1889.] 



Reaction between Copper and JYitric Acid. 



219 



not properly sheltered, nitrous acid was formed by the decomposition 

 of the nitric acid at a rate faster than that at which it was oxidised 

 by the air current. The acid was subsequently diluted with the 

 required quantity of water and preserved in a dark cupboard. When 

 3 — 4 c.c. of this diluted acid were mixed with about 100 c.c. of water, 

 no blue colour was produced on addition of starch and potassium 

 iodide solutions, and the faintest possible orange tinge imparted to an 

 aqneous solution of nieta-phenylene diamine hydrochloride. The acid 

 was thus free from any considerable trace of nitrous acid. Experi- 

 ments similar to the above were repeated, but the current of carbonic 

 acid omitted, in order to more precisely fix the conditions. 



After introduction of a copper sphere there was no evolution of 

 gas for three minutes, and five minutes after the reaction had set in 

 a considerable quantity of nitrous acid was shown to be present. As 

 regards the production of nitrous from nitric acid under these con- 

 ditions, Professor Armstrong writes, in a note appended to a paper by 

 Dr. Divers :* " With reference to the formation of ^oO i during the dis- 

 solution of metals, &c, I some time ago satisfied myself by experiment 

 that it is produced .... and there is, I believe, no doubt that, 

 whatever the nature of the reducing agent, be it hydrogen or metal, 

 . . . . the primary product of the reduction of nitric acid is 

 nitrous acid." This observation, made some time ago, is amply 

 confirmed by the above and succeeding experiments. I would, 

 therefore, merely wish to call attention to the short interval of time 

 which elapses between the commencement of the reaction and the 

 formation in considerable quantities of nitrous acid. Again, when 

 the copper sphere was introduced into the acid containing the copper 

 salt and the nitrous acid, the evolution of gas commenced at once. 



In another experiment, in which the copper sphere was previously 

 heated and then allowed to cool in a current of hydrogen to remove 

 any superficial coating of cupric or cuprous oxide, no gas was evolved 

 for 1' 50" after the introduction of the sphere into the acid, and 

 4' 30" after the reaction had set in an abundance of nitrous acid was 

 shown to be present. 



At this point, Mr. Harcourt, from a recollection of some experi- 

 ments by the late Sir Benjamin Brodie, suggested to me to place into 

 the acid a substance which should remove the nitrous acid as fast as 

 it might be formed ; urea fulfils this requirement, in that it reacts 

 immediately with nitrous acid to form nitrogen and carbonic acid, 

 according to the equation 



00(NH 2 ) 2 + 2HXO, = COo + 2X 2 + 3H,0. 



Accordingly, 1 gram of urea, dissolved in 1 c.c. of water, was 

 added to a litre of nitric acid (sp. gr. -^f = 1*1662, percentage of 



* f Cliern. Soc. Journ.,' 1883 (Trans.), p. 456. 



