$50 PRINCIPLES OF CHEMISTRY 



Potassium ferricyanide, or red prussiate of potash, K 3 FeC 6 N 6 , is 

 called 'Gmelin's salt,' because this savant obtained it by the action 

 of chlorine on a solution of the -yellow prussiate : K 4 FeC 6 N 6 + Cl 

 e= K 3 FeC 6 N G 4- KC1. The reaction is due to the ferrous salt being 

 changed by the action of the chlorine into a ferric salt. It separates 

 from solutions in anhydrous, well-formed prisms of a red colour, but 

 the solution has an oliye colour ; LOO parts of water, at 10, dis- 

 solve 37 parts of the salt, and at 100, 78 parts. 30 The red prus- 

 siate gives a blue precipitate with ferrous salts, called TurnbidVs blue, 

 very much like Prussian blue (and the soluble variety), because it also 

 contains ferrous cyanide and ferric cyanide, although in another propor- 



in both the ferric and ferrous states. With ferrous chloride it forms Prussian blue, and 

 with ferric chlbfide Turnbull's blue. 



Prussian blue was discovered in the beginning of the last century by a Berlin 

 manufacturer, Diesbach. It was then prepared, as it sometimes is also at present, 

 directly from potassium cyanida obtained by heating animal charcoal with potassium 

 carbonate. The mass thus obtained Is dissolved in water, alum is added to the 

 solution in order to saturate the- free alkali, and then a solution of green vitriol is added 

 which has previously been sufficiently exposed to tho air to contain both ferric and 

 ferrous salts. If the solution of potassium cyanide be mixed with a solution containing 

 both salts, Prussian blrte will be formed, because it is a -compound of ferrous cyanide, 

 FeC 8 N 2 , and ferric cyanide, Fe 2 C 6 N 6 . A ferric salt with potassium ferrocyanide forma 

 a blue colour, because ferrous cyanide is obtained from the first salt and ferric cyanide 

 from the second. During the preparation of this compound alkali must be avoided, aa 

 otherwise the precipitate would contain oxides of iron. Prussian blue has not a crystal- 

 line structure ; it forms a blue mass with a copper-red metallic lustre. Both acids and 

 alkalis act on it. The action is at first confined to the ferric salt it contains. Thus 

 alkalis form ferric oxide and ferrocyanide in solution: 2Fe 2 C 6 N 6 ,8FeC 2 N 2 +'12KHO 

 , = 2(Fe< 2 O-,,3H. 4 0) + 8K 4 FeC 6 N 6 . Various ferrocyanides may thus be prepared. Prussian 

 blue is soluble in an aqueous solution of oxalic acid, forming blue ink. In air, when 

 exposed to the action of light, it fades; but in the dark again absorbs oxygen and 

 becomes blue, which fact is also sometimes noticed in blue cloth. An excess of potassium 

 ferrocyanide renders Prussian blue soluble in water, although insoluble in various saline 

 solutions that is, it converts it into the soluble variety. Strong hydrochloric acid also 

 Dissolves Prussian blue. 



80 An excess ;f chlorine must not be employed in preparing this compound, otherwise 

 '-'the reaction goes further. It is easy to find out when the action of the chlorine on potassium 

 ierrocyanide must cease ; it is only necessary to take a sample of the liquid and add a 

 solution of a ferric salt to it'. If a precipitate of Prussian blue is formed, more chlorine 

 must be added, as there is still- some undecomposed ferrocyanide, for the ferricyanide 

 does not give a precipitate with ferric salts. Potassium' ferricyanide, like the ferro- 

 cyanide, easily exchanges its potassium for hydrogen and various metals by double 

 decomposition. With the salts of tin, silver, and mercury it forms yellow precipitates, 

 and with those of uranium, nickel, cobalt, copper, and bismuth brown precipitates. The 

 lead salt under the action of sulphuretted hydrogen forms lead sulphide and a hydrogen 

 salt or acid, H 3 FeC b N 6 , corresponding with potassium ferricyanide, which is soluble, 

 crystallises in red needles, and resembles hydroferrocyanic acid, H 4 FeC 6 N G . Under th* 

 action of reducing agents for instance, sulphuretted hydrogen, copper potassium ferri- 

 .cyanide is changed into ferrocyanide, especially in the presence of alkalis, and thus forma 

 a rather energetic oxidising agent capable, for instance, of changing manganous oxide 

 into dioxide, bleaching tissues, &c. 



