Nitric acid

The monobasic strong acid, which is a colorless liquid under standard conditions, which turns yellow when stored, can be in a solid state, characterized by two crystalline modifications (monoclinic or rhombic lattice), at temperatures below minus 41.6 ° C. This substance with a chemical formula - HNO3 - is called nitric acid. It has a molar mass of 63.0 g / mol, and its density corresponds to 1.51 g / cm³. The boiling point of the acid is 82.6 ° C, the process is followed by decomposition (partial): 4HNO3 → 2H2O + 4NO2 + O2. A solution of acid with a mass fraction of the basic substance equal to 68% boils at a temperature of 121 ° C. The refractive index of a pure substance corresponds to 1.397. The acid is able to mix with water in any ratio and, being a strong electrolyte, almost completely break up into H + and NO3- ions. Solid forms - trihydrate and monohydrate have the formulas: HNO3 • 3H2O and HNO3 • H2O, respectively.

Nitric acid is a corrosive, toxic substance and a strong oxidizer. From the Middle Ages, the name "strong water" (Aqua fortis) is known. The alchemists, who discovered acid in the 13th century, gave this name, making sure of its extraordinary properties (corroded all metals except gold), exceeding by a million times the strength of acetic acid, which in those days was considered the most active. But three centuries later, it was found that a mixture of acids such as nitric and hydrochloric acid in a volumetric ratio of 1: 3, which for this reason was called "royal vodka", could be eroded, even gold. The appearance of a yellow shade during storage is explained by the accumulation of nitrogen oxides in it. On sale, acid is more often at a concentration of 68%, and when the content of the main substance is more than 89% it is called "fuming".

The chemical properties of nitric acid distinguish it from dilute sulfuric or hydrochloric acids in that HNO3 is a stronger oxidizer, so hydrogen is never released in reactions with metals. Due to its oxidizing properties, it also reacts with many non-metals. In both cases, NO2 is always formed. In redox reactions, nitrogen reduction occurs to various degrees: HNO3, NO2, N2O3, NO, N2O, N2, NH3, which is determined by acid concentration and metal activity. The molecules of the compounds formed contain nitrogen with an oxidation state of +5, +4, +3, +2, +1, 0, +3, respectively. For example, copper is oxidized with concentrated acid to copper (II) nitrate: Cu + 4HNO3 → 2NO2 + Cu (NO3) 2 + 2H2O, and phosphorus to metaphosphoric acid: P + 5HNO3 → 5NO2 + HPO3 + 2H2O.

Otherwise, dilute nitric acid reacts with nonmetals. Using the example of the reaction with phosphorus: 3P + 5HNO3 + 2H2O → 3H3PO4 + 5NO, it can be seen that nitrogen is reduced to a divalent state. As a result, nitrogen monoxide is formed, and phosphorus is oxidized to phosphoric acid. Concentrated nitric acid in admixture with hydrochloric acid dissolves gold: Au + 4HCl + HNO3 → NO + H [AuCl4] + 2H2O and platinum: 3Pt + 18HCl + 4HNO3 → 4NO + 3H2 [PtCl6] + 8H2O. In these reactions, at the initial stage, hydrochloric acid is oxidized with nitrogen and chlorine is released, and then the metals form complex chlorides.

Nitric acid on an industrial scale is obtained in three main ways:

1. The first is the interaction of salts with sulfuric acid: H2SO4 + NaNO3 → HNO3 + NaHSO4. Previously, this method was the only one, but, with the advent of other technologies, it is now used in the laboratory to produce fuming acid.
2. The second is the arc method. When air is blown through an electric arc with a temperature of 3000 to 3500 ° C, some of the nitrogen of the air reacts with oxygen, with the formation of nitrogen monoxide: N2 + O2 → 2NO, which after cooling is oxidized to nitrogen dioxide (at high temperatures, the monoxide does not react with oxygen) : O2 + 2NO → 2NO2. Then, practically, all nitrogen dioxide, with an excess of oxygen, dissolves in water: 2H2O + 4NO2 + O2 → 4HNO3.
3. The third is the ammonia method. Ammonia is oxidized on a platinum catalyst to nitrogen monoxide: 4NH3 + 5O2 → 4NO + 6H2O. The formed nitroses are cooled, and nitrogen dioxide is formed, which is absorbed by water. This method produces an acid with a concentration of 60 to 62%.

Nitric acid in industry is widely used for the production of drugs, dyes, explosives, nitrogen fertilizers and nitric acid salts. In addition, it is used to dissolve metals (for example, copper, lead, silver) that do not react with other acids. In the jewelry business is used to determine gold in the alloy (this method is the main one).