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Nitric Acid

For a long time all nitric acid (HNO3) used to be produced from nitrates occurring in nature. For example, nitric acid can be obtained by adding sulphuric acid to saltpeter (potassium nitrate), whereby the latter undergoes decomposition.

In the pure state the acid is a colorless liquid which boils at 87 oC. The boiling point rises with increasing dilution. As pure nitric acid will always undergo some decomposition when left to stand, especially when exposed to the action of light, acid with a concentration exceeding 90% almost invariably contains some dissolved NO2. This decomposition process can be stopped by diluting the acid with water.

Nitric Acid

Nitric acid produces a very convincing aged effect on maple and especially on pine. It is an extremely dangerous chemical to work with full-strength, so basically recommend 10-20% solution. After applying the nitric acid, heat the wood with an electric heat gun to develop the color effect. Then neutralize the nitric acid with a solution of baking soda dissolved in water. Allow at least one week drying time before finishing.

Nitric Acid

The various processes for the commercial manufacture of nitric acid are based on any of three principles: the decomposition of nitrates more particularly Chile saltpeter with sulphuric acid: the direct synthesis of NO from nitrogen and oxygen in an electric arc, followed by the last two reactions of the ammonia process or the catalytic oxidation of ammonia.

The ammonia process is now most widely employed, more particularly with platinum as the catalyst. In certain variants of the process the catalyst may, however, be Fe2O3, Mn2O3 or Bi2O3. In this process liquid ammonia is vaporized in an evaporator and is mixed with air. The gas mixture (10% ammonia and 90% air) makes its way through a filter and a preheater to the contact reactor, in which it passes over platinum gauze (the catalyst) heated initially to about 900 oC. Heat liberated in the reaction maintains the temperature of the catalyst. In the reactor about 90% of the ammonia is oxidized to nitric oxide: 4NH3 + 5O2 -> 4 NO +6H2O

Directly after the platinum gauze are filtering agents which trap the unstable platinum compounds present in the gas discharged from the reactor: the platinum precipitated in this way is recovered. This gas, of which 97 ½% is NO, is cooled in the gas cooler. On leaving the cooler, it is passed through absorption towers, filled with rings made of ceramic material, in which two reactions take place. In the first tower the nitric oxide is oxidized to nitrogen dioxide. 2 NO+O2 -> 2 NO2. In the following towers (four in all) the dioxide reacts with water to form nitric acid: 3 NO2 +H2O -> 2 HNO3.

The requisite water is added in the last tower. In the preceding towers the NO2 is brought into contact not with water, but with nitric acid solution, which is circulated by pumps and passed through cooling apparatus to remove the heat evolved in the reaction, as low temperatures are favorable to the absorption reactions. In a degasifying tower air is blown through the acid to remove such amounts of NO gas as are still present in it. The exhaust gas from the final absorption tower contains 0.3 to 0.4 % NO.

The installation described here produces nitric acid in a concentration of between 40 and 60%. A higher concentration (Upto 99.5%) is obtainable by distillation with concentrated sulphuric acid. Nitric acid is used in the manufacture of fertilizers, explosives, lacquers, dyes, plastics and synthetic fibers.

Alone, it is useful in metallurgy and refining as it reacts with most metals, and in organic syntheses. When mixed with hydrochloric acid, nitric acid forms Aqua Regia, one of the few reagents capable of dissolving gold and platinum.

Nitric acid is a powerful oxidizing agent, and the reactions of nitric acid with compounds such as cyanides, carbides, and metallic powders can be explosive. Reactions of nitric acid with many organic compounds, such as turpentine, are violent and hypergolic (i.e., self-igniting). Concentrated nitric acid dyes human skin yellow due to a reaction with the keratin. These yellow stains turn orange when neutralized.