|Molecular formula||CO 2|
colorless, odorless gas
|External identifiers / databases|
|Molar mass||44.01 g mol −1|
1.98 kg m −3 (0 ° C and 1013 hPa)
no melting point (triple point at −56.6 ° C and 5.19 bar)
−78.5 ° C / 1013 mbar
5.73 MPa (20 ° C)
in water: 3.3 g l −1 at 0 ° C, 1.7 g l −1 at 20 ° C, each at 1013 hPa
|Global warming potential||
1 (by definition)
|ΔH f 0||
−393.5 kJ mol −1 (g)
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C|
Carbon dioxide or carbon dioxide is a chemical compound of carbon and oxygen with the empirical formula CO 2 , an incombustible, acidic and colorless gas . It dissolves well in water: colloquially, it is often incorrectly called " carbonic acid ", especially in connection with beverages containing carbon dioxide . With basic metal oxides or - hydroxides it forms two types of salts , the carbonates and bicarbonates are called.
CO 2 is an important part of the global carbon cycle and, as a natural component of the air, an important greenhouse gas in the earth's atmosphere : Human activities, above all the burning of fossil fuels , increased the proportion in the earth's atmosphere from approx. 280 parts per million (ppm, parts per million ) at the beginning of industrialization to 407.8 ppm in 2018. In May 2019, a monthly average of around 415 ppm was measured at the NOAA monitoring station Mauna Loa in Hawaii, and the trend is rising. This increase causes an intensification of the greenhouse effect , which in turn is the cause of the current global warming . Around 100 million tons of carbon dioxide are released into the atmosphere every day through human activities (as of 2020).
With an adequate supply of oxygen , CO 2 is produced both when carbon-containing substances are burned and in the organism of living beings as a product of cellular respiration . Plants , algae and some bacteria and archaea convert CO 2 into biomass through fixation ( carbon dioxide assimilation ) . During photosynthesis , glucose is created from inorganic CO 2 and water .
CO 2 can be toxic; however, the concentrations and amounts in the air or through the consumption of lemonade, for example, are far from sufficient for this. It has a wide range of technical applications: In the chemical industry e.g. B. it is used to extract urea . In solid form as dry ice , it is used as a coolant , while supercritical carbon dioxide serves as a solvent and extractant .
Carbon dioxide was one of the first gases to be given a name. In the 17th century, the Flemish chemist Johan Baptista van Helmont observed that the mass of charcoal decreased when it was 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 that he called gas or spiritus sylvestre ("forest spirit").
The properties of carbon dioxide have been studied more thoroughly by the Scottish doctor Joseph Black . In 1754 he found out that when calcium carbonate solutions are mixed with acids, a gas is released which he called fixed air . He realized that this was heavier than air and did not support combustion processes. When this gas was introduced into a solution of calcium hydroxide , it was able to generate a precipitate . With this phenomenon he showed that carbon dioxide occurs in the breath of mammals and is released through microbiological fermentation. His work proved that gases can be involved in chemical reactions and contributed to the case of the phlogiston theory .
Joseph Priestley succeeded in making the first soda water in 1772 by adding sulfuric acid to a calcareous solution and dissolving the resulting carbon dioxide in a beaker of water. However, William Brownrigg recognized the connection between carbon dioxide and carbonic acid much earlier. In 1823, Humphry Davy and Michael Faraday liquefied carbon dioxide by increasing the pressure. The first description of solid carbon dioxide comes from Adrien Thilorier , who opened a pressurized container with liquid carbon dioxide in 1834 and found that the spontaneous evaporation takes place with cooling, which leads to solid CO 2 .
Carbon dioxide is found in the atmosphere , the hydrosphere , the lithosphere, and the biosphere . The exchange of carbon between these earth spheres takes place largely through carbon dioxide. Around 2015 there were around 830 gigatons (830 billion tons ) of carbon in the atmosphere in the form of carbon dioxide. The hydrosphere contains around 38,000 gigatons of carbon in the form of physically dissolved carbon dioxide as well as dissolved hydrogen carbonates and carbonates . The lithosphere contains by far the largest proportion of chemically bound carbon dioxide. Carbonate rocks such as calcite and dolomite contain around 60,000,000 gigatons of carbon. In addition, large amounts of carbon are stored in permafrost areas such as the tundras of the Arctic and Antarctic polar regions, in boreal coniferous forests or high mountains and in bogs .
