Everything about Nitric Acid totally explained
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Nitric acid (
HNO3), also known as
aqua fortis and
spirit of nitre, is a highly
corrosive and
toxic strong acid that can cause severe burns. The synthesis of nitric acid was first recorded circa
800 AD by the Muslim
alchemist Jabir ibn Hayyan.
Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of
oxides of nitrogen. If the solution contains more than 86% nitric acid, it's referred to as
fuming nitric acid. Fuming nitric acid is characterized as
white fuming nitric acid and
red fuming nitric acid, depending on the amount of
nitrogen dioxide present.
Properties
Pure anhydrous nitric acid (100%) is a colorless liquid with a density of 1522 kg/m³ which solidifies at -42 °C to form white crystals and boils at 83 °C. When boiling in light, even at room temperature, there's a partial
decomposition with the formation of nitrogen dioxide following the reaction:
» 4HNO
3 → 2H
2O + 4NO
2 + O
2 (72°C)
which means that anhydrous nitric acid should be stored below 0 °C to avoid decomposition. The
nitrogen dioxide (NO
2) remains dissolved in the nitric acid coloring it yellow, or red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapours, leading to the common name "red fuming acid" or "fuming nitric acid".
Nitric acid is
miscible with water in all proportions and
distillation gives an
azeotrope with a concentration of 68% HNO
3 and a boiling temperature of 120.5 °C at 1 atm. Two solid hydrates are known; the monohydrate (HNO
3·H
2O) and the trihydrate (HNO
3·3H
2O).
Nitrogen oxides (NO
x) are soluble in nitric acid and this property influences more or less, all the physical characteristics depending on the concentration of the oxides. These mainly include the vapor pressure above the liquid and the boiling temperature, as well as the color mentioned above.
Nitric acid is subject to
thermal or light decomposition with increasing
concentration and this may give rise to some non-negligible variations in the vapour pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.
Acidic properties
Being a typical acid, nitric acid reacts with
alkalis,
basic oxides, and
carbonates to form
salts, such as
ammonium nitrate. Due to its oxidizing nature, nitric acid generally doesn't donate its proton (that is, it doesn't liberate
hydrogen) on reaction with
metals and the resulting salts are usually in the higher oxidized states. For this reason, heavy
corrosion can be expected and should be guarded against by the appropriate use of corrosion resistant metals or
alloys.
Nitric acid has an
acid dissociation constant (pK
a) of −1.4: in
aqueous solution, it almost completely (93% at 0.1 mol/L)
ionizes into the
nitrate ion NO
3− and a
hydrated proton, known as a
hydronium ion, H
3O
+.
» HNO
3 + H
2O → H
3O
+ + NO
3-
Oxidizing properties
Reactions with metals
Being a powerful oxidizing agent, nitric acid reacts violently with many organic materials and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction then takes place with all metals except the
precious metal series and certain alloys. As a general rule of course, oxidizing reactions occur primarily with the concentrated acid, favouring the formation of
nitrogen dioxide (NO
2).
» Cu + 4HNO
3 → Cu(NO
3)
2 + 2NO
2 + 2H
2O
The acidic properties tend to dominate with dilute acid, coupled with the preferential formation of
nitrogen oxide (NO).
» 3Cu + 8HNO
3 → 3Cu(NO
3)
2 + 2NO + 4H
2O
Since nitric acid is an oxidizing agent,
hydrogen (H
2) is rarely formed. Only
magnesium (Mg), Manganese (Mn) and
calcium (Ca) react with
cold,
dilute nitric acid to give hydrogen:
» Mg
(s) + 2HNO
3 (aq) → Mg(NO
3)
2 (aq) + H
2 (g)
Passivation
Although
chromium (Cr),
iron (Fe) and
aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the metal from further oxidation, which is called
passivation.
Reactions with non-metals
Reaction with non-metallic elements, with the exception of
silicon and
halogens, usually oxidizes them to their highest
oxidation states as acids with the formation of
nitrogen dioxide for concentrated acid and
nitrogen oxide for dilute acid.
» C + 4HNO
3 → CO
2 + 4NO
2 + 2H
2O
or
» 3C + 4HNO
3 → 3CO
2 + 4NO + 2H
2O
Grades
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% water and a maximum of 0.5% dissolved NO
2.
Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO
2) leaving the solution with a reddish-brown color. One formulation of RFNA specifies a minimum of 17% NO
2, another specifies 13% NO
2.
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).
Industrial production
Nitric acid is made by mixing
nitrogen dioxide (NO
2) with
water in the presence of oxygen or air to oxidize the
nitrous acid also produced by the reaction.
Dilute nitric acid may be concentrated by distillation up to 68% acid, which is an
azeotropic mixture with 32% water. Further concentration involves distillation with
sulfuric acid which acts as a dehydrating agent. In the laboratory, such distillations must be done with all-glass apparatus at reduced pressure, to prevent decomposition of the acid.
Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid. Commercial production of nitric acid is via the
Ostwald process, named after
Wilhelm Ostwald.
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 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.
Laboratory synthesis
In laboratory, nitric acid can be made from
copper(II) nitrate or by reacting approximately equal masses of
potassium nitrate (KNO
3) with 96%
sulfuric acid (H
2SO
4), and
distilling this mixture at nitric acid's boiling point of 83 °C until only a white crystalline mass,
potassium hydrogen sulfate (KHSO
4), remains in the reaction vessel. The obtained red fuming nitric acid may be converted to the white nitric acid.
» H
2SO
4 + KNO
3 → KHSO
4 + HNO
3
The dissolved
NOx are readily removed using reduced pressure at room temperature (10-30 min at 200
mmHg or 27
kPa) to give white fuming nitric acid. This procedure can also be performed under reduced pressure and temperature in one step in order to produce less
nitrogen dioxide gas.
Uses
IWFNA may be used as the
oxidizer in
liquid fuel rockets.
IRFNA was one of 3 liquid fuel components for the
BOMARC missile
A solution of nitric acid and alcohol,
Nital, is used for etching of metals to reveal the microstructure.
Commercially available aqueous blends of 5-30% nitric acid and 15-40% phosphoric acid are commonly used for cleaning food and dairy equipment primarily to remove precipitated calcium and magnesium compounds (either deposited from the process stream or resulting from the use of hard water during production and cleaning).
Digestion
In
elemental analysis by
ICP-MS and
ICP-AES, dilute nitric acid (0.5 to 2.0 %) is used as a matrix compound for determining metal traces in solutions. Ultrapure acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.
Woodworking
In a low concentration (approximately 10%), nitric acid is often used to artificially age
pine and
maple. The color produced is a grey-gold very much like very old wax or oil finished wood (
wood finishing).
Other uses
Alone, it's 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.
Safety
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 (for example, self-igniting).
Concentrated nitric acid dyes human
skin yellow due to a reaction with the
keratin. These yellow stains turn orange when neutralized.
Further Information
Get more info on 'Nitric Acid'.
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