848 PRINCIPLES OF CHEMISTRY 



and they, so to speak, saturate each other. Of course, all this is not a 

 definite explanation, but then the supposition of a special complex radicte 

 can even less be regarded as such. 



Potassium ferrocyanide, K 4 FeC 6 N 6 , is very easily formed by mixing 

 solutions of. ferrous sulphate and potassium cyanide. First, a white 

 precipitate of ferrous cyanide, FeC 2 N 2 , is formed, which becomes blue 

 on exposure to air, but is soluble in an excess of potassium cyanide, 

 forming the ferrocyanide. The same yellow prussiate is obtained on 

 heating animal nitrogenous charcoal or animal matters such as 

 horn, leather cuttings, &c. with potassium carbonate in iron 

 vessels, 27 bls the mass formed being afterwards boiled with water with 

 exposure to air, potassium cyanide first appearing, which gives yellow 

 prussiate. The animal charcoal may be exchanged for wood charcoal, 

 permeated with potassium carbonate and heated in nitrogen or 

 ammonia ; the mass thus produced is then boiled in water with ferric 

 oxide. 28 In this manner it is manufactured on the large scale, and is 

 called 'yellow prussiate' ('prussiate de potasse/ Blutlaugensalz). 



It is easy to substitute other metals for the potassium in the yellow 

 prussiate. The hydrogen salt or hydroferrocyanic acid, H 4 FeC 6 N 6 , is 

 obtained by mixing strong solutions of yellow prussiate and hydro- 

 chloric acid. If ether be added and the air excluded, the acid is 

 obtained directly in the form of a white scarcely crystalline precipitate 

 which becomes blue on exposure to air (as ferrous cyanide does from the 

 formation of blue compounds of ferrous and ferric cyanides, and it is 

 on this account used in cotton printing). It is soluble in water and 

 alcohol, but not in ether, has marked acid properties, and decomposes 

 carbonates, which renders it easily possible to prepare ferrocyanides of 



27 bU The sulphur of the animal refuse here forms the compound FeKS 2 , which 

 by the action of potassidm cyanide yields potassium sulphide, thiocyanate, and ferro- 

 cyanide. 



28 Potassium ferrocyanide may also be obtained from Prussian blue by boiling with a 

 solution of potassium hydroxide, and from the ferricyanide by the action of alkalis and 

 reducing substances (because the red pruasiate is a product of oxidation produced by 

 the action of chlorine : a ferric salt is reduced to a ferrous salt), &c. In many works 

 (especially in Germany and France) yellow prussiate is prepared from the mass, con- 

 taining oxide of iron, and employed for purifying coal gas (Vol. L, p. 861), which 

 generally contains cyanogen compounds. About 2 p.c. of the nitrogen contained in coal is 

 converted into cyanogen, which forms Prussian blue and thiocyanates in the mass used 

 for purifying the gas. On evaporation the solution yields large yellow crystals containing 

 8 molecules of water, which is easily expelled by heating above 100. 100 parts of water 

 at the ordinary temperature are capable of dissolving 25 parts of this salt ; its sp. gr. is 

 1-83. Wnen ignited it forms potassium cyanide and iron carbide, FeC 2 (Chapter XIII., 

 Note 12). Oxidising substances change it into potassium ferricyanide. With strong 

 sulphuric acid it gives carbonic oxide, and with dilute sulphuric acid, when heated, 

 prussic acid is evolved according to the equation: 2K 4 FeC 6 N 6 + 8H 2 S0 4 = Ka 



+ 3K.,SO 4 + 6HCN; hence in the yellow prussiate K* replaces Fe. 



