Nitric acid

The chemical compound nitric acid (H N O₃), otherwise known as aqua fortis, is a colorless, corrosive liquid, a toxic acid which can cause severe burns.

At room temperature it gives off red or yellow fumes. Commonly used as a laboratory reagent, it is used in the manufacture of explosives such as nitroglycerin and trinitrotoluene (TNT), and as well as of fertilizers such as ammonium nitrate. It has additional uses in metallurgy and refining as it reacts with most metals, and in organic syntheses. When combined with hydrochloric acid it forms aqua regia, one of the few reagents capable of dissolving gold and platinum. Nitric acid is also a component of acid rain.

Nitric acid is a strong acid with a pK_a of -2: in aqueous solution, it completely dissociates into the nitrate ion NO₃⁻ and a hydrated proton, known as a hydronium ion, H₃O⁺. The salts of nitric acid (which contain the nitrate ion) are also known as nitrates. The overwhelming majority of them are very soluble in water.

Nitric acid is made by mixing nitrogen dioxide (N O₂) with water. Creating a very pure nitric acid usually involves distillation with sulfuric acid, as nitric acid forms an azeotrope with water with a composition of 68% nitric acid and 32% water. Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid. If the nitric acid solution contains more than 86% nitric acid, it is referred to as fuming nitric acid, and can be separated into two kinds of fuming acids, white fuming nitric acid, and red fuming nitric acid.

White fuming nitric acid, also called 100% nitric acid or WFNA, is very close to the anhydrous nitric acid product. One specification for white fuming nitric acid is that it has a maximum of 2 percent water and a maximum of 0.5 percent dissolved NO₂. Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO₂) leaving the solution with a reddish-brown color. One formulation of RFNA specifies a minimum of 17% NO₂, another specifies 13% NO₂. In either event, an inhibited fuming nitric acid (either IWFNA, or IRFNA) can be made by the addition of 0.6 to 0.7% hydrogen fluoride, HF. This fluoride is added for corrosion resistance in metal tanks (the fluoride creates a metal fluoride layer that protects the metal). The obvious use for such a corrosion inhibited product is as an oxidizer in liquid fuel rockets.

Nitric acid is a very 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 on contact, due to interactions with the skin protein keratin. Strangely, these yellow stains turn orange when alkalised.

Commercial production of nitric acid is via the Ostwald process after Wilhelm Ostwald.

Nitric acid and its salts, the nitrates, should not be confused with nitrous acid and its salts, the nitrites.

Synthesis

Nitric acid can be made by reacting 200 g of potassium nitrate (KNO₃) in 106 ml of 96% sulphuric acid (H₂SO₄), and distilling this mixture at nitric acid's boiling point of 83 °C until only a white crystalline mass, potassium hydrogen sulfate or potassium bisulfate (KHSO₄), remains in the reaction vessel. The obtained red fuming nitric acid may be converted to the white nitric acid.
The dissolved NO_x are readily removed using reduced pressure at room temperature (10-30 min at 20 mmHg or 27 kPa). Obtained white fuming nitric acid has density 1.51 g/cm³. This procedure can also be performed under reduced pressure and temperature in one step in order to produce less nitrogen dioxide gas.

The acid can also be synthesized by oxidizing ammonia, but the product is diluted by the water also formed as part of the reaction. However, this synthesization method is important in producing ammonium nitrate from ammonia derived from the Haber process, because the final product can be produced from nitrogen, hydrogen, and oxygen as the sole feedstocks.

Nitric acid was first synthesized circa 800 CE by alchemist Jabir ibn Hayyan, who also invented modern distillation and numerous other basic chemical processes still in use today.

Reactions

Pour 10 mL of cyclohexanol into a beaker under a fume hood. Add 10 mL of red fuming nitric acid to the beaker and quickly close the fume hood. Nitrogen dioxide gas is evolved, filling the fume hood. NO₂ is poisonous, causing pulmonary edema. Possibly the largest danger of NO₂ inhalation is that effects can take up to two days to manifest.

Place 10 g of activated charcoal into a beaker under a fume hood. Add 10 mL of red fuming nitric acid to the beaker and quickly close the fume hood. The nitric acid will oxidize the charcoal, and the reaction is very exothermic.

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Categories: Nitrates | Acids