Occurrence in the atmosphere and man-made climate change
- For the influence of humans, see also specifically anthropogenic increase in CO2 concentration and Keeling curve .
Carbon dioxide is a naturally occurring trace gas in the earth's atmosphere that has an impact on the climate , but its concentration increases in particular through the combustion of fossil fuels . Ice core data showed that atmospheric CO 2 levels fluctuated between 190 ppm during the height of the ice ages and 280 ppm during the warm periods over the past 420,000 years up to the start of industrialization in the mid-18th century .
With industrialization , there was a sharp increase in the amount of carbon dioxide in the atmosphere as a result of human activity, which continues. Between 1750 and 1958 (the beginning of systematic measurements by Charles David Keeling ), the CO 2 value initially rose moderately to 315 ppm, and then increased to 401 ppm by 2015. The concentration exceeded the threshold of 400 ppm (0.04%) in the local daily mean on May 9, 2013, according to a measurement by the National Agency for Ocean and Atmospheric Research (NOAA) of the United States on Mauna Loa (Hawaii). The monthly global mean value measured by NOAA exceeded the 400 ppm limit for the first time in March 2015; in February 2018 this value was 408 ppm (provisional status, as the data from the previous year is still being checked). Finally, the data for 2017 show a new record high of 405.5 ppm, which is 46 percent above the pre-industrial value. In 2018, a new record high of 407.8 ppm was reached. The main sources are the burning of fossil fuels for energy production and in the industrial sector. The release of carbon dioxide stored in soils and forests through changes in land use, for example through clearing of forests, also contributes to the increase to a much lesser extent. In 2014, energy use and industrial use of fossil fuels as well as land use accounted for 70% and 5%, respectively, of total man-made greenhouse gas emissions (measured in carbon dioxide equivalents ).
The total mass of carbon dioxide in the atmosphere is around 3000 gigatons or around 800 Gt carbon (the ratio of the molar masses of CO 2 to C is 44:12). The concentration varies seasonally and locally, especially near the ground. In urban areas, the concentration is generally higher; indoors, the concentration can be up to ten times the average value.
Carbon dioxide absorbs part of the thermal radiation ( infrared radiation ), while the shorter-wave part of the solar radiation can pass almost unhindered. An absorbent body also emits according to its temperature. These properties make carbon dioxide a so-called greenhouse gas . After water vapor , carbon dioxide is the second most effective of the greenhouse gases in terms of its proportion, although the specific effectiveness of methane and ozone are higher. All greenhouse gases together increase the mean temperature on the earth's surface from around −18 ° C to +15 ° C due to the natural greenhouse effect . Carbon dioxide has a relatively large share in the overall effect and thus contributes to the earth's friendly climate.
The proportion of carbon dioxide in the earth's atmosphere has been subject to considerable fluctuations over the course of the earth's history, which have various biological, chemical and physical causes. 500 million years ago, the concentration of carbon dioxide was at least ten times higher than it is today. As a result, the CO 2 concentration steadily decreased and was around 300 million years ago during the Permocarbon Ice Age , at the transition from the Carboniferous to the Permian , at an average of around 300 ppm and fell briefly in the early Permian to a low of probably 100 ppm during the In the Mesozoic era , the CO 2 level was mostly between 1,000 and 2,000 ppm, only to fall well below 1,000 ppm in the New Earth Age , after a climatic optimum in the early Eocene , up to the beginning of the Cenozoic Ice Age about 34 million years ago.
