PICRIC ACID AND PICRATES. 277 



whereas that of nitrobenzene 



C 6 H 6 + HN0 3 = C 6 HJlsr0 2 + H 2 0, gives off + 36'6. 

 The difference between these two quantities, -f 22 '2 4- a, is 

 enormous and cannot be compensated, either by the difference 

 in the quantities of heat disengaged by the union of H 2 O with 

 the excess of nitro-sulphuric acid, in the two experiments, or 

 by the difference in the respective heats of solution, in the same 

 liquid, of nitrobenzene and benzene-sulphonic acid. The differ- 

 ence is further increased by the heat of formation of the 

 secondary sulphuric acid hydrate. Thus the formation of nitro- 

 benzene gives off much more heat than that of benzene-sul- 

 phonic acid ; the formation of the nitro-derivative, in preference 

 to a sulphuric derivative, is therefore a natural consequence 

 of the general principles of thenno-chemistry. 



6. Trinitrophenol, or Picric Acid and its Salts. 



1. Let us apply these formulae to picric acid. This acid is 

 derived from phenol, by the replacement of three atoms of 

 hydrogen 



C 6 H 6 + 3HN0 3 = C 6 H 3 (N0 2 ) 3 +3H 2 0. 



Now the heat of formation of phenol may be estimated either 

 at + 34 Gal., or at -f 28 Cal., according to whether we adopt 

 the heat of combustion of Favre and Silbermann (737) or that 

 of M. Eechenberg (743), the difference between which values 

 does not amount to quite one-hundredth. 



We will take the mean, 31 Cal., for an equivalent, 229 grms. 

 This being allowed, the heat of formation of picric acid from its 

 elements, C 6 (diamond) -f H 3 + N 3 +0 7 , will be + 31 -f 27 = 

 + 58 Cal. for 229 grms. ; the heat of combustion being + 609'5 Cal., 

 according to our formulae. 



2. It is easy to proceed from this to the heat of formation of 

 picrates. Let ammonium picrate be 



C 6 H 2 (N0 2 ) 3 0]SrH 4 = 246 grms. 



According to the calculations, 1 the formation of this body by 

 means of pure acid and ammonia gas 



C 6 H 3 (N0 2 ) 3 (solid) + NH 3 (gas), 



disengages -f 22*9 Cal., which gives, for the heat of formation of 

 the salt from its elements, for 246 grms. 



C + H 6 + 2N 2 + 7 ; + 58 + 12'2 + 22-9 = + 831 Cal. 



Messrs. Sarrau and Vieille 2 found + 80*1 Cal. for combustion in 

 oxygen, a value agreeing with the former within the limit 



1 Table v. p. 127. 



2 "Comptes rendus des stances de 1'Acad^mie des Sciences," torn, xciii. 

 p. 270. 



