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Commercially available nitric acid is an azeotrope with water at a concentration of 68% HNO3. This solution has a boiling temperature of 120.5 °C (249 °F) at 1 atm. It is known as “concentrated nitric acid”. The azeotrope of nitric acid and water is a colourless liquid at room temperature.
Two solid hydrates are known: the monohydrate HNO3·H2O or oxonium nitrate [H3O]+[NO3]− and the trihydrate HNO3·3H2O.
An older density scale is occasionally seen, with concentrated nitric acid specified as 42 Baumé[22]
Fuming nitric acid contaminated with yellow nitrogen dioxide
Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:
4 HNO3 → 2 H2O + 4 NO2 + O2
This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.
The nitrogen dioxide (NO2) and/or dinitrogen tetroxide (N2O4) remains dissolved in the nitric acid coloring it yellow or even 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 vapors, leading to the common names “red fuming nitric acid” and “white fuming nitric acid”. Nitrogen oxides (NOx) are soluble in nitric acid.
Main article: Red fuming nitric acid
Commercial-grade fuming nitric acid contains 98% HNO3 and has a density of 1.50 g/cm3. This grade is often used in the explosives industry. It is not as volatile nor as corrosive as the anhydrous acid and has the approximate concentration of 21.4 M.
Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO2) leaving the solution with a reddish-brown color. Due to the dissolved nitrogen dioxide, the density of red fuming nitric acid is lower at 1.490 g/cm3.
An inhibited fuming nitric acid, either white inhibited fuming nitric acid (IWFNA), or red inhibited fuming nitric acid (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.
White fuming nitric acid, pure nitric acid or WFNA, is very close to anhydrous nitric acid. It is available as 99.9% nitric acid by assay, or about 24 molar. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved NO2. Anhydrous nitric acid is a colorless, low-viscosity (mobile) liquid with a density of 1.512–3 g/cm3 that solidifies at −42 °C (−44 °F) to form white crystals. Its dynamic viscosity under standard conditions is 0.76 cP As it decomposes to NO2 and water, it obtains a yellow tint. It boils at 83 °C (181 °F). It is usually stored in a glass shatterproof amber bottle with twice the volume of head space to allow for pressure build up, but even with those precautions the bottle must be vented monthly to release pressure.
Two major resonance representations of HNO3
The two terminal N–O bonds are nearly equivalent and relatively short, at 1.20 and 1.21 Å This can be explained by theories of resonance; the two major canonical forms show some doubl bond character in these two bonds, causing them to be shorter than N–O single bonds. The third N–O bond is elongated because its O atom is bonded to H atomwith a bond length of 1.41 Å in the gas phase.The molecule is slightly aplanar (the NO2 and NOH planes are tilted away from each other by 2°) and there is restricted rotation about the N–OH single bond.
Nitric acid is normally considered to be a strong acid at ambient temperatures. There is some disagreement over the value of the acid dissociation constant, though the ph value is usually reported as less than −1. This means that the nitric acid in diluted solution is fully dissociated except in extremely acidic solutions. The pKa value rises to 1 at a temperature of 250 °C
Nitric acid can act as a base with respect to an acid such as sulfuric acid:
HNO3 + 2 H2SO4 ⇌ [NO2]+ + [H3O]+ + 2 HSO−4; Equilibrium constant: K ≈ 22
The nitronium ion, [NO2]+, is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties, it can undergo an autoprotolysis reaction, similar to the self-ionization of water:
2 HNO3 ⇌ [NO2]+ + NO−3 + H2O
Nitric acid reacts with most metals, but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese, and zinc liberate :
Mg + 2 HNO3 → Mg(NO3)2 + H2
Mn + 2 HNO3 → Mn(NO3)2 + H2
Zn + 2 HNO3 → Zn(NO3)2+ H2
Nitric acid can oxidize non-active metals such as copper and silver. With these non-active or less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry:
3 Cu + 8 HNO3 → 3 Cu(NO3)2 + 2 NO + 4 H2O
The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide. With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry:
Cu + 4 H+ + 2 NO−3 → Cu2+ + 2 NO2 + 2 H2O
Upon reaction with nitric acid, most metals give the corresponding nitrates. Some metalloids and metals give the oxides; for instance, Sn, As, Sb, and Ti are oxidized into SnO2 , AsO5 , Sb2O5 and TiO2 respectively .
Some precious metals, such as pure gold and platinum-group metals do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, …) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to colour changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (< 14 karats) and to rapidly assess the gold purity.
Being a powerful oxidizing agent, nitric acid reacts with many non-metallic compounds, sometimes explosively. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction takes place with all metals except the noble metals series and certain alloys. As a general rule, oxidizing reactions occur primarily with the concentrated acid, favoring the formation of nitrogen dioxide (NO2). However, the powerful oxidizing properties of nitric acid are thermodynamic in nature, but sometimes its oxidation reactions are rather kinetically non-favored. The presence of small amounts of nitrous acid (HNO2) greatly increases the rate of reaction.
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 bulk of the metal from further oxidation. The formation of this protective layer is called passivation .Typical passivation concentrations range from 20% to 50% by volum .Metals that are passivated by concentrated nitric acid are iron, cobalt, chromium, nickel, and aluminium
Being a powerful oxidizing acid, nitric acid reacts with many organic materials, and the reactions may be explosive. The hydroxyl group will typically strip a hydrogen from the organic molecule to form water, and the remaining nitro group takes the hydrogen’s place. Nitration of organic compounds with nitric acid is the primary method of synthesis of many common explosives, such as nitroglycerin and trinitrotoluene (TNT). As very many less stable byproducts are possible, these reactions must be carefully thermally controlled, and the byproducts removed to isolate the desired product.
Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen, noble gases, silicon, and halogens other than iodine, usually oxidizes them to their highest oxidation states as acids with the formation of nitrogen dioxide for concentrated acid and nitric oxide for dilute acid.
C (graphite) + 4 HNO3 → CO2 + 4 NO2 + 2 H2O
3 C (graphite) + 4 HNO3 → 3 CO2 + 4 NO + 2 H2O
Nitric acid reacts with proteins to form yellow nitrated products. This reaction is known as the xanthoproteic reaction. This test is carried out by adding concentrated nitric acid to the substance being tested, and then heating the mixture. If proteins that contain amin acids with aromatic rings are present, the mixture turns yellow. Upon adding a base such as ammonia, the color turns orange. These color changes are caused by nitrated aromatic rings in the protein. Xanthoproteic acid is formed when the acid contacts epithelial cells. Respective local skin color changes are indicative of inadequate safety precautions when handling nitric acid.
Industrial nitric acid production uses the Ostwald process. The combined Ostwald and Haber processes are extremely efficient, requiring only air and natural gas feedstocks .
The Ostwald process’ technical innovation is the proper conditions under which anhydrous ammonia burns to nitric oxide (NO) instead of dinitrogen (N2)]The nitric oxide is then oxidized, often with atmospheric oxygen, to nitrogen dioxide (NO2):
2 NO + O2 → 2 NO2
The dioxide then disproportionates in water to nitric acid and the nitric oxide feedstock:
3 NO2 + H2O → 2 HNO3 + NO
The net reaction is maximal oxidation of ammonia:
NH3 + 2 O2 → HNO3 + H2O
Primary Entry Routes : Inhalation, skin, eyes and ingestion
Acute Effects ; Inhalation of vapours can cause breathing difficulties,severe
exposure may lead to pneumonia and pulmonary edema. Ingestion
can cause immediate pain & burns of mouth, throat and
gastrointestinal tract,
Skin contact can cause redness,pain and skin burns.
Eye contact – vapours are irritating and may cause damage to
eyes.
Carcinogenicity : Not listed as carcinogenic.
Chronic Effects : Long term exposures seldom occur due to corrosive properties .
Handling Precautions : Protect from physical damage.
Storage Requirements : Store in a cool dry ventilated storage area with acid resistance
floors. Keep away from heat, water and incompatible materials.
SECTION 8 – EXPOSURE CONTROLS / PERSONAL PROTECTION
Engineering Controls : Provide proper ventilation so as to maintain environment below
air borne exposure limit.
Respiratory Protection : If exposure limit is exceeded,use respiratory protection.
Hazard statement(s)
H272 May intensify fire; oxidizer.
H290 May be corrosive to metals.
H314 Causes severe skin burns and eye damage.
H331 Toxic if inhaled.
Precautionary statement(s)
P210 Keep away from heat, hot surfaces, sparks, open flames and
other ignition sources. No smoking.
P220 Keep away from clothing and other combustible materials.
P280 Wear protective gloves/ protective clothing/ eye protection/ face
protection.
P303 + P361 + P353 IF ON SKIN (or hair): Take off immediately all contaminated
clothing. Rinse skin with water.
P304 + P340 + P310 IF INHALED: Remove person to fresh air and keep comfortable
for breathing. Immediately call a POISON CENTER/ doctor.
P305 + P351 + P338 IF IN EYES: Rinse cautiously with water for several minutes.
Remove contact lenses, if present and easy to do. Continue
rinsing.
Supplemental Hazard information (EU)
EUH071 Corrosive to the respiratory tract.
Reduced Labeling (<= 125 ml)
Pictogram
Signal Word Danger
Hazard statement(s)
H331 Toxic if inhaled.
H314 Causes severe skin burns and eye damage.
Precautionary statement(s)
P280 Wear protective gloves/ protective clothing/ eye protection/ face
protection.
P303 + P361 + P353 IF ON SKIN (or hair): Take off immediately all contaminated
clothing. Rinse skin with water.
P304 + P340 + P310 IF INHALED: Remove person to fresh air and keep comfortable
for breathing. Immediately call a POISON CENTER/ doctor.
P305 + P351 + P338 IF IN EYES: Rinse cautiously with water for several minutes.
Remove contact lenses, if present and easy to do. Continue
rinsing
General advice
First aiders need to protect themselves. Show this material safety data sheet to the doctor
in attendance.
If inhaled
After inhalation: fresh air. Immediately call in physician. If breathing stops: immediately
apply artificial respiration, if necessary also oxygen.
In case of skin contact
In case of skin contact: Take off immediately all contaminated clothing. Rinse skin with
water/ shower. Call a physician immediately.
In case of eye contact
After eye contact: rinse out with plenty of water. Immediately call in ophthalmologist.
Remove contact lenses.
If swallowed
After swallowing: make victim drink water (two glasses at most), avoid vomiting (risk of
perforation). Call a physician immediately. Do not attempt to neutralise.
4.2 Most important symptoms and effects, both acute and delayed
The most important known symptoms and effects are described in the labelling (see section
2.2) and/or in section 11
4.3 Indication of any immediate medical attention and special treatment needed
No data available
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