330 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



As a proof, 



J[6HC1] (1 equiv. = 2 litres) 



was added to the liquid ; this gave off + 25'0 Cal., with the 

 production of an abundant precipitate of Prussian blue; the 

 heat disengaged varied, during this precipitation, from 23 to 

 25. To sum up, the hydrochloric acid should produce the 

 following reactions : 



[2HC1 (diluted) + K 2 (diluted) = 2KC1 (diluted)] ... + 



[6HC1 (diluted) + Fe 2 3 (precipitated) = Fe 2 Cl 6 (diluted)] + 11-4 , oc . 

 K2Fe 2 Cl 6 (diluted) + 3K 4 Fe(CN) 6 (in solution) = Fe 7 (CN) 18 | + ^ 



+ 12KC1 (diluted)] ............... + 1-4J 



The approximate consistence between the values 25 and 2 6 '4 

 is as close as can be hoped for in the study of such precipitates, 

 the state of which varies with the conditions. 



9. We may conclude, further 



i[18HCN (diluted) + 3FeO (precipitated) + 2Fe 2 3 (precipitated) 



= Fe 7 (CN)w (precipitated)] gives off + 24-9. 



i[6HCN (diluted) + FeO (precipitated) = H 4 Fe (CN) 6 (in solu- 



tion)], + 12-3. 



These values were verified by means of the direct formation 

 of Prussian blue from potassium cyanide and the two sulphates 

 of iron : 



equiv. = 2 litres) + 3FeS0 4 (1 equiv. = 2 litres) + 

 2Fe2(S0 4 ) 3 (1 equiv. = 2 litres) = Fe 7 (CN) 18 (precipitated) 

 + 9K 2 S0 4 (diluted)] gives off + 37*5. 



The difference between the heat of formation of the alkaline 

 sulphate and that of the iron sulphates, reckoning from the 

 oxides, being 12'5 4- 111 - 471 = - 23'2, and the heat of 

 formation of 3KCN from potash being 8 '9, we can easily 

 determine from these data the heat disengaged in the formation 

 of Prussian blue from hydrocyanic acid : 



i[18HCN (diluted) + 3FeO + 2Fe 2 3 = Fe 7 (CN) 18 ] gives off 

 37-5 + 8-9 -23-2 = +23-2; 



a value sufficiently near to 24*9, which was obtained in another 

 way, but is a less accurate result. 



10. It is now possible to draw a few general conclusions 

 from these results. 



The first that occurs to us relates to the heat disengaged in 

 the formation of the ferrocyanide, starting from hydrocyanic 

 acid or from potassium cyanide. 



J[6HCN" (in solution) + 3K 2 (diluted)] gives off + 8'7. 



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



gives off + 39-3. 



We see that the substitution of ferrous oxide for potash, with 



