PROTOCOMPOUNDS OF IIU 



as the protoxide of iron. When cyanide of ^ 



to a protosalt of iron, a yellowish red precipita 

 dissolves in an excess of the alkaline cyanide 

 ferrocyanide of potassium (page 450). A grey \ 

 on distilling the ferrocyanide of ammonium by c 

 and a white insoluble substance on digesting rea 

 tated prussian blue in sulphuretted hydrogen waU Drained 

 in a well-stopped phial, which, although they differ consider- 

 ably in properties, have both been looked upon as protocyanide 

 of iron. The most remarkable property of this cyanide is its 

 tendency to combine with other cyanides of all classes, and to 

 form double cyanides, or to enter as a constituent into the salt- 

 radicals ferrocyanogen and ferricyanogen Cy 3 Fe, and Cy 6 Fe 2 . 

 The two following compounds are obtained when the ferrocya- 

 nide and the ferricyanide of potassium are added to a protosalt 

 of iron. 



Ferrocyanide of potassium and iron; 3Fe. K + 2 (Cy 3 Fe). 

 The bluish white precipitate which falls on testing a protosalt 

 of iron with the ferrocyanide of potassium or yellow prussiate of 

 potash. Of the four equivalents of potassium contained in two 

 equivalents of the latter salt (page 456), three are replaced by 

 three equivalents of iron in the formation of this precipitate, 

 while the three potassium unite with the former salt-radical of 

 the iron. This salt is represented above as consisting of 2 eq. 

 of ferrocyanogen with 4 eq. of metal (3FeH-K), ferrocyanogen 

 being bibasic. Exposed to the air it absorbs oxygen, and be- 

 comes blue. It then affords ferrocyanide of potassium to 

 water, and after all soluble salts are removed, a compound re- 

 mains, which Liebig names the basic sesquiferro cyanide of iron, 

 and represents by the formula Fe 4 , 3(Cy 3 Fe) +Fe 2 O 3 , which 

 corresponds, as will be seen afterwards, with 1 eq. of prussian 

 blue + 1 eq. of peroxide of iron. This basic compound is dissolved 

 entirely by continued washing, and affords a beautiful deep blue 

 solution. The addition of any salt causes the separation of this 

 compound. Its solution may be evaporated to dryness without 

 decomposition. 



Ferricyanide of iron, Turnbull's blue ; 3Fe + (Cy 6 Fe 2 ). 

 This is the beautiful blue precipitate that falls on adding the fer- 

 ricyanide of potassium (red prussiate of potash) to a protosalt 

 of iron. It is formed by the substitution of 3 eq. of iron for 

 the 3 eq. of potassium of the latter salt (page 450). The same 



