nitric acid 89-8-3

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(Applied Chemistry)

Inorganic Industrials Chemistry

Nitric Acid R. Pourata

In the Name of God

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Outline

Introduction

Properties

Production

Industrial Production

Manufacture of Highly Concentrated Nitric Acid

Uses of nitric acid

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Introduction

Nitric acid is a strong acid that occurs in nature only in the form of

nitrate salts. When large-scale production of nitric acid began, sodium

nitrate (soda saltpetre, Chile saltpetre) was used as the feedstock. At

the beginning of the 20th century the reserves of Chile saltpeter were

thought to be nearing exhaustion, so processes were developed for

replacing nitrogen from natural nitrates with atmospheric nitrogen.

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Properties

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Properties

Nitric acid

IUPAC name Nitric acid

Other names Aqua fortis; Spirit of nitre; Salpetre acid; Hydrogen Nitrate

Properties

Molecular formula HNO3

Molar mass 63.012 g/mol

Appearance Clear, colorless liquid

Density 1.51 g/cm³, colorless liquid

Melting point -42 °C, 231 K, -44 °F

Boiling point 83 °C, 356 K, 181 °F (bp of pure acid. 68% solution boils at 120.5°C)

Solubility in water miscible

Hazards

EU classification Oxidant (O) Corrosive (C)

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Properties

Nitric acid is miscible with water in all proportions. At a

concentration of 69.2 wt %, it forms a maximum-boiling azeotrope

with water. The azeotropic mixture boils at 121.8 °C. Pure anhydrous

nitric acid boils at 83

87 °C; the reason a range of boiling points are

cited in the literature is that the acid decomposes on heating:

4HNO3 2H2O + 4NO2 + O2 (72°C)

In the pure anhydrous state, nitric acid is a colorless liquid.

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Properties

Chemical Properties. Concentrated nitric acid, with nitrogen in the

+ 5 oxidation state, acts as a strong oxidizing agent. The reaction

4NO3 + 4H+ 4NO + 2H2O +3O2

goes to the right for all substances with oxidation potentials more

negative than + 0.93 V. For example, copper (+ 0.337 V) and silver

(+ 0.799 V) are dissolved by nitric acid, whereas gold (+ 1.498 V)

and platinum (+ 1.2 V) are resistant.

In practice, 50 % nitric acid (aqua foris) is used for separating gold

from silver.

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Properties

Highly diluted nitric acid is almost completely dissociated

HNO3+ H2O H3O+ + NO3

and does not attack copper and more noble metals.

Due to its acid nature, however, it reacts with base metals, liberating

hydrogen and forming nitrates.

A mixture (volume ratio 3:1) of concentrated nitric acid and

concentrated hydrochloric acid (aqua regia) also dissolves gold.

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Properties

Physical properties of aqueous nitric acid as a function of composition

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Production

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Production of Acid Nitric from Saltpetre

Production of Acid Nitric by Electric Arc Process

1- Production of Nitrogen Oxide

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1- Production of Nitrogen Oxide


2-Oxidation of Nitrogen Oxide:

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Nitrogen Oxides

Nitric oxide (NO), nitrogen(II) oxide

Nitric oxide

Molecular formula

NO

Molar mass 30.0061

Appearance colourless gas

Density 1.3

103 kg m 3 (liquid) 1.34 g dm 3 (vapour)

Melting point 163.6 C (109.6 K) (-262.48 F)

Boiling point 151.7 C (121.4 K) (-241.06 F)

Hazards

EU classification Toxic (T), corrosive (C)

Nitrogen Oxides

Nitrogen dioxide (NO2), nitrogen(IV) oxide

Nitrogen dioxide

Molecular formula NO2

Molar mass 46.0055

Appearance brown gas

Density 1443 kg/m³, liquid 3.4 kg/m³, gas at 294.25 K

Melting point -11.2 C (261.95 K)

Boiling point 21.1 C (293.25 K)

Hazards

EU classification Highly toxic (T+)

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Nitrogen Oxides

Dinitrogen tetroxide (N2O4),

nitrogen(IV) oxide

Dinitrogen tetroxide

IUPAC name Dinitrogen Tetroxide

Properties

Molecular formula

N2O4

Molar mass 92.011 g mol 1

Appearance Transparent gas

Density 1443 kg/m³ (liquid at 1.013 bar, boiling point)

Melting point 261.9 K (-11.2 C)

Boiling point 294.3 K (21.1 C)

Solubility in other solvents

reacts with water

Vapor pressure 96 kPa (20 C) [1]

Hazards

Main hazards

Inhalation: Corrosive & toxic Skin: Corrosive Eyes: Corrosive

Nitrogen Oxides

Nitrous oxide (N2O), nitrogen (I) oxide

Nitrous oxide

Molecular formula

N2O

Molar mass 44.0128 g/mol

Appearance colorless gas

Density 1222.8 kg m-3 (liquid) 1.8 kg m-3 (gas STP)

