1892.] Formation and Decomposition of Nitrous Acid. 3y 



such an acid, however, would contain nearly 50 per cent, of nitrous 

 acid. 



After most of the experimental work above detailed had been per- 

 formed, a paper appeared by F. Marehlewski,* entitled " Zur 

 Kenntniss der verschiedenen Farbungen der Salpetersaure," in which 

 the author describes various experiments upon the reaction between 

 nitric oxide and nitric acid. The main object of the paper was to 

 determine the composition of the various coloured liquids produced 

 when nitric oxide, nitrous fumes, and nitrogen peroxide are passed 

 into nitric acid of different concentration. The method employed 

 consisted in preparing such liquids and then decolorising them by a 

 current of carbonic anhydride ; the gases evolved were passed into 

 sulphuric acid and the solution subjected to analysis. The process is 

 based upon the observations of Lunge that nitrous fumes give under 

 these conditions nitrosyl sulphuric acid, while nitrogen peroxide 

 gives a mixture of nitrosyl sulphuric and nitric acids, thus : 



(I.) 2H 2 S0 4 +N 2 3 = 2S0 2 .N0 2 . 

 (II.) H 2 S0 4 -f N 2 4 = S0 2 .N0 2 .OH + HN0 3 . 



It might appear open to question whether the composition of these 

 liquids can be ascertained by this indirect method, and whether a 

 liquid apparently decolorised contains nothing but the residual nitric 

 acid (more or less diluted) of the operation. My own experience has 

 shown that it is extremely difficult to remove the last traces of the 

 yellow colour from the more concentrated acids; though, on the other 

 hand, it must be allowed that the method of analysis adopted in the 

 text would not distinguish between nitrous acid on the one hand and 

 nitrogen peroxide on the other, the latter of which would appear in 

 the course of analysis as an equimolecular mixture of nitrous and 

 nitric acids. These remarks would, however, apply probably only to 

 the two last series of experiments. Marchlewski gives no observa- 

 tions of time, temperature, or degree of humidity of gases, whether 

 oxides of nitrogen or carbonic anhydride, nor a single control 

 analysis of the residual nitric acid. Some proof is given that the 

 difference between the green and blue acids is dependent upon not 

 only the water present but also upon the dissolved nitric oxide. 

 Marchlewski seems to be quite unaware of the observations of 

 Peligot,t made nearly forty years ago, upon the same subject. 



* Ber. Deut. Chem. Ges.,' vol. 24, p. 3271. 



f ' Ann. Chim. Phys.' [Ill], vol. 2, p. 58. The remarks of Peligot seem worth 

 transcribing, as having possibly fallen into undeserved oblivion : " Comme 1'acide 

 azoteux pur parait etre bleu tandis que 1'acide azotique et 1'acide hypoazotique 

 melanges sont jaunes, on obtint par cette action de 1'eau " (viz., upon nitrogen per- 

 oxides) " selon les proportions employees les differents nuances de vert et de bleu 



