IRON, COBALT, AND NICKEL 849 



the metals of the alkalis and alkaline earths ; these are readily soluble, 

 have a neutral reaction, and resemble the yellow prussiate. Solutions 

 of these salts form precipitates with the salts of other metals, because the 

 ferrocyanides of the heavy metals are insoluble. Here either the whole 

 pf the potassium of the yellow prussiate, or only a part of it, is exchanged 

 lor an equivalent quantity of the heavy metal. Thus, when a cupric 

 salt is added to a solution of yellow prussiate, a red precipitate is obtained 

 which still contains half the potassium of the yellow prussiate : 



K 4 FeC 6 N 6 + CuS0 4 = K 2 CuFeC 6 N 6 + K 2 S0 4 . 



But if the process be reversed (the salt of copper being then in excess) 

 the whole of the potassium will be exchanged for copper, forming a 

 reddish-brown precipitate, Cu 2 FeC 6 N 6 ,9H 2 O. This reaction and 

 those similar to it are very sensitive and may be used for detecting 

 metals in solution, more especially as the colour of the precipitate 

 very often shows a marked difference when one metal is exchanged 

 for another. Zinc, cadmium, lead, antimony, tin, silver, cuprous and 

 aurous , salts form white precipitates ; cupric, uranium, titanium 

 and molybdenum salts reddish-brown; those of nickel, cobalt, 

 and chromium, green precipitates ; with ferrous salts, ferrocyanide 

 forms, as has been already mentioned, a white precipitate namely, 

 uFe 2 Fe0 6 N 6 , or FeC 2 N 2 which turns blue on exposure to air, and 

 with ferric salts a blue precipitate called Prussian blue. Here the 

 potassium is replaced by iron, the reaction being expressed thus : 

 2Fe 2 Cl c + 3K 4 FeC 6 N 6 = 12KCl4-Fe 4 Fe 3 C !8 N 18 , the latter formula 

 expressing the composition of Prussian blue. It is therefore the 

 compound 4Fe(CN) 3 + 3Fe(CN) 2 . The yellow prussiate is prepared in 

 chemical works on a large scale especially for the manufacture 'of this 

 blue pigment, which is used for dyeing cloth and other fabrics and 

 also as one of the ordinary blue paints. It is insoluble in water, and 

 the stuffs are therefore dyed by first soaking them in a solution of a 

 ferric salt and then in a solution of yellow prussiate. If however 

 an excess of yellow prussiate be present complete substitution between 

 potassium and iron does not occur, and soluble Prussian blue is 

 formed ; KFe 2 (CN) 6 = KCN,Fe(CN) 2 ,Fe(CN) 3 . This blue salt is 

 colloidal, is soluble in pure water, but insoluble and precipitated 

 when other salts for instanpe, potassium or sodium chloride are 

 present even in small quantities, and is therefore first obtained as a 

 precipitate. 29 



** Skraup obtained this salt both from potassium ferrocyanide with ferric chloride 

 and from ferrjcyanide with ferrous chloride, which evidently shows that it contains iron 



