CARBON DIOXIDE 183

Coke is obtained by destructive distillation or carbonization of bituminous coal, coal-tar pitch and petroleum produced during petroleum cracking. Coke from bituminous coal is used to reduce iron ore in blast furnaces; and to produce synthesis gas. Petroleum coke or that obtained from coal-tar pitch is used in electrolytic reduction of aluminum oxide to aluminum and in the preparation of several metal carbides. .

CARBON DIOXIDE

[124–38–9]

Formula: CO2; MW 44.009; structure O=C=O, linear molecule, bond angle 180°C; net dipole moment zero.

Occurrence and Uses

Carbon dioxide is found throughout nature. Its concentration in the air is 0.036% by volume. It is the primary component of exhaled air of all animals. It also is the product of oxidation of all carbonaceous matter and an end product of complete combustion. It also is found dissolved in natural waters. It occurs in the earth’s crust and in volcanic eruptions.

All plants depend on carbon dioxide and water for their survival, making their food by the process of photosynthesis. Carbon dioxide is found in abundance in the atmospheres of many other planets and their moons throughout the solar system.

Carbon dioxide is a greenhouse gas, which traps the infrared radiation reradiated back by the earth’s surface, causing global warming and, therefore, changing the climate. The CO2 concentration in the atmosphere over a 30year period from 1960 to 1990 has increased significantly from about 320 to 356 ppm by volume, which is widely attributed to the growth of industrial and automobile CO2 emission during this period.

Carbon dioxide has extensive commercial applications. Some important applications of this compound include carbonation of beverages; as a fire extinguishing agent; in the manufacture of carbonates; as dry ice (solid CO2) for refrigeration; as an aerosol propellant; as a shielding gas for welding; as

184 CARBON DIOXIDE

an inert atmosphere in preparation and handling of flammable substances; in cloud seeding; in fumigation of rice; to produce harmless smoke on stage; as an antiseptic; and as a supercritical fluid to extract organic pollutants for their analyses.

Physical Properties

Colorless, odorless and tasteless gas; 1.53 times heavier than air; density 1.80 g/L at 25°C; can be liquefied under pressure; liquefies at –56.6°C at 5.2 atm; density of liquid CO2 at 0°C and 34 atm 0.914 g/mL; solidifies to white snow-like flakes known as dry ice, density 1.56 g/cm3 at –79°C; dry ice sublimes to CO2 gas at –78.5°C; critical temperature 31°C; critical pressure 72.79 atm, critical density 94 cm3/mol; moderately soluble in water, solubility 173 mL and 88mL CO2/100 mL water at 0°C and 20°C, respectively; solubility increases with pressure.

Thermochemical Properties

Production

Carbon dioxide is produced as a by-product in many processes. It is produced as a by-product in the manufacture of lime from calcium carbonate:

CaCO3 calcination→ CaO + CO2

CO2 also is derived from synthesis gas which is a mixture of CO, CO2, H2 and N2 from air obtained by steam reforming. Carbon dioxide also is obtained by combustion of natural gas:

CH4 + 2O2 → CO2 + 2H2O

It also is obtained as a by-product in the Haber–Bosch process for the manufacture of ammonia. The method involves passing steam and air over hot coke.

Carbon dioxide also is produced along with ethanol from fermentation of carbohydrates by yeast:

C6H12O6 yeast→ 2CO2 + 2C2H5OH

In the laboratory, CO2 may be produced by the reaction of any carbonate with a dilute mineral acid:

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

CARBON DIOXIDE 185

Reactions

Carbon dioxide is slightly acidic in nature. Its aqueous solution is carbonic acid, H2CO3 , a weak unstable acid:

CO2 + H2O → H2CO3

Reactions with alkalis yield carbonates:

NaOH + CO2 → Na2CO3

It turns lime water milky due to the formation of calcium carbonate:

Ca(OH)2 + CO2 → CaCO3 + H2O

When passed through a solution of alkali or alkaline earth metal sulfide, the corresponding carbonate is produced with deposition of sulfur:

BaS + 3CO2 → 2BaCO3 + S

Many metal oxides form carbonates. When passed into an aqueous solution of carbonate, the corresponding bicarbonate is produced:

Na2CO3 + CO2 + H2O → 2NaHCO3

When passed into an aqueous solution of chloride salt of alkali or alkaline earth metal, the product is bicarbonate:

CaCl2 + CO2 + H2O → CaHCO3 + 2HCl

Carbon dioxide reacts with heated calcium metal, forming calcium carbide and calcium oxide:

3Ca + CO2 heat→ 2CaO + CaC

The photosynthetic conversion of carbon dioxide and water in the presence of sunlight and chlorophyll produces carbohydrates, such as glucose and other sugars, and oxygen:

6CO2 + 6H2O light → C6H12O6 + 6O2

chlorophyll

Analysis

Carbon dioxide may be readily analyzed by various instrumental techniques, such as IR, GC, and GC/MS. Many portable infrared analyzers are available commercially for rapid, on site monitoring of CO2. Also, it can be analyzed by GC using a TCD or an FID. It readily may be identified by mass spectrometry from its characteristic ionic mass 44. Dissolved CO2 in water

186 CARBON DISULFIDE

may be calculated nomographically from temperature, pH, total dissolved solids, and alkalinity. (APHA, AWWA and WEF.1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, D.C.: American Public Health Association.)

Toxicity

Although CO2 is nontoxic, exposure to a high concentration can cause asphyxiation due to lack of oxygen. Exposure to 5 to 10% volume in air can be fatal.

CARBON DISULFIDE

[75–15–0]

Formula: CS2; MW 76.139

Synonym: carbon bisulfide; dithiocarbonic anhydride

Uses

Carbon disulfide is used in the manufacture of rayon, cellophane, electronic vacuum tubes, and xanthogenates. It is used to make carbon tetrachloride. It also is used as an industrial solvent; and as an analytical solvent. Because of its low response to GC-FID, it is used widely in air analysis of organic compounds.

Physical Properties

Colorless liquid; commercial grade has a pungent disagreeable odor, in its purest form the odor is sweet and pleasant; flammable; refractive index 1.6295; density 1.261 g/mL at 20°C; boils at 46.3°C; freezes at –110.8°C; critical temperature 279°C, critical pressure 77.97 atm, critical volume 173 cm3/mol; slightly soluble in water, 0.29 g/100g at 20°C; soluble in alcohol, ether, benzene, chloroform, and oils; forms an azeotrope with water (CS2: H2O = 97.2%)

Preparation

Carbon disulfide is manufactured by heating sulfur vapor with charcoal, and condensing vapors of the compound formed. Alternatively, it may be obtained by heating sulfur with natural gas or petroleum fractions. Instead of sulfur, H2S may be used. The reaction occurs at very high temperatures. The