Melting point -90.86 C, 182 K, -132 F

Boiling point -88.48 C, 185 K, -127 F

Structure

Molecular shape

linear

Dipole moment 0.166D

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Nitrogen Oxides

Dinitrogen trioxide (N2O3), nitrogen(II,

IV) oxide

Dinitrogen trioxide

Molecular formula

N2O3

Molar mass 76.01

Appearance blue liquid

Density 1.4

103 kg m 3, liquid

Melting point 100.1 C (173.05 K)

Boiling point 3 C (276 K)

Hazards

EU classification

Highly toxic (T+)

Nitrogen Oxides

Dinitrogen pentoxide (N2O5),

nitrogen(V) oxide

Dinitrogen pentoxide

Other names dinitrogen pentoxidednpo

Properties

Molecular formula N2O5

Molar mass 108.01 g mol-1

Appearance white solid

Density 2.05 g cm-3, solid

Melting point 41 C (under pressure to suppress sublimation)

Boiling point decomposes

Solubility in water decomp. to HNO3

Structure

Coordination geometry

linear at N2O and planar at NO3

Hazards

Main hazards strong oxidizer, forms strong acid in contact with water

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3-Absorption of The Nitrous Gases in Water

3 NO2 + H2O 2 HNO3 + NO H = -73 kJ/mol

N2O4 + H2O HNO3 + HNO2 H = -65 kJ/mol


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Industrial Production

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Production of nitric acid by the Ostwald process

The industrial production of nitric acid by the Ostwald process

involves three chemical steps.

Catalytic oxidation of ammonia with atmospheric oxygen to yield

nitrogen monoxide:

4 NH3+ 5 O2 4 NO + 6 H2O (1)

Oxidation of the nitrogen monoxide product to nitrogen dioxide or

dinitrogen tetroxide:

2 NO + O2 2 NO2 N2O4 (2)

Absorption of the nitrogen oxides to yield nitric acid:

3 NO2+ H2O 2 HNO3+ NO (3)

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Schema of the Ostwald process for the manufacture of nitric acid.

The overall reaction corresponds to:

NH3 + 2 O2 HNO3 + H2O H = -369 kJ/mol

(heat of reaction for 60% acid)

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Catalytic Combustion of ammonia to Nitrogen(II) oxide:

The oxidation of ammonia (combustion) with (excess) atmospheric

oxygen to nitrogen(II) oxide (NO) is carried out in the presence of a

catalyst at 820 to 950°C either at atmospheric pressure or at pressures

up to 12 bar:

4 NH3+ 5 O2 4 NO + 6 H2O H = -904 kJ/mol

NO-yield in ammonia combustion is between 94 and 98% depending

upon temperature, pressure and flow rate.

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The oxidation of ammonia benefits slightly from pressure reduction,

since less nitrogen and dinitrogen(I) oxide (N2O) is then produced in

side reactions:

4 NH3+ 3 O2 2 N2 + 6 H2O H = -1268 kJ/mol

4 NH3+ 4 O2 2 N2O + 6 H2O H = -1 105 kJ/mol

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The adverse influence of pressure, necessary in the case of reduced

apparatus size (to reduce investment costs), upon yield, can to some

extent be compensated by increasing the combustion temperature, but

with increased catalyst losses. The yield is generally 94 to 98% (e.g.

97 to 98% at 1 bar, 95 to 96% at 5 bar, 94% at 8 to 10 bar).

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Conversion of ammonia to nitrogen monoxide on a platinum gauze as a function of temperature a) 100 kPa; b) 400 kPa

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The combustion mixture contains up to 13% by volume of ammonia,

being below the lower explosion limit for ammonia-air mixtures

(15.5% by volume at I bar). At higher operating pressures the

concentration of ammonia in the combustion mixture is lower still

(below 1 l%), since the lower explosion limit decreases with

increasing operating pressure.

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The ammonia oxidation catalyst is usually a platinum alloy gauze

containing 5 to 10% rhodium, or additionally with 5% palladium, with

a diameter of up to 4 m (with 1024 meshes/cm2 and a wire thickness

of 0.06 to 0.076 mm, the latter for higher pressures). The higher the

pressures and flow rates the larger the number of gauzes incorporated

into the reactor (up to 50 one above another).


Losses of precious metals in the combustion of ammonia to nitrogen monoxide as a function of temperature and catalyst composition [5] a) Pt; b) Pt Rh 98/2; c) Pt Rh 90/10

Ammonia Oxidation Catalyst

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production of nitric acid by the Ostwald process

Photograph of platinum rhodium gauze (Degussa, FRG) taken with a scanning electron microscope (enlargement 100:1) A) Initial stage; B) Highly activated stage

Ammonia Oxidation Catalyst

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Oxidation of Nitrogen(II) Oxide:

The hot nitrogen(II) oxide-containing gas from the combustion step

(e.g. with ca. 10 to 12% NO) is cooled, the heat content being utilized

for steam production or waste gas-heating. It is then reacted with

additional atmospheric oxygen (secondary air) to nitrogen(IV) oxide

(NO2):

2NO+ O2 2NO2 H = -1 14 kJ/mol

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Oxidation of Nitrogen(II) Oxide:

2NO+ O2 2NO2 H = -1 14 kJ/mol

This reaction is favored by low temperatures, the temperature

coefficient of the rate constant being negative, and still more strongly

by increased pressure due to the volume reduction during the reaction.

