Carbon dioxide

Carbon dioxide is a chemical compound of carbon and oxygen with the molecular formula _CO2, an incombustible, acidic and colorless gas. It dissolves well in water: Here it is often colloquially - especially in connection with carbon dioxide-containing beverages - mistakenly also called "carbonic acid". With basic metal oxides or hydroxides, it forms two types of salts called carbonates and hydrogen carbonates.

_CO2 is an important part of the global carbon cycle and, as a natural component of the air, is a major greenhouse gas in the Earth's atmosphere.Human activities, most notably the burning of fossil fuels, have increased the amount in the Earth's atmosphere from about 280 parts per million (ppm) at the beginning of industrialization to 407.8 ppm in 2018. In May 2019, NOAA's Mauna Loa monitoring station in Hawaii measured a monthly average of about 415 ppm, and the trend continues to rise. This increase causes an amplification of the greenhouse effect, which in turn is the cause of the current global warming. Every day, approximately 100 million tons of carbon dioxide are released into the atmosphere as a result of human activities (as of 2020).

Under sufficient oxygen supply, _{CO2 is} produced both during the combustion of carbon-containing substances and in the organism of living organisms as a product of cellular respiration. Plants, algae and some bacteria and archaea convert _CO2 into biomass by fixation (carbon dioxide assimilation). During photosynthesis, glucose is formed from inorganic _CO2 and water.

_CO2 can have a toxic effect. However, the concentrations in the air or quantities ingested with lemonade, for example, are by no means sufficient for this. _CO2 has a wide range of technical applications: In the chemical industry, for example, it is used to obtain urea. In solid form as dry ice it is used as a coolant, supercritical carbon dioxide serves as a solvent and extraction agent.

History

_CO2 was one of the first gases to be given a name. The Flemish chemist Johan Baptista van Helmont (1580-1644) observed that the mass of charcoal decreased when burned because the mass of the remaining ash was less than that of the charcoal used. His interpretation was that the rest of the charcoal had turned into an invisible substance, which he called gas or spiritus sylvestre ("forest spirit").

The Scottish physician Joseph Black (1728-1799) studied the properties of _CO2 more thoroughly. In 1754 he discovered that when calcium carbonate solutions were mixed with acids, a gas was released which he called fixed air. He realized that this was heavier than air and did not support combustion processes. When he introduced this gas into a solution of calcium hydroxide, he was able to produce a precipitate. With this phenomenon he showed that carbon dioxide was present in the breath of mammals and was released by microbiological fermentation. His work proved that gases can be involved in chemical reactions and contributed to the case for phlogiston theory.

Joseph Priestley succeeded in making soda water for the first time in 1772 by introducing sulphuric acid into a solution containing lime and dissolving the resulting carbon dioxide in a beaker of water. The relationship between carbon dioxide and carbonic acid had been recognized earlier by William Brownrigg. In 1823, Humphry Davy and Michael Faraday liquefied carbon dioxide by increasing the pressure. Henry Hill Hickman operated on animals from 1820, which was painless after inhaling carbon dioxide to achieve anaesthesia. He also described the physiological processes during anesthesia. The first description of solid carbon dioxide was made by Adrien Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide and found that spontaneous evaporation occurs under cooling, resulting in solid _CO2.

Extraction and presentation

Carbon dioxide is produced when carbon-containing fuels, especially fossil fuels, are burned. This produces around 36 gigatonnes (billion tonnes) of carbon dioxide per year worldwide, which is released into the atmosphere. Processes to capture the carbon dioxide and store it in deep rock strata are currently (2016) in the early stages of development and are not yet ready for series production; their effectiveness and economic efficiency, particularly in sustainable energy systems, are being critically evaluated.

Carbon dioxide is produced when carbon reacts with oxygen:

Technically, carbon dioxide is produced when coke is burned with excess air. In coal gasification and steam reforming of natural gas, carbon dioxide is produced, among other things, as a product of the water gas shift reaction in synthesis gas production.

For use in ammonia synthesis and methanol production, the synthesis gas is washed in the rectisol process, for example, and carbon dioxide is thus produced in large quantities in a very pure form. Carbon dioxide is a by-product of lime burning. Through subsequent purification via the formation of potassium carbonate to hydrogen carbonate and subsequent release by heating, about 530 million tons are recovered per year.

In the laboratory, carbon dioxide can be released from calcium carbonate and hydrochloric acid, for example in a Kipp apparatus. The device was used in laboratories in the past. The method is hardly used any more, as carbon dioxide is available in gas cylinders or as dry ice.

Carbon dioxide is also extracted from the air using the direct air capture (DAC) process.