HEAT OP FORMATION OF PEKCHLORIO ACID. 355 



deemed advisable to operate in an atmosphere of oxygen, which 

 completes the combustion, as was proved by the determination 

 of the carbonic acid. 



Three series of experiments were made; namely, with 

 potassium picrate, ammonium picrate, and picric acid ; but 

 only the first two gave satisfactory results, as the combustion 

 of the picric acid was never complete, probably owing to its 

 being, to a certain extent, volatile. 



The numbers obtained with potassium picrate burnt, in the 

 one case by pure oxygen, and in the other by perchlorate, 

 differ by 8*6 Cal. ; the numbers obtained with ammonium 

 picrate, by 6*5 Cal. ; results which agree as closely as could 

 be expected for values which represent the difference of 

 numbers that are far too high. We shall adopt the mean 

 7'5 Cal. as corresponding to the reaction 



KC10 4 (solid) = KC1 (solid) + 20 2 gas. 



This decomposition, when effected at the ordinary temperature, 

 would then absorb heat, contrary to what takes place in the 

 decomposition of potassium chlorate, which liberates heat to 

 the amount of + ll'O Cal. 



It is easy to calculate the heat of formation of potassium 

 perchlorate from the elements. For, if we admit that 



K + Cl = KC1 (solid) liberates + 105-0 Cal. 



K + Cl + 4 = KC10 4 (solid) liberates + 112-5 



From this figure and the preceding data it follows that 



H + Cl + 20 2 = HC10 4 (liquid, pure) disengages +19-1 



H + Cl + 20 2 + water = HC10 4 (diluted) 



K + Cl + 20 2 = KC10 4 (solid) + 112-5 dissolved 



Na + Cl + 20 2 = NaC10 4 (solid) + 100-2 dissolved 



N + H 4 + Cl + 20 2 = NH 4 C10 4 (solid) + 79-7 dissolved 



+ 39-35 

 + 100-4 

 + 96-7 

 + 73-3 

 KC10 3 + = KC10 4 (solid) + 17-9 



9. We derive from these figures 



HC10 4 (pure, liquid) = HC1 (gas) + 4 liberates ... ... + 2'9 



i[2HC10 4 (pure, liquid) = C1 2 + 7 + H 2 (gas)], + 9'9 ; H 2 



(Kquid) + 14-9 



HC10 4 (diluted) = HC1 + 4 nil 



J[2HC10 4 (diluted) = Cl, + 7 + H 2 liquid)] - 4-9 



numbers which account for the difference between the stability 

 of the concentrated and diluted acids, and also for the easy decom- 

 position of the concentrated acid. 



10. The perchlorates in solution are converted into chlorides 

 with scarcely any thermal phenomenon, but it is different with 

 the solid salts. In fact 



KC10 4 (solid) = KC1 (solid) + 20 2 - 7-5 



NaC10 4 (solid) = NaCl (solid) +20 2 - 3-0 



[Ba(C10 4 ) 2 solid = BaCl 2 (solid) + 40 2 ] ... - 1-1 



The conversion of a solid perchlorate into chloride, at the 

 Ordinary temperature, therefore, absorbs heat ; that is to say, it 



2 A2 