For at least 800,000 years, the carbon dioxide content has always been below 300 ppm. The carbon dioxide concentration has remained relatively constant at 300 ppm over the past 10,000 years. The balance of the carbon dioxide cycle was thus balanced during this time. With the beginning of industrialization in the 19th century, the proportion of carbon dioxide in the atmosphere increased. The current concentration is likely the highest in 15 to 20 million years. In the period from 1960 to 2005, the carbon dioxide content rose by an average of 1.4 ppm per year. In 2017, the 10-year average increase was a good 2 ppm per year.
The anthropogenic , i.e. man-made carbon dioxide emissions amount to around 36.3 gigatons per year and are only a small proportion of the carbon dioxide, which comes mainly from natural sources, of around 550 gigatons per year. However, since the natural carbon sinks absorb the same amount of CO 2 again, the carbon dioxide concentration remained relatively constant before industrialization. About half of the additional carbon dioxide is absorbed by the biosphere and the oceans (this leads to their acidification ), so that these now absorb more carbon dioxide than they give off. As a result, the earth has been “greening” since 1982 (Leaf Area Index), as has been proven by satellite data from NASA. Overall, the increase in regions covered by foliage increased by around twice the land area of the USA during this period, which is partly due to the fertilization of the plants through the increase in CO 2 . The other half of the carbon dioxide emitted remains in the atmosphere and leads to a measurable increase in concentration, which Charles Keeling was able to demonstrate for the first time in the early 1960s with the Keeling curve named after him .
It is widely recognized scientifically that there is a statistically significant human impact on climate that is the primary cause of global warming . Most of this warming is most likely due to the anthropogenic amplification of the natural greenhouse effect through the emission of greenhouse gases. The additionally generated carbon dioxide contributes about 60% to the intensification of the greenhouse effect.
Occurrence in oceans
The water of the oceans contains carbon dioxide in dissolved form and as carbonic acid in equilibrium with hydrogen carbonates and carbonates. The amount dissolved changes with the season, as it depends on the temperature and salt content of the water: Cold water dissolves more carbon dioxide. Since cold water has a higher density, the carbon dioxide-rich water sinks into deeper layers. Only at pressures over 300 bar and temperatures over 120 ° C (393 K) is it the other way round, for example in the vicinity of deep geothermal vents .
The oceans contain around 50 times more carbon than the atmosphere. The ocean acts as a large carbon dioxide sink and absorbs around a third of the amount of carbon dioxide released by human activities. In the upper layers of the oceans, it is partially bound by photosynthesis. As the solution of carbon dioxide increases, the alkalinity of the salt water falls , which is known as acidification of the oceans and which is very likely to have negative consequences for the marine ecosystems. Many sea creatures are sensitive to fluctuations in the acidity of the oceans; Acidification events in the history of the earth led to mass extinctions and a sharp decline in biodiversity in the world's oceans. Organisms that build up calcium carbonate structures are particularly affected , as this dissolves as the acidity of the oceans increases. Corals , mussels and echinoderms such as starfish and sea urchins are considered particularly vulnerable .
Among other things, it is feared that this has a negative effect on the formation of mussel shells, among other things . These effects are already visible in coral reefs and certain oyster farms; with increasing acidification, stronger ecological consequences are expected. On the other hand, there is evidence that an increased concentration of carbon dioxide stimulates some species to produce more mussel shells.
Occurrence in fresh water
Aerobic bacteria and animals living in (under) water consume oxygen and exhale CO 2 . If there is sufficient contact with the atmosphere, this gas can be released into the air and oxygen can be absorbed at the same time. For this purpose, a surface that freely borders on the air without a layer of ice or oil is beneficial, as are wave movements, turbulence with air, i.e. the formation of foam and spray, water currents that also include deeper layers and wind. Without sufficient gas exchange, a body of water can become low in oxygen and high in CO 2 on the surface . They say "it tips over".
