be formed, and is itself 

 changed into sulphuric acid, and hence the oxidation of the ferrous compound does not 

 take place in its presence. Unless these precautions are -taken, green vitriol turns 

 brown, partly changing into the ferric salt. "When turned brown, it is not completely 

 Soluble in water, because during its oxidation a certain amount of free insoluble ferric 

 oxide is formed: 6FeSO 4 + O3 = 2Fe 2 (SO 4 ) 3 + Fe 2 O3. In order to cleanse such mixed 

 green vitriol from the oxide, it is necessary to add some sulphuric acid and iron and boil 

 the mixture; the ferric salt is then transformed into the ferrous state: Fe 8 (S0 4 ) 3 + Fe 

 = 8FeS6 4 . 



Green vitriol is used for the manufacture of Nordhausen sulphuric acid (Chapter 

 XX.), for preparing ferric .oxfde, in many dye works (for preparing the indigo vats and 

 reducing blue indigo to white), and in many other processes, it is also a very good 

 disinfectant, ''and is the cheapest salt from which other compounds of iron may be 

 obtained. 



The other ferrous salts (excepting the yellow prussiate, which will be mentioned later 

 are but little used, and it is therefore unnecessary to dwell upon them. We will only 

 mention ferrous chloride, which, in the crystalline state, has the composition 

 FeCl 2 ,4H 2 O. It is easily prepared . for instance, by the action of hydrochloric acid on 

 iron, and in the anhydrous state by the action of hydrochloric acid gas on metallic iron 

 at a red heat. The anhydrous ferrous chloride then volatilises in the form of colourless 

 cubic crystals. Ferrous oxalate (or the double potassium salt) acts as a powerful 

 reducing agent, and is frequently employed in photography (as a developer). 



