PRUSSIAN BLUE. 329 



heat disengaged gradually increasing with the time; as fre- 

 quently happens in the formation of amorphous precipitates. 1 

 Similarly : 



1 i 2 [3(K 4 Fe(C]Sr) 6 = 4 litres) + 2Fe 2 (N0 3 ) 6 (1 equiv. = 2 litres) = 

 Fe 7 (CN) 18 ) (precipitated) 4- 2KN0 3 (in solution)] disengages 

 + 0-725. 



According to the result furnished by the ferric sulphate, the 

 substitution of potash for ferric oxide (K 2 for Fe^) in Prussian 

 blue gives off -f 7'2 ; and according to the result furnished by 

 the nitrate, + 7'2 ; thus the two results agree. Admitting 

 that in the formation of potassium ferrocyanide, 



[H 4 Fe(CN) 6 (diluted) + 2K 2 (diluted)], disengages + 13'5 

 X 2 = + 27, 



we conclude that the formation of Prussian blue, with the same 

 acid and precipitated ferric oxide 



[3H 4 Fe(CN) 6 (diluted) + 2Fe 2 O 3 (precipitated)], gives off 

 + 6-3x2=-}- 12-6. 



The value 6-3 is little different from the value 5*7, which 

 represents the heat of combination of nitric and hydrochloric 

 acids with ferric oxide ; this is a fresh proof of the analogy 

 existing between hydroferrocyanic acid and the mineral acids. 

 However + 6'3 is greater than 5'7, which explains why diluted 

 hydrochloric acid does not decompose Prussian blue, with 

 formation of ferric chloride. 



7. Hydrocyanic acid, one of the weakest acids known, thus 

 constitutes, when combined with ferrous cyanide, a powerful 

 acid, comparable in every respect with nitric, acetic, and hydro- 

 chloric acids. This is a fresh proof, which helps to establish 

 the theory that the acid properties that are most marked, even 

 in compounds containing carbon and hydrogen, are not 

 necessarily connected in any way with the presence or propor- 

 tion of oxygen. 



8. We now have to measure the heat disengaged in the 

 formation of ferrocyanide. The following results were found : 



i[FeS0 4 (1 equiv. = 2 litres) + 2Fe2(S0 4 ) 3 (1 equiv. = 2 litres) 

 + 6K 2 0(1 equiv. = 2 litres)] gives off -f 23'2. 



Adding to the above mixture ^[6HCN (1 equiv. = 4 litres)], 

 we observe a fresh disengagement of + 3 9 -3, which represents 

 the formation of ferrocyanide from hydrocyanic acid and the 

 two oxides 



[6HCN(in solution) + 2K 2 (in solution) + FeO (precipitated) 

 = K 4 Fe(CN) 6 (in solution)] gives off + 39'3. 



1 " Annales de Chimie et de Physique," 5 e s&ie, torn. iv. pp. 174, 181. 