Due to special geological conditions, fresh water can be loaded with considerable amounts of carbon dioxide from volcanic sources, such as water from mineral springs or in lakes on extinct volcanoes, so-called maars . Under the pressure of great water depth, CO 2 can be dissolved in a much higher mass concentration than under atmospheric pressure at the surface of the water. If a lake is not (sufficiently) traversed by water or is mixed by wind and / or heat convection currents and if more CO 2 is brought in from below than mixing and diffusion can transport upwards , then CO 2 -rich deep water forms , which has the potential to increase a catastrophic CO 2 release into the air. A local outgassing once triggered locally under water leads to the rise of a body of water, the relief of hydrostatic pressure that takes place in the process increases the outgassing. This self-reinforcing process can lead to the release of large amounts of CO 2 , which can kill people and animals near the lake.
One of these natural disasters occurred in 1986 at Lake Nyos in Cameroon . The lake is located in an old volcanic crater in the Oku volcanic area. A magma chamber feeds the lake with carbon dioxide and thus saturates its water. Probably triggered by a landslide in 1986, large amounts of carbon dioxide were released from the lake, killing around 1,700 residents and 3,500 livestock in the surrounding villages. Another catastrophe occurred in 1984 at Lake Manoun , the water of which is saturated with carbon dioxide by a similar mechanism. 37 people were killed in this carbon dioxide release. The Kiwu Lake in Central Africa also has high concentrations of dissolved gases in its deep water. It is estimated that around 250 km³ of carbon dioxide is dissolved in this lake.
The atmosphere of Venus consists of 96.5% carbon dioxide, has about 90 times the mass of the earth's atmosphere and a pressure of about 90 bar. The high proportion of carbon dioxide is one of the causes of the strong greenhouse effect. In addition, the distance from the sun is on average 41 million kilometers shorter than the earth, which leads to a surface temperature of around 480 ° C. With a share of 95%, carbon dioxide also makes up the main part of the Martian atmosphere . At the Mars poles, atmospheric carbon dioxide is partially bound as dry ice. Due to the low atmospheric pressure of about seven millibars, the greenhouse effect only leads to an increase of about 5 K despite the high carbon dioxide content. The atmospheres of the outer planets and their satellites contain carbon dioxide, which originates from impacts from comets such as Shoemaker-Levy 9 and cosmic dust is attributed. Using the instruments of the Hubble Space Telescope , NASA found carbon dioxide on extrasolar planets like HD 189733 b .
Carbon dioxide is found both in interstellar space and in protoplanetary disks around young stars. Formation occurs through surface reactions of carbon monoxide and oxygen on water ice particles at temperatures around −123 ° C (150 K). The carbon dioxide is released when the ice evaporates. The concentration in free interstellar space is relatively low, as reactions with atomic and molecular hydrogen form water and carbon monoxide.
Extraction and presentation
Carbon dioxide is produced when carbon-containing fuels are burned, particularly fossil fuels . Around the world, around 36 gigatons (billion tons) of carbon dioxide are released into the atmosphere each year. Processes for separating carbon dioxide and storing it in deep rock layers are currently (2016) at the beginning of their development and are not yet ready for series production; their effectiveness and profitability, especially in sustainable energy systems, are critically assessed.
Technically, carbon dioxide is produced when coke is burned with excess air. In coal gasification and the 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 in the production of methanol , the synthesis gas is washed using the Rectisol process , for example , which means that large amounts of carbon dioxide are produced in a very pure form. Carbon dioxide is a by-product of lime burning . Subsequent purification via the formation of potassium carbonate into hydrogen carbonate and subsequent release by heating, around 530 million tons per year are obtained.
In the laboratory, carbon dioxide can be released from calcium carbonate and hydrochloric acid , for example in a Kipp apparatus . The device was previously used in laboratories. The method is rarely used any more, as carbon dioxide is available in gas bottles or as dry ice .
At normal pressure below −78.5 ° C, carbon dioxide is present as a solid, called dry ice . If this is heated, it does not melt, but rather sublimates , i.e. it changes directly into the gaseous state of aggregation. It therefore has no melting point and no boiling point under these conditions.
The critical temperature is 31.0 ° C, the critical pressure 73.8 bar and the critical density 0.468 g / cm³. Below the critical temperature, carbon dioxide can be compressed to a colorless liquid by increasing the pressure. A pressure of about 60 bar is required for this at room temperature.
