Nitric Acid, HNO₃

Nitric acid was one of the earliest nitrogen compounds to be used, as it was known to the Egyptians. Its preparation was first described by Geber in a.d. 778, who obtained it by distilling a mixture of nitre, blue vitriol, and alum. In the thirteenth century Lullius prepared it from nitre and iron sulphate, while Glauber in 1648 showed its production from nitre and oil of vitriol. Known under such names as "aqua-fortis," "spiritus nitri acidus," and "spiritus nitri fumans Glauberi," it was largely used by the alchemists for the separation of silver and gold. Mayow in 1669 was the first to propound a theory as to its composition, as he recognised that the same substance was present in nitre as in the air which supported combustion, namely, dephlogisti- cated air (oxygen). In 1776 Lavoisier showed that oxygen was a definite constituent of nitric acid, although he was unable to show how it was combined with the other constituents. Cavendish in 1784 showed that nitric acid was produced by passing electric sparks through a mixture of nitrogen and oxygen, and about the same time it was proved by Lavoisier and Priestley that it consisted only of nitrogen, oxygen, and water.

Free nitric acid is only found in nature in small amounts, chiefly in rain-water, where it has been produced by electrical discharges in the atmosphere. Neutralisation of rain-water occurs by means of soil bases with the formation of nitrates, and this salt production is also the result of free nitric-acid formation by the oxidation of nitrogenous organic matter.

Main article: Manufacture of Nitric Acid from Sodium Nitrate

1. The laboratory method for preparing nitric acid is to distil dry potassium nitrate with an equal weight of concentrated sulphuric acid in a retort, and to collect the nitric acid in a well-cooled flask:
   
   KNO₃ + H₂SO₄ = KHSO₄ + HNO₃.
   
   The distillation is carried out at as low a temperature as possible, and the product is of a distinctly yellow colour owing to the presence of nitrogen peroxide. Purification may be effected by redistilling with an equal volume of concentrated sulphuric acid, and blowing a current of air through this distillate, which has been gently warmed.
2. Combination of nitrogen and oxygen by passing through the electric arc, and absorbing the oxides of nitrogen in water.
3. Oxidation of ammonia by means of a catalyst.
   
   Nitric acid is obtained by a number of other reactions which can scarcely be called methods of preparation:
   1. Burning hydrogen in air.
   2. Passing electric sparks through moist air.
   3. Action of silent electric discharge on a mixture of nitrogen and oxygen in the presence of water.

The outstanding property of nitric acid is its oxidising nature. Its corrosive action on the skin produces painful wounds, and even dilute nitric acid causes an intense yellow coloration due to the formation of xantho-proteic acids. Hair, wool, silk, and many other organic substances are coloured similarly. The decomposition of nitric acid by heat is readily shown by supporting a churchwarden clay pipe so that the stem is inclined downwards with the end under a beehive shelf in a trough of water. An inch or two of the stem is raised to a red heat by a burner, and a few cubic centimetres of nitric acid poured into the bowl. Decomposition occurs according to the equation

4HNO₃ = 4NO₂ + O₂ + 2H₂O.

The oxygen can be collected in a gas cylinder over water, while the nitrogen peroxide dissolves with the formation of nitric and nitrous acids.

A large number of products result from the reduction of nitric acid, and the following can be identified under different conditions: nitrous acid HNO₂, nitrogen peroxide NO₂, nitric oxide NO, nitrous oxide N₂O, nitrogen N₂, ammonia NH₃, and hydroxylamine NH₂OH.

Action of Nitric Acid on Non-Metals

For details about Action of Nitric Acid on Metals read main article Action of Nitric Acid on Metals

Most non-metals are quickly oxidised in many cases to their highest state of oxidation, and often this occurs violently. Thus phosphorus is converted first into phosphorous and then into phosphoric acid:

6P + 10HNO₃ + 4H₂O = 6H₃PO₄ + 10NO.

Sulphur is oxidised to sulphuric acid:

S + 2HNO₃ = H₂SO₄ + 2NO.

Iodine is oxidised to iodic acid:

3l₂ + 10HNO₃ = 6HIO₃ + 10NO + 2H₂.

Boron is converted into boric acid and selenium into selenious acid.

Action of Nitric Acids on Compounds

Sulphuretted hydrogen is oxidised to sulphur, sulphur dioxide to sulphuric acid, with production of nitric oxide. In the presence of concentrated sulphuric acid, nitrosyl-sulphuric acid is produced.

