IKON, NICKEL, COBALT 497 



Ferrous oxide FeO is a black substance made by cautious 

 reduction of ferric oxide by a stream of hydrogen. 



Ferrous Carbonate FeCO 3 . The carbonate occurs in nature 

 as an impurity in clay, in clay iron stone, and pure as siderite. 

 Water containing carbonic acid dissolves it, giving the bicar- 

 bonate : 



FeC0 3 + H 2 C0 3 - Fe(HC0 3 ) 2 . 



Thus well and river waters all contain at least traces of ferrous 

 bicarbonate as a part of their hardness. Exposure to the air 

 causes oxidation and, as ferric carbonate is not stable, rust (ferric 

 hydroxide or a hydrated ferric oxide) is deposited : 



4Fe(HCO 3 ) 2 + O 2 + 10H 2 O -> 4Fe(OH) 3 + 8H 2 CO 3 . 



This red deposit is seen in white vessels in which such water drips 

 or stands. It also " yellows " goods washed in such water, if the 

 carbonate of iron is not previously precipitated by soda or some 

 other softening agent (see p. 391). 



Ferro- and Ferri-cyanides. Potassium ferrocyanide IQFe- 

 (CN) 6 , or yellow prussiate of potash, is a pale yellow, soluble salt. 

 The iron is contained in the negative radical and ion [Fe(CN) 6 ] ', 

 and the solution therefore gives the reactions of this ion, and not 

 of ferrous- or ferric-ion. One of the double decompositions of this 

 salt namely, that with ferric salts is important because it 

 gives a gelatinous precipitate of Prussian blue (ferric ferrocy- 

 anide) : 



4FeCl 3 + 3K 4 Fe(CN) 6 -> Fe 4 m (Fe(CN) 6 yv J + 12KC1. 



Prussian blue is employed in making paints, and is the usual 

 pigment in laundry blueing. Although insoluble, it is such a fine 

 powder that it appears to dissolve in the water. It is used in the 

 laundry to correct the yellowish tint derived from the ferrous 

 bicarbonate in the water (p. 391). If the goods are not freed 

 by rinsing from soap and soda, however, the alkali liberated by 



