Structure of Nitric acid
Nitric acid is a strong mineral acid. It is a mono–basic acid, strong oxidizing agent and can oxidize metals and non–metals easily. It is used in the manufacture of fertilizers, silk industry, explosive materials such as (T.N.T) etc. On industrial scale, nitric acid can be prepared by the following methods.
Principle reactions involved in Ostwald's process
Pure anhydrous nitric acid is a colorless liquid. But for prolonged storage even at room temperature it partially decomposes into nitrogen dioxide(NO2). Color of nitrogen dioxide is brown. Due to this dissolved nitrogen dioxide conc. nitric acid (HNO3) is brownish red in color.
General properties
Pure anhydrous nitric acid (100%) is a colorless mobile liquid with a density of 1.512 g/cm3 which solidifies at –42°C to form white crystals and boils at 83°C. When boiling in light, and slowly even at room temperature, there is a partial decomposition with the formation of nitrogen dioxide following the reaction:
Thus, anhydrous nitric acid should be stored below 0oC to avoid decomposition. The nitrogen dioxide (NO2) remains dissolved in the nitric acid coloring it yellow or red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish–brown vapors, leading to the common name “red fuming acid” or “fuming nitric acid”. Fuming nitric acid is also referred to as 16 molar nitric acid. It is the most concentrated form of nitric acid at Standard Temperature and Pressure (STP).
Nitric acid is miscible with water. The two solid hydrates of nitric acid known are the monohydrate (HNO3.H2O) and the trihydrate (HNO3.3H2O).
Nitration mixture
Nitration mixture is the 1:1 mixture of concentration H2SO4 and concentration HNO3. Sulphuric acid ionises the nitric acid into nitronium ion. This nitronium ion (electrophilic) attacks the benzene to given nitro benzene
Acid–base properties
Nitric acid is normally considered to be a strong acid at ambient temperatures, the pKa value is usually reported as less than –1. Nitric acid can act as a base with respect to an acid such as sulfuric acid.
The nitronium ion, NO+2 is the active reagent in aromatic nitration reactions.
Since nitric acid has both acidic and basic properties it can undergo an autoprotolysis reaction, similar to the self–ionization of water
Nitric acid used as oxidizing agent
Formation of Xanthoprotein (yellow)
Oxidizing properties
Nitric acid is a strong oxidizing agent as shown by its large positive reduction potential (E0= +ve ).
Being a powerful oxidizing agent, nitric acid reacts violently with many non–metallic compounds and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable.
Reaction takes place with all metals except the noble metals series and certain alloys. Some precious metals, such as pure gold do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid.
As a general rule, oxidizing reactions occur primarily with the concentrated acid, favoring the formation of nitrogen dioxide (NO2).
Nitric acid is used as a cheap means in jewellery shops to quickly spot low–gold alloys ( < 14 carats) and to rapidly assess the gold purity.
Xanthoproteic test
Nitric acid reacts with proteins to form yellow nitrated products. This reaction is known as the xanthoproteic reaction. This test is performed to identify proteins that contain aminoacids with aromatic rings.