The solubility in water is comparatively high. At 20 ° C under normal pressure, the saturation is in equilibrium with the pure carbon dioxide phase at 1688 mg / l. For comparison, the solubility of oxygen or nitrogen is shown below : With a pure oxygen phase, saturation is already reached at 44 mg / l and with a pure nitrogen phase at 19 mg / l. Under standard conditions the density of carbon dioxide is 1.98 kg / m³.
The carbon dioxide molecule is linear, all three atoms are in a straight line. The carbon is bound to the two oxygen atoms with double bonds , with both oxygen atoms having two free electron pairs . The carbon-oxygen distance is 116.32 pm. The carbon-oxygen bonds are polarized by the different electronegativities of carbon and oxygen ; the electric dipole moments cancel each other out due to the molecular symmetry , so that the molecule does not have an electric dipole moment. The (bending) vibration mode of the molecule, in which the carbon atom moves perpendicular to the axis and the oxygen atoms move in the opposite direction (and vice versa), corresponds to an infrared wavelength of 15 μm . This 15 μm radiation is the main component of carbon dioxide in the greenhouse effect.
Carbon dioxide is an incombustible, acidic and colorless gas; At low concentrations it is odorless, at high concentrations one perceives a pungent to sour odor, although here too (similar to, for example, hydrogen cyanide ) there are people who cannot perceive this odor. Carbon dioxide dissolved in water forms carbonic acid (H 2 CO 3 ), whereby more than 99% of the carbon dioxide is only physically dissolved; the aqueous solution therefore reacts slightly acidic. The carbonic acid as such and the dissolved carbon dioxide are in equilibrium with their dissociation products ( species ) hydrogen carbonate ( bicarbonate , HCO 3 - ) and carbonate (CO 3 2− ), which are in a proportion to each other depending on the pH value . In water this equilibrium is predominantly on the side of carbon dioxide and hydrogen carbonate ions are only formed to a small extent. If the oxonium ions (H 3 O + ) formed during dissociation are intercepted with hydroxide ions (OH - ) by adding an alkali , the quantitative ratio shifts in favor of carbonate.
Due to the positive partial charge on carbon, carbon dioxide reacts as an electrophile in the carboxylation of carbon nucleophiles such as metal alkynylidene or alkyl magnesium compounds to form a carbon-carbon bond. Carbon dioxide reacts with phenolates to form phenol carboxylic acids .
In industry, carbon dioxide is used in a variety of ways. It is inexpensive, non-flammable and is used physically as a compressed gas, in liquid form, solid as dry ice or in supercritical phase. The chemical industry uses carbon dioxide as a raw material for chemical synthesis. This CO 2 comes from z. B. from fertilizer production, where it is very pure, or from exhaust gases that require downstream cleaning in order to remove unwanted accompanying substances.
Use in food technology
Carbon dioxide contained in beverages stimulates the taste sensory cells when drinking, which has a refreshing effect. In drinks such as beer or sparkling wine , it is produced through alcoholic fermentation , in others such as lemonade or soda water it is added artificially or natural mineral water containing carbon dioxide is used. During production, carbon dioxide is pumped into the beverage under high pressure, around 0.2% of which reacts with water to form carbonic acid, while most of it dissolves as a gas in the water. As a food additive , it bears the designation E 290. In private households, carbon dioxide from pressure cartridges is passed through the drink to be enriched with soda makers .
Baker's yeast develops carbon dioxide through the fermentation of sugar and is used as a leavening agent in the production of yeast dough . Baking powder , a mixture of sodium hydrogen carbonate and an acidic salt, releases carbon dioxide when heated and is also used as a leavening agent.
In winemaking, dry ice is used as a coolant to cool freshly picked grapes without diluting them with water, thus avoiding spontaneous fermentation. The winemakers in Beaujolais use carbonic acid maceration to produce Beaujolais Primeur .
