NITRIC ACID. 173 



6. HEAT OF FORMATION OF DILUTE NITRIC ACID, OF 

 MONOHYDRATED NlTRIC ACID AND OF ANHYDROUS 

 NITRIC ACID. 



1. The heat of formation of nitric acid from its elements is 

 a datum of the first importance, and may be deduced from that 

 of nitric oxide. Several methods were employed. We shall 

 first indicate them in principle and then give them in detail. 



(1) BY THE NITRITES. Having first formed a nitrite in 

 presence of a base, such as baryta, the barium nitrite is trans- 

 formed into nitrate ; and this by four distinct processes. 



(2) BY NITRIC ACID AND BARIUM DIOXIDE. The nitric oxide 

 is dissolved in concentrated nitric acid, which changes it into 

 nitrogen trioxide, the latter being further oxidised after dilution 

 by barium dioxide. 



(3) BY NITRIC PEROXIDE. This body is further oxidised by 

 various agents, which have already been indicated. They may 

 be employed either to measure the heat of formation of nitric 

 peroxide, deduced from that of nitric acid, which is known, or 

 to measure the heat of formation of nitric acid, that of nitric 

 peroxide being known. 



2. First method. Transformation of barium nitrite into 

 nitrate. In this reaction barium nitrite is oxidised, and the 

 heat liberated is measured by four processes distinct and inde- 

 pendent of one another. 



First process. Gaseous chlorine. Initial system: Ba(N0 2 ) 2 

 dissolved ; C1 4 gas ; H 4 gas ; 2 gas ; nBaO dissolved ; nHCl dis- 

 solved, these bodies being all separate from one another. Final 

 system : Ba(N0 3 ) 2 dissolved ; 4HC1 dissolved ; n(BaC! 2 + H 2 0) 

 dissolved, these bodies being mixed. 



It is first of all supposed that H 2 is allowed to react on 0, 

 which forms water, liberating -f 69 Cal. ; then the following 

 experiments are carefully carried out. Dry chlorine is agitated 

 with baryta water, of known strength and weight, in a calori- 

 metric flask (p. 163) ; the heat liberated, Q, is measured, and the 

 chlorine absorbed, p, is directly weighed to within O'OOl grm. 

 Care is taken that there shall remain a considerable excess of 

 free baryta, and agitation is kept up incessantly during the 

 operation, in order to prevent the formation of any other oxide 

 of chlorine than hypochlorous acid; the measurement of the 

 heat liberated supplies a verification in this respect. 1 



A quantity of barium nitrite strictly equivalent to the weight 

 of chlorine absorbed (Ba(N0 2 ) 2 for C1 2 ) is then taken and 

 dissolved separately in water, the solution is then mixed with 

 that of the hypochlorite, an operation which liberates a quantity 

 of heat, , which can almost be neglected. 



1 " Annales de Chimie et de Physique," 5 e sdrie, torn. v. pp. 335-338. 