Dimerization to dinitrogen(1V) oxide is also promoted by low

temperatures and high pressures.

2NO2 N2O4 H = -57 kJ/mol

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Conversion of Nitrogen(IV) Oxide into Nitric Acid:

The gas mixture obtained by oxidation of nitrogen(II) oxide,

containing nitrogen(1V) oxide and dinitrogen(1V) oxide (so-called

nitrous gases), is reacted in the third reaction step with water as

follows:

3 NO2 + H2O 2 HNO3 + NO H = -73 kJ/mol

N2O4 + H2O HNO3 + HNO2 H = -65 kJ/mol

to nitric acid, nitrogen(II) oxide and nitrous acid. The nitrous acid is

further oxidized to nitric acid by the (atmospheric) oxygen present,

either in the liquid or vapor phase.

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Conversion of Nitrogen(IV) Oxide into Nitric Acid:

The absorption of the nitrous gases in the process water is favored by

low temperatures, high pressures and longer contact times. The

quantity of process water, of which the acid condensate is a part, is

dependent upon the required nitric acid concentration. Higher

pressures permit the production of higher nitric acid concentrations

(up to 70% HNO3), since under pressure almost complete absorption

of nitrous gases can be attained in a small quantity of process water

with low emission of residual gas. Only 45 to 50% nitric acid can be

produced at atmospheric pressure.

Production of nitric acid by the Ostwald process Absorption tower

a) Nitrous gas inlet; b) Inner compartment; c) Outer compartment

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The 50 to 70% nitric acid produced in conventional nitric acid plants

is suitable for industrial purposes e.g. the manufacture of fertilizers,

the synthesis of ammonium nitrate, for example, requiring 60% acid.

However, for nitration reactions in organic synthesis a highly

concentrated (ca. 98 to 99%) nitric acid is required. Since nitric acid

forms an azeotrope with water at 69.2% nitric acid, concentration of

weak acid by distillation is not possible.

Highly concentrated nitric acid can be produced by direct and indirect

processes. Direct processes are favored in Western Europe, whereas

indirect processes are favored in the USA.

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Direct Processes

In the direct highly concentrated nitric acid processes, of which there

are many variants, the nitrous gases resulting from the catalytic

combustion of ammonia and oxidation of the resulting nitrogen(I1)

oxide are either separated and the dinitrogen(1V) oxide reacted with

oxygen and water forming nitric acid, or dissolved in concentrated

nitric acid and the superazeotropic acid distilled.

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Direct Processes

Uhde process:

4 NH3+ 5 O2 4 NO + 6 H2O

2 NO + O2 2 NO2 N2O4

2HNO3+ NO 3NO2 + H2O

2 NO2 N2O4

N2O4 + H2O + 0.5 O2 2 HNO3

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Direct Processes

Davy McKee's Sabar process:

4 NH3+ 5 O2 4 NO + 6 H2O

2 NO + O2 2 NO2 N2O4

Dinitrogen(1V) oxide dissolved in concentrated nitric acid

Superazeotropic acid distilled

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Indirect Extractive Distillation Processes

Of the various indirect processes for the manufacture of highly

concentrated acid only two are industrially important: the sulfuric acid

process and the magnesium nitrate process.

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Sulfuric Acid Process:

In the sulfuric acid process, which poses considerable corrosion

problems, medium Concentrated nitric acid is first produced using

conventional methods (e.g. in a M/M-type unit) as in the magnesium

nitrate process. Concentrated sulfuric acid is fed in at the head of the

concentrating tower. During the extractive distillation, diluted sulfuric

acid accumulates in the sump and 99% nitric acid is driven off. The

diluted sulfuric acid is then concentrated by vacuum distillation and

recycled.

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Sulfuric Acid Process:

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Magnesium Nitrate Process:

In the magnesium nitrate process weak acid is distilled with 72%

magnesium nitrate solution, whereupon highly concentrated nitric

acid is driven off at the head of the dehydration tower. The sump

product is then concentrated by vacuum distillation.

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Uses of nitric acid

Uses of nitric acid

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HNO3-consumption spectrum in the USA in 1992 according to use:

Total consumption 8.9 . 106 t

Ammonium nitrate 77.6%

Adipic acid 7.9%

Nitrobenzene 4.0%

Toluene diisocyanate 4.2%

various 6.3%

Uses of nitric acid

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Explosives like trinitrotoluene (T.N.T.) nitro glycerine, gun cotton,

ammonal etc. Ammonal is a mixture of ammonium nitrate and

aluminum powder.

Fertilizers such as calcium nitrate, ammonium nitrate etc.

Nitrate salts such as calcium nitrate, silver nitrate, ammonium

nitrate.

Dyes, perfumes, drugs etc. from coal tar products.

Uses of nitric acid

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It is used in the purification of silver, gold, platinum etc.

Nitric acid is used in etching designs on copper, brass, bronze ware

etc.

It is used to prepare "aqua regia" to dissolve the noble elements.

It is used as a laboratory reagent.

Uses of nitric acid

The End

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nitric acid 89-8-3

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