Fathers. — Composition of Nitric Acid. 



299 



Art. XXXI. — Note on the Composition of Nitric Acid. 



By H. T. M. Fathers. 



Communicated by Professor Easterfield. 



[Read before the Wellington Philosophical Society, 4th October, 1911.] 



Though the composition and molecular weight of nitric acid are known 

 to be represented by the formula HN0 3 , I have been unable to find in 

 any text-book a method of showing to a class that the substance really 

 has the formula assigned to it. I have therefore worked out the details 

 of a comparatively simple method whereby the demonstration may without 

 difficulty be carried out. 



1. Strong nitric acid is prepared by distilling a mixture of sodium- 

 nitrate with an excess of 98 per cent, sulphuric acid at as low a temperature 

 as possible, and the resulting acid is further dehydrated by distilling at 

 about 20 mm. pressure (vacuum of the water-ejector pump) with three 

 times its volume of strong sulphuric acid, and condensation of the acid- 

 vapours by means of a good freezing-mixture. Diffusion of the aqueous 

 vapour from the pump into the distillate is prevented by means of a tube 

 containing pumice moistened with sulphuric acid. An acid prepared in 

 this way will be found to be practically colourless, and to have a specific 

 gravity and titration value corresponding to over 99 per cent, of pure 

 nitric acid. 



2. As soon as the acid is prepared a number of thin-walled glass bulbs, 

 each capable of holding about 0-2 grams of acid, and blown on capillary 

 stems about 3 cm. in length, are filled with the acid by placing the bulbs 

 with the open end of the stem downwards in a beaker containing the 

 acid and placing the beaker in a desiccator, which is then evacuated with 

 the aid of the filter pump. Upon readmitting the air the bulbs will be 

 found to be completely filled with acid, except for the presence of a very 

 minute air-bubble. The ends of the capillaries are now sealed over a small 

 flame, and the weight of the contained acid ascertained. 



3. The apparatus in which the analysis of the acid is actually carried 

 out is as follows : — 



dry; CO 7 . _f ' 



IjUJ 



flisaU tube in which one of the bulbs containing a weighed quantity 

 of nitric acid is placed. & is a short and narrow piece of combustion tubing 

 containing a 10 cm. spiral of copper gauze which has been heated to redness 





