PURIFICATION OF NITRIC ACID. 
117 
ART. XXIII. — PURIFICATION OF NITRIC ACID. 
BY M . E . MILLON. 
The ordinary methods of purifying nitric acid by nitrate 
of silver, nitrate of baryta, or litharge, and desiccation, al- 
ways leaves in the product a certain quantity of nitrous 
acid, which exercises a marked influence on the properties 
of the acid. The aqueous solution of hydrosulphuric acid 
has been adopted by M. Millon, as the best test for the pre- 
sence of nitrous acid. The nitric acid to be tried should be 
diluted with once or twice its volume of water; the least 
trace of nitrous acid causes the liberation of sulphur, which 
gives an opalescent appearance to the liquor. At the same 
time, a small quantity of ammonia is formed. 
When the nitric acid contains no organic matter, it may 
be obtained free from nitrous acid, by setting apart the first 
products of its distillation, which contains the whole of the 
nitrous acid; but the best and surest method to adopt, even 
when the acid contains organic matter, is, according to M. 
Millon, to mix bichromate of potash with the acid to be 
purified, in the proportions of one part to one hundred of 
acid. The nitrous acid is transformed into nitric acid at the 
expense of the oxygen of the chromic acid. 
This addition is sufficient for the purification of nitric 
acid, the density of which does not surpass 1.48; but for 
acid of a greater density than this, it is necessary to resort 
to other means, for the acid is decomposed under the influ- 
ence of heat, and cannot be distilled. 
In preparing the concentrated acid from the acid of com- 
merce, it is well to separate, by distillation, the first third 
of the acid, putting into the retort some platinum wire, or 
what is better, spongy platinum. There will thus be drawn 
VOL. IX. — NO. II. 1 1 
