NITRIC ACID. 
§§ 84, 85.] 
113 
clecinormal soda last used to make it alkaline—that is to say, if the 
total acidity was equal to 3-6 d.n. soda, and 5*6 d.n. soda was added to 
the 10 c.c. evaporated to dryness and burned, then 5-6 c.c. of d.n. 
sulphuric acid is added to the ash. The solution is now warmed to get 
rid of carbon dioxide, and, after addition of a little phenolphthalein, 
titrated with d.n. soda solution until the change of colour shows satura¬ 
tion : the number of c.c. used, multiplied by 0-00365, equals the HC1. 
§ 84. In investigating the stains from hydrochloric acid on fabrics, or 
the leaves of plants, any free hydrochloric acid may be separated by 
boiling with water, and then investigating the aqueous extract. Should, 
however, the stain be old, all free acid may have disappeared, and yet 
some of the chlorine remain in organic combination with the tissue, or in 
combination with bases. Dr Angus Smith has found weighed portions 
of leaves, etc., which had been exposed to the action of hydrochloric 
acid fumes, richer in chlorides than similar parts of the plants not thus 
exposed. 
The most accurate method of investigation for the purpose of separat¬ 
ing chlorine from combination with organic matters is to cut out the 
stained portions, weigh them, and burn them up in a combustion tube, 
the front portion of the tube being filled with caustic lime known to be 
free from chlorides ; a similar experiment must be made with the 
unstained portions. In this way a considerable difference may often be 
found ; and it is not impossible, in some instances, to thus detect, after 
the lapse of many years, that certain stains have been produced by a 
chlorine-holding substance. 
III.—Nitric Acid. 
§ 85. General Properties. —Nitric acid—commonly known in England 
as aqua fortis, chemically as nitric acid, hydric nitrate, or nitric mono- 
hydrate —is a mono-hydrate of nitrogen pentoxide (N 2 0 5 ), two equi¬ 
valents, or 126 parts, of nitric acid containing 108 of N 2 0 5 , and 18 of 
H 2 0. Anhydrous nitric acid, or nitrogen pentoxide, can be obtained by 
passing, with special precautions, dry chlorine over silver nitrate ; the 
products are free oxygen and nitrogen pentoxide, according to the fol¬ 
lowing equation :— 
Silver Nitrate. Chlorine. Silver Chloride. Nitrogen Pentoxide. Oxygen. 
Ag 2 0,N 2 0 5 + 2C1 = 2AgCl -f N 2 0 5 + O 
By surrounding the receiver with a freezing mixture, the acid is con¬ 
densed in crystals, which dissolve in water, with emission of much heat, 
forming nitric acid. Sometimes the crystals, though kept in sealed tubes, 
decompose, and the tube, from the pressure of the liberated gases, bursts 
with a dangerous explosion. 
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