`=>` In the laboratory, nitric acid is prepared by heating `color{red}(KNO_3)` or `color{red}(NaNO_3)` and concentrated `color{red}(H_2SO_4)` in a glass retort.

`color{red}(NaNO_3+H_2SO_4 → NaHSO_4+HNO_3)`

`=>` On a large scale it is prepared mainly by Ostwald’s process. This method is based upon catalytic oxidation of `color{red}(NH_3)` by atmospheric oxygen.

`color{red}(4NH_3(g) +undersettext{(from air)} (5O_2 (s)) underset(500K , 9 text(bar)) oversettext(Pt /Rh gauge catalyst)→ 4NO(g) +6H_2O (g))`

Nitric oxide thus formed combines with oxygen giving `color{red}(NO_2)`

`color{red}(2NO (g) + O_2 (g) ⇌ 2NO_2 (g))`

Nitrogen dioxide so formed, dissolves in water to give `color{red}(HNO_3)`.

`color{red}(3NO_2(g) +H_2O(l) → 2HNO_3 (aq) +NO (g))`

● `color{red}(NO)` thus formed is recycled and the aqueous `color{red}(HNO_3)` can be concentrated by distillation upto `~ 68%` by mass.

● Further concentration to `98% `can be achieved by dehydration with concentrated `color{red}(H_2SO_4)`.

● It is a colourless liquid (f.p. `231.4 K` and b.p. `355.6 K`).

● Laboratory grade nitric acid contains `~ 68%` of the `color{red}(HNO_3)` by mass and has a specific gravity of `1.504`.

● In the gaseous state, `color{red}(HNO_3)` exists as a planar molecule with the structure as shown.

● In aqueous solution, nitric acid behaves as a strong acid giving hydronium and nitrate ions.

`color{red}(HNO_3 (aq) +H_2O(l) → H_3O^+ (aq) + NO_3^(-) (aq))`

● Concentrated nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum.

● The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation.

`color{red}(3Cu +8 HNO_3 text(dilute) → 3Cu (NO_3)_2+2NO+4H_2O)`

`color{red}(Cu +4HNO_3 text(Conc.) → Cu(NO_3)_2 +2NO_2+2H_2O)`

● Zinc reacts with dilute nitric acid to give `color{red}(N_2O)` and with concentrated acid to give `color{red}(NO_2)`.

`color{red}(4Zn+10HNO_3 text(dilute) → 4Zn (NO_3)_2+5H_2O +N_2O)`

`color{red}(Zn +4HNO_3 (text(Conc.)) → Zn (NO_3)_2 +2H_2O +2NO_2)`

● Some metals (e.g., `color{red}(Cr, Al)` ) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface.

● Concentrated nitric acid also oxidises non–metals and their compounds. Iodine is oxidised to iodic acid, carbon to carbon dioxide, sulphur to `color{red}(H_2SO_4)`, and phosphorus to phosphoric acid.

`color{red}(I_2+10HNO_3 → 2HIO_3 +10NO_2+4H_2O)`

`color{red}(C +4HNO_3 → CO_2 +2H_2O+4NO_2)`

`color{red}(S_8+48HNO_3 (text(Conc.)) → 8H_2SO_4+48NO_2+16 H_2O)`

`color{red}(P_4 +20 HNO_3 ( text(conc.)) → 4H_3PO_4 +20NO_2+4H_2O)`



● `color{green}("Brown Ring Test" )`: The familiar brown ring test for nitrates depends on the ability of `color{red}(Fe^(2+))` to reduce nitrates to nitric oxide, which reacts with `color{red}(Fe^(2+))` to form a brown coloured complex.

`color{green}("Method")` : The test is usually carried out by adding dilute ferrous sulphate solution to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicate the presence of nitrate ion in solution.

`color{red}(NO_3 +3Fe^(2+) +4H^(+) → NO +3Fe^(3+) +2H_2O)`

`color{red}([Fe(H_2O)_6]^(2+) +NO → undersettext{(brown)} ([ Fe (H_2O)_5 (NO)]^(2+) + H_2O)`

`=>` The major use of nitric acid is in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives and pyrotechnics.

`=>` It is also used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds.

`=>` Other major uses are in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels.