Carbon Dioxide - The Chemical Compound

Carbon dioxide is a chemical compound composed of one carbon and two oxygen. It is often referred to by its formula CO₂. It is present in the Earth's atmosphere at a low concentration of approximately 0.04% and is an important greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle.

Origins
Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms ; man-made sources of carbon dioxide come mainly from the burning of fossil fuels for heating, power generation and transport. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to carbohydrates during a process called photosynthesis. They produce the energy needed for this reaction through the photolysis of water. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms, forming a cycle.

Chemical and physical properties
Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces 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. One may notice this sensation if one attempts to stifle a burp after drinking a carbonated beverage.

Its density at standard temperature and pressure is around 1.98 kg/m³, about 1.53 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. As it is fully oxidized, it is not very reactive and is non-flammable.

Carbon dioxide pressure-temperature phase diagram
At temperatures below -78 °C, carbon dioxide changes directly from a gas to a white solid called dry ice through a process called deposition. Liquid  carbon dioxide forms only at pressures above 5.1 atm; at atmospheric pressure, it passes directly between the solid phase and the gaseous phase in a process called sublimation.

Synthesis and chemistry
Carbon dioxide may be obtained from air distillation, however this yields only very small quantities of CO₂. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. As an example, the reaction between sulfuric acid and calcium carbonate (limestone or chalk) is depicted below:

H₂SO₄ + CaCO₃ ? CaSO₄ + H₂CO₃
The H₂CO₃ then decomposes to water and CO₂. Such reactions are accompanied by foaming and/or bubbling. In industry such reactions are widespread because it 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 ^oC also produces CO₂:

CaCO₃ ? CaO + CO₂
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.

CH₄ + 2 O₂ ? CO₂ + 2 H₂O
Iron is reduced from its oxides with coke in blast furnace, producing pig iron and carbon dioxide:

2 Fe₂O₃ + 3 C ? 4 Fe + 3 CO₂
Carbon dioxide can be used in chemistry to create a carboxylic acid from a Grignard reagent.

R-MgX + CO₂ ? R-COOH
Yeast produces carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits:

Glucose ? 2 CO₂ + 2 C₂H₅OH
All aerobic organisms produce CO₂ when they burn carbohydrates, fatty acids and proteins; it 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 amount of reactions involved are exceedingly complex and not described easily. Photoautotrophs (i.e. plants, cyanobacteria) utilize another modus operandi: They absorb the CO₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH₂O ? (CH₂O)_n + nO₂
Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H₂CO₃ (carbonic acid). The relative concentrations of CO₂, H₂CO₃, and the deprotonated forms HCO₃^- (bicarbonate) and CO₃^2-(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 CO₂;
From combustion of carbonaceous fuels;
As a byproduct of fermentation;
From thermal decomposition of CaCO₃;
As a byproduct of sodium phosphate manufacture;
Directly from natural carbon dioxide gas wells.

Wikipedia.org
05 July 2007
www.wikipedia.org

Articles on Science Matters:

Mind Matters - Brainwaves Power Their Will to Move

Science has made it possible for the lame to move again and it only takes imagination.

A young woman, confined to a wheelchair, is told to think about moving another wheelchair in front of her, first to the left and then forward. As if by magic, the wheelchair follows her mental commands.

"She was controlling the chair with her imagination" said Mr Timothy Surgenor, president and chief executive of Cyberkinetics Neurotechnology Systems. Mr Surgenor was using the video of the woman, who was paralysed by a brain stem stroke, to demonstrate a technology called BrainGate to some 900 researchers, physicians and investors attending a meeting at the Cleveland Clinic last month.

The woman had a tiny sensor that anlyses brain signals implanted on the part of her brain that controls hand movements. A small plug protruding from just above her ear is connected toa computer that in turn has a wireless connection to the electronic wheelchair she was controlling.

This concept is evolving. For people living with paralysis, the technology has the potential to be life-changing. A lot of the technology that supports BrainGate is already out there. Cyberkinetics provides the operating system. the goal is to make the components small enough and wireless, thus eliminating the need for a plug on the scalp.

Northstar Neuroscience, another company attending the meeting at Cleveland Clinic, is testing a device that aims to help stroke victims recover from disabilities such as impairment of hand and arm movement. the therapy identifies specific areas of the brain that are trying to compensate for the lost function and implants electrodes there. Electronic stimulation theoretically strengthens connections between neurons. The system works by taking advantage of a naturally occuring phenomenon called neuroplasticity - the brain's ability to reorganize in response to an injury. When part of the brain dies because of a stroke, another part of the brain attempts to take over that function. The trouble is, in most cases the process does not go far enough and relatively little function is recovered, explained by Alan Levy, Northstar's chief executive.

For several years, doctors have been implanting brain pacemakers into patients with Parkinson's disease or other disorders that causes severe tremors. The stopwatch-size device, made by Medtronic, is implanted in the chest and connected to leads threaded into the brain. Known as deep brain stimulation, it delivers electrical pulses to targeted areas in the brain to interrupt the signals that causes tremors. Medtronic is also testing to seeif it might also help cases of obesessive comppulsive disorder (OCD), depression and obesity.

Cleveland Clinic's Dr Rezai said using electricity to stimulate various parts of the nervous system or organs may soon help people who suffer such varied afflictions as OCD, migraine headaches, sleep apnoea, incontinence, obesity, impotence, hypertension and even heart failure.

Article abstracted from,
The Straits Times, Mind Your Body on december 20, 2006.
Singapore
Reuters
22 Dec 2006

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No Sleep Means No New Brain Cells

Mission out on sleep may cause the brain to stop producing new cells, a study has suggested. The work on rats, by a team from Prinsceton Unversity, found that a lack of sleep affected the hippocampus, a brain region involved in forming memories.

The research in Proceedings of the National Academy of Science (PNAS) showed a stress hormone causes the effect. A British expert said it would be interesting to see if too little rather than no sleep had the same consequence.

The researchers compared animals which were deprived of sleep for 72 hours with others which were not. They found that those which missed out on rest had higher levels of the stress hormone corticosterone.  These animals also produced significantly fewer new brain cells in a particular region of the hippocampus.

When animals' corticosterone levels were kept at a constnt level, the reduction in cell proliferation ended. The result suggest that elevated stress hormone levels resulting from sleep deprivation could explain the reduction in cell production in the adult brain.

Sleep patterns were restored to normal within a week. However, levels of nerve cell production (neurogenesis) were not restored for two weeks, adn the brain appears to boosts its efforts in order to counteract the shortage.

Writing in PNAS, the researchers, led by Dr Elizabeth Gould, said that although the role of nerve cell production in adults remained unknown, "the suppression of adult neurogenesis may underlie some of the cognitive deficits associated with prolonged sleep deprivation."

People whoexperience a lack of sleep face concentration problems and other difficulties. Sleep expert Neil Stanley, based at the Norfolk and Norwich University Hospital, said that the study's findings could not be directly translated to humans because people did not go without sleep for 72 hours, unless they were in extreme circumstances.

But he added, "It is an interesting finding. It would be interesting to see if partial sleep deprivation - getting a little bit less sleep every night that you need - had the same effect."

By,
BBC News