COMPOSITION OF PRUSSIAN BLUE. 99 



ARTICLE V. 



On the Composition of Prussian Blue, prepared from different 

 oxides of Iron. By T. Phillips Allkn, Corresponding 

 Member of the Maryland Academy of Science and Literature. 



[Read before the Academy, January 26, 1836.1 



Is Prussian blue, prepared by decomposing a salt of per- 

 oxide of iron with ferro-prussiate of potash identical with the 

 Prussian blue obtained by decomposing a salt of joro^oxide of 

 iron with ferro-prussiate of potash, and oxidating the pre- 

 cipitate? 



This question has not yet been satisfactorily answered. 

 Considering it worthy of investigation, I undertook a series of 

 experiments to endeavour to determine it in my own mind. 

 Many conflicting opinions have been advanced on this in- 

 teresting subject. The observing manufacturer has always 

 contended that to obtain a blue of first quality, it was necessary 

 to precipitate the solution of ferro-prussiate of potash, with a 

 solution ofjyro^o-sulphate of iron, and then to oxidize the preci- 

 pitate by washing it with water, or in preference with acidu- 

 lated water. The theoretical chemist, on the other hand, has 

 advanced that the blue obtained in this manner was different 

 from thai obtained by decomposing a solution of ferro prussiate 

 of potasU with a solution of /peroxide of iron ; that the former 

 is inferior to the latter in intensity of colour, and that it is not 

 a neutral prussiate of iron, but is a sub-salt containing an 

 excess of oxide of iron. 



Having had occasion to prepare some prussian blue on a 

 large scale, I dissolved 300 lbs. of ferro-prussiate of potash, to 

 which I added a quantity of solution of proto-sulphate of iron, 

 containing 390 lbs. of crystallized salt. On testing the 

 liquor after the precipitate had subsided, I found it contained 

 a large excess of sulphate of iron; whereas, had the decompo- 

 sition taken place as has generally been supposed, that is, had 

 all the cyanogen in combination with the potassium of the 

 cyanide of potassium and iron, combined with the iron 

 of the 390 lbs. of proto-sulphate of iron, neither the proto- 

 sulphate of iron nor the ferro-prussiate of potash should have 

 been in excess; for I had employed the proto-sulphate of iron 



