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Nitric Acid is a colourless, highly
corrosive, poisonous liquid (freezing point : -42° C, boiling point:
83° C) that will react with water or steam to produce heat and toxic, corrosive, and
flammable vapors. It is toxic and can cause severe
burns. It is an important industrial chemical for the
manufacture of fertilizers, dyes, drugs, plastics, and explosives. Nitric acid is prepared
commercially by the two-stage
oxidation of ammonia (Ostwald process) to nitrogen dioxide.
Ammonia gas is successively oxidized to nitric oxide and nitrogen dioxide by air
or oxygen in the presence of a platinum gauze catalyst, which is
then absorbed in water to form nitric acid. The resulting acid-in-water solution
(about 50-78% by weight acid) can be dehydrated by distillation with
sulfuric acid. It is
miscible with water in all proportions. The nitric acid of commerce is typically a solution of 52% to 68%
nitric acid in water. More concentrated solutions are available. It forms an azeotrope that has the composition 68% nitric acid and 32% water and that boils
at 120.5°C. Solutions
containing over 86% nitric acid are commonly called fuming nitric acid; they
often have a reddish-brown color from dissolved nitrogen oxides. In aqueous
solution it is both a strong acid and a powerful oxidizing agent. Among the many important reactions of nitric acid are:
neutralization with ammonia to form ammonium nitrate, used widely in fertilizers and explosives;
nitration of glycerol and toluene, forming the explosives nitroglycerin and trinitrotoluene, respectively;
preparation of nitrocellulose; and oxidation of metals to the
corresponding oxides or nitrates.
Many compounds are oxidized by nitric
acid. Nonmetallic elements such as carbon, iodine, phosphorus and sulfur are oxidized by concentrated nitric
acid to their oxides or oxyacids
with the formation of NO2
. Hydrochloric
acid, aqueous HCl, is readily oxidized by concentrated nitric acid to
chlorine and chlorine dioxide. The action of nitric acid on a metal usually results in
reduction of the acid (i.e., a decrease in the oxidation state of the
nitrogen). The products of the reaction are determined by the concentration of
nitric acid, the metal involved (i.e., its reactivity), and the
temperature. In most cases, a mixture of nitrogen oxides, nitrates, and other
reduction products is formed. Relatively unreactive metals such as copper,
silver, and lead reduce concentrated nitric acid primarily to
NO2. The reaction of dilute nitric acid with copper produces NO,
while more reactive metals, such as zinc and iron, react with dilute
nitric acid to yield N2O. When extremely dilute nitric acid
is used, either nitrogen gas or the ammonium ion
(NH4+) may be formed. Nitrates, which are salts or esters
of nitric acid contains NO3- (nitrate
ion), are formed by replacing the hydrogen with a
metal (e.g., sodium or potassium) or a radical (e.g., ammonium or ethyl). Some
important inorganic nitrates are potassium
nitrate (used in explosives, fireworks, matches, and fertilizers, and as a preservative
in foods. It is sometimes used in medicine as a diuretic.), sodium nitrate (used in making potassium nitrate, fertilizers, and explosives.), silver nitrate
(used in the preparation of silver salts for photography, in chemical analysis, in silver
plating, in inks and hair dyes, and to silver mirrors.), and ammonium nitrate (Major uses are in fertilizers and explosives). Calcium nitrate
is used in fertilizers; barium and strontium nitrates are used to color
fireworks and signal flares; bismuth nitrate is used in making pharmaceuticals.
Most nitrates are soluble in water, and a major use of nitric acid is to produce
soluble metal nitrates. All nitrates decompose when heated and may do so
explosively. The presence of
nitrates in the soil is of great importance, since it is from these compounds
that plants obtain the nitrogen necessary for their growth. Organic
nitrates are esters formed by reaction of nitric acid with the hydroxyl (-OH)
group in an alcohol. Nitrocellulose (or cellulose nitrate) is
a highly flammable compound formed when cellulose materials are treated with concentrated nitric acid. The extent of the reaction between the cotton and acid can be varied to give a
range of compounds, from the highly explosive gun-cotton to the flammable
collodion cotton or pyroxilin. These are now used worldwide as propellants in cartridges and other ammunition.
Collodion cotton and other less reactive forms of nitrocellulose are used
chiefly in lacquers. They also form the basis of one of the earliest plastics,
celluloid,
made by the action on nitrocellulose of a solution of
camphor in ethanol. Guncotton, fully nitrated cellulose, is used for explosives.
Nitroglycerin is the nitric acid triester of glycerol and is
more correctly called glycerol trinitrate. It is mixed with an absorbent material to form dynamite and is also used as a component of smokeless powder. Guncotton, fully nitrated cellulose is
used for explosives.
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