Hydrogen bromide yields bromine and nitrogen peroxide, while hydrogen iodide gives iodine and nitric oxide. Hydrogen chloride produces a solution which contains nitrosyl chloride and chlorine (aqua-regia=4 vols. conc. HCl + 1 vol. conc. HNO₃):

HNO₃ + 3HCl = NOCl + Cl₂ + 2H₂O.

Ferrous salts are oxidised to ferric with production of nitric oxide; stannous salts give stannic salts, hydroxylamine, and ammonia. Arsenious chloride and arsenious oxide give arsenic acid, and similarly antimony chloride is oxidised to antimonic acid.

Free nitric acid is detected by the ready production of brown fumes when warmed with metallic copper. Nitrates behave similarly when treated with concentrated sulphuric acid and copper.

The brown-ring test consists in adding a solution of ferrous sulphate to the solution of nitric acid or nitrate, and carefully pouring concentrated sulphuric acid down the side of the test-tube. The strong acid collects at the bottom of the tube, and at the junction of the two liquids a brown ring is formed containing the compound FeSO₄.NO.

Brucine, in the presence of concentrated sulphuric acid, develops a red colour.

"Nitron" gives a white precipitate with nitric acid or nitrates; the test is sensitive to 1 part per 60,000.

Estimation

There are a large number of methods in use for estimating nitric acid and nitrates. The more important of these methods may be classified as follows: -

1. Titration Methods.
2. Gasometric Methods.
3. Gravimetric Method.

1. Titration Methods. -
   1. The simplest method of estimation of free nitric acid is by titrating with standard alkali. Any indicator may be used in the absence of nitrous acid, but if this is present, then methyl red is preferable, as it is only slowly acted upon.
   2. The Bowman and Scott method consists in reducing the nitric acid (or nitrate) to N₂O₃ by means of a solution of ferrous sulphate in strong sulphuric acid, the end point being indicated by the appearance of a pinkish-brown coloration. Nitrous acid and nitrites do not interfere, but chlorates, bromates, iodates, chlorides, bromides, and iodides must be absent.
   3. The Pelouze-Fresenius method is based upon the reduction of nitrate by means of ferrous chloride, and the estimation of the excess of ferrous salt by titration with standard potassium permanganate:
      
      2KNO₃ + 6FeCl₂ + 8HCl = 6FeCl₃ + 2KCl + 2NO + 4H₂O.
      
      This method appears to be accurate with dilute solutions of nitrates.
   4. Many modifications of the reduction of nitrates to ammonia have been proposed. The original method was by heating the nitrate with Devarda's alloy in alkaline solution, and subsequently distilling the ammonia into standard acid solution.
   
   
   Other reducing agents are, reduced iron and sulphuric acid, aluminium amalgam in alkaline solution, and titanous sulphate. The last-named method has the advantage of quickness with accuracy. Nitrates can also be estimated by the electrolytic reduction in a solution of copper sulphate containing a known quantity of standard acid.
2. Gasometric Methods. -
   1. The Lunge nitrometer is very largely used for the estimation of nitric acid and nitrates, but is liable to a number of errors due to the solubility of nitric oxide in sulphuric acid, temperature fluctuations, period of shaking, etc.
   2. The Schloesing-Grandeau method depends upon the reduction of nitrates to nitric oxide (which is measured) by boiling with ferrous chloride and hydrochloric acid:
      
      NaNO₃ + 3FeCl₂ + 4HCl = NaCl + 3FeCl₃ + NO + 2H₂O.
3. Gravimetric Method. - Nityon has already been mentioned as affording a delicate means of detecting nitrates. This substance is 1:4 diphenyl, 3:5 endoanilido, 4:5 dihydro, 1:2:4 triazole, and gives a crystalline insoluble nitrate even with minute quantities of nitric acid. The precipitate obtained by using an acetic acid solution of nitron is filtered and weighed in a Gooch crucible.
   
   Nitrous and nitric acids in a mixture may be estimated by first estimating the nitrous acid by means of potassium permanganate, and then oxidation of the nitrous acid in another portion with hydrogen peroxide and estimating the total nitric acid with nitron.
   
   The nitron method for nitrates is inapplicable in the presence of hydrobromic, hydriodic, chloric, perchloric, thiocyanic, ferrocyanic, ferricyanic, and picric acids.