Carbon dioxide Totally Explained

| Section2 = | Section8 = }} Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. It is currently at a globally averaged concentration of approximately 375 ppm by volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide's chemical formula is .
   In general, it's exhaled by animals and utilized by plants during photosynthesis. Additional carbon dioxide is created by the combustion of fossil fuels or vegetable matter, among other chemical processes.
   Carbon dioxide is an important greenhouse gas because of its ability to absorb many infrared wavelengths of the Sun's light, and because of the length of time it stays in the Earth's atmosphere. Due to this, and the role it plays in the respiration of plants, it's a major component of the carbon cycle.
   In its solid state, carbon dioxide is commonly called dry ice. Carbon dioxide has no liquid state at pressures below 5.1 atm.

Chemical and physical properties

Carbon dioxide is a colorless, odorless gas. When inhaled at concentrations higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 800 ppm are considered unhealthy, amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm are considered dangerous to animal life.
   At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole, and as it's fully oxidized, it's moderately reactive and is non-flammable, but will support the combustion of metals such as magnesium.
   At −78.51° C, carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Solid carbon dioxide is normally called "dry ice", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a versatile cooling agent, and it's relatively inexpensive. As it warms, solid carbon dioxide sublimes directly into the gas phase, making its use convenient as it leaves no liquid. It can often be found in groceries and laboratories, and it's also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning.
   Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6°C (See phase diagram, above). The critical point is 7,821 kPa at 31.1°C.
   An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure. This form of glass, called carbonia, was produced by supercooling heated at extreme pressure (40–48 GPa or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica and germania glasses, however, carbonia glass isn't stable at normal pressures and reverts back to gas when pressure is released.

History of human understanding

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).
The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that the fixed air was denser than air and didn't support either flame or animal life. He also found that when bubbled through an aqueous solution of lime (calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.
Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday. The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid .

Isolation

Carbon dioxide may be obtained from air distillation. However, this yields only very small quantities of . A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. For example, the reaction between sulfuric acid and calcium carbonate (limestone or chalk) is depicted below:
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The then decomposes to water and . Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.
The production of quicklime (CaO) a chemical that has widespread use, from limestone by heating at about 850 °C also produces :
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The combustion of all carbon containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below. �

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:
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Yeast produces carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits:
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All aerobic organisms produce when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria of cells. is the prime energy source and the main metabolic pathway in heterotroph organisms such as animals, and also a secondary energy source in phototroph organisms such as plants when not enough light is available for photosynthesis. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (for example plants, cyanobacteria) use another modus operandi: They absorb the from the air, and, together with water, react it to form carbohydrates:
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Carbon dioxide is soluble in water, in which it spontaneously interconverts between and (carbonic acid). The relative concentrations of, and the deprotonated forms (bicarbonate) and (carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

Industrial production

Carbon dioxide is manufactured mainly from six processes:

As a byproduct in ammonia and hydrogen plants, where methane is converted to ;
From combustion of carbonaceous fuels;
As a byproduct of fermentation;
From thermal decomposition of ;
As a byproduct of sodium phosphate manufacture;
Directly from natural carbon dioxide gas wells.

Uses

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. High levels of carbon dioxide in the atmosphere effectively exterminate many pests. Greenhouses will raise the level of to 10,000 ppm (1%) for several hours to eliminate pests such as whiteflies, spider mites, and others.
   In medicine, up to 5% carbon dioxide is added to pure oxygen for stimulation of breathing after apnea and to stabilize the balance in blood.
   A common type of industrial gas laser is the carbon dioxide laser.
   Carbon dioxide can also be combined with limonene oxide from orange peels or other epoxides to create polymers and plastics.
   Carbon dioxide is used in enhanced oil recovery where it injected into or adjacent to producing oil wells, usually under supercritical conditions. It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.
   In the chemical industry, carbon dioxide is used for the production of urea, carbonates and bicarbonates, and sodium salicylate.
   Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they're employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment isn't practical.
   Liquid carbon dioxide (industry nomenclature R744 / R-744) was used as a refrigerant prior to the discovery of R-12 and is likely to enjoy a renaissance due to environmental concerns. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bars, systems require highly resistant components that have been already developed to serial production in many sectors. In car air conditioning, in more than 90% of all driving conditions, R744 operates more efficiently than systems using R-134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Some applications: Coca-Cola has fielded -based beverage coolers and the US Army is interested in refrigeration and heating technology.
   By the end of 2007, the global car industry is expected to decide on the next-generation refrigerant in car air conditioning. is one discussed option.(see The Cool War)

In the Earth's atmosphere

Carbon dioxide in earth's atmosphere is considered a trace gas, and is measured in parts per million. Current concentration levels average approximately 385 ppmv, which represents a total of around 800 gigatons of carbon. Its concentration can vary considerably on a regional basis: in urban areas it's generally higher, and indoors can reach 10 times the atmospheric concentration.
   Carbon dioxide is a greenhouse gas; see greenhouse effect for more.
   It is observed that due to human activities such as the combustion of fossil fuels and deforestation, the concentration of atmospheric carbon dioxide has increased by about 35% since the beginning of the age of industrialization.
   Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4-5 times during the Jurassic Period and then maintained a slow decline until the industrial revolution..
   Up to 40% of the gas emitted by a volcano during a subaerial volcanic eruption is carbon dioxide. However, human activities currently release more than 130 times the amount of emitted by volcanoes. According to the best estimates, volcanoes release about 130-230 million tonnes (145-255 million tons) of into the atmosphere each year. Emissions of by human activities amount to about 27 billion tonnes per year (30 billion tons).

In the oceans

There is about 50 times as much carbon dissolved in the oceans in the form of CO₂ and CO₂ hydration products as exists in the atmosphere .

Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it's exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Role in photosynthesis

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration. Carbon dioxide gas must be introduced into greenhouses to maintain plant growth, as even in vented greenhouses the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm, at which level photosynthesis is significantly reduced. Venting can help offset the drop in carbon dioxide, but will never raise it back to ambient levels of 340 ppm. Carbon dioxide supplementation is the only known method to overcome this deficiency. Direct introduction of pure carbon dioxide is ideal, but rarely done because of cost constraints. Most greenhouses burn methane or propane to supply the additional, but care must be taken to have a clean burning system as increased levels of nitrogen oxides result in reduced plant growth. Sensors for sulfur dioxide and are expensive and difficult to maintain; accordingly most systems come with a carbon monoxide (CO) sensor under the assumption that high levels of carbon monoxide mean that significant amounts of are being produced. Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm when compared with ambient conditions.
Plants also emit during respiration, so it's only during growth stages that plants are net absorbers. For example a growing forest will absorb many tonnes of each year, however a mature forest will produce as much from respiration and decomposition of dead specimens (for example fallen branches) as used in biosynthesis in growing plants. Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved in the upper ocean and thereby promotes the absorption of from the atmosphere.

Animal toxicity

Carbon dioxide content in fresh air varies between 0.03% (300 ppm) and 0.06% (600 ppm), depending on the location (see graphical map of in real-time

).
   A person's exhaled breath is approximately 4.5% carbon dioxide.
   Internal combustion engines, as used in automobiles, when fuel and ignition are correctly set for best and smoothest power, exhaust % rates of 9.9% to 10.1% by volume are produced. When California Air Resources Board (aka CARB) regulations regarding CO% and HC% are imposed, the CO2% rises to 12% to 15% by volume.
   It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that's considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less.
   These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.
   Acute carbon dioxide toxicity is sometimes known as by the names given to it by miners: blackdamp (also called choke damp or stythe). Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before reached levels toxic to people.
   Carbon dioxide caused a great loss of life at Lake Nyos in Cameroon in 1986, when an upwelling of -laden lake water quickly blanketed a large surrounding populated area. The heavier carbon dioxide forced out the life-sustaining oxygen near the surface, killing nearly two thousand people.
   Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space. OSHA limits concentrations in the workplace to 5,000 for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 30,000 and considers CDPL exceeding 40,000 as "immediately dangerous to life and health." People who breathe 50,000 for more than half an hour show signs of acute hypercapnia, while breathing 70,000 – 100,000 can produce unconsciousness in only a few minutes. Accordingly, carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

Human physiology

is carried in blood in three different ways. (The exact percentages vary depending whether it's arterial or venous blood).

Most of it (about 80% – 90%) is converted to bicarbonate ions HCO₃⁻ by the enzyme carbonic anhydrase in the red blood cells.

5% – 10% is dissolved in the plasma

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