When storing fruit and vegetables, temperature and the composition of the atmosphere play a major role. In the warehouses of fruit producers and retailers, apples have been stored in controlled atmospheres for many decades. The realization that ripening fruit consumes oxygen and gives off carbon dioxide, and that an atmosphere without oxygen brings ripening to a standstill, dates back to the early 19th century. In the 1930s, the UK first set up a warehouse with the ability to regulate the levels of oxygen and carbon dioxide in the air. The economic importance of precisely adapted controlled atmospheres in fruit storage is considerable. By adding carbon dioxide to the atmosphere, the shelf life can be extended by months, thus reducing dependence on imports from warmer regions for part of winter and spring. On the other hand, an improper addition of carbon dioxide can cause defects in the pulp and render an entire inventory or container shipment worthless. The biochemical processes that lead to the delayed maturity have not yet been deciphered. It is currently assumed that both the slowing down of the maturation process and the formation of the various types of damage are controlled by stress reactions at the cellular level.
Fruit, vegetables and mushrooms packaged in foil for retail purposes, unprocessed or cut, are given a protective atmosphere in order to extend their shelf life and not to lose the impression of freshness on the way to the consumer. Today meat, fish and seafood , pasta, baked goods and dairy products are also offered in this way. The proportion of carbon dioxide in the protective atmosphere is significantly higher in packaged products that are not intended to be stored for months than in stored fruit and vegetables (1–5%, rarely up to 20%), for which carbon dioxide can cause damage. Typical proportions are 20% carbon dioxide for beef, 50% for veal, pork and pasta , 60% for baked goods and 80% for fish. Packaging under pure carbon dioxide is avoided, however, as it would encourage the development of pathogenic, anaerobic germs and in many cases would impair the color and taste of the products. Determining the optimal protective atmosphere for a product is the subject of intensive research in the food industry .
Supercritical carbon dioxide has a high solubility for non-polar substances and can replace toxic organic solvents . It is used as an extraction agent, for example to extract natural substances such as caffeine in the production of decaffeinated coffee through decaffeination .
Carbon dioxide is due to its oxygen-displacing properties fire-fighting purposes , especially in portable fire extinguishers and automatic fire extinguishing systems, as extinguishing agent used. CO 2 extinguishing systems flood the entire room with carbon dioxide to protect silos or storage halls for flammable liquids. This led to repeated accidents, some of which resulted in death from suffocation. A study by the US Environmental Protection Agency (EPA) identified 51 accidents between 1975 and 1997 with 72 deaths and 145 injuries.
As a refrigerant , carbon dioxide is used under the designation R744 in vehicle and stationary air conditioning systems, in industrial refrigeration, supermarket and transport refrigeration, and in beverage machines. It has a large volumetric cooling capacity and therefore a higher efficiency for a given volume. Carbon dioxide is environmentally friendly, as the global warming potential is only a fraction of the refrigerants used today. It has no ozone depletion potential. Carbon dioxide is used in heating cycles as well as in hot water pumps and vehicle heating. In gas-cooled nuclear reactors of the type AGR , carbon dioxide is used as a coolant .
Carbon dioxide is used as a protective gas in welding technology, either in its pure form or as an additive to argon or helium . At high temperatures it is thermodynamically unstable, which is why it is not called an inert gas , but an active gas .
With the carbon dioxide laser, laser gas, a mixture of nitrogen , helium and carbon dioxide, flows continuously through the discharge tube. In addition to the solid-state lasers , these gas lasers are among the most powerful lasers in industrial use with outputs between 10 watts and 20 kilowatts. The efficiency is around 10 to 20%.
In liquid form, carbon dioxide is traded in pressurized gas cylinders . There are two types: Riser tubes for withdrawing liquids and bottles without a riser tube for withdrawing gaseous carbon dioxide. Both must be vertical for removal. The riser cylinder is always operated without, the other with a pressure reducing valve . As long as there is still liquid carbon dioxide in the pressure bottle, the internal pressure is only dependent on the temperature. A measurement of the filling level is therefore only possible with both types of bottle by weighing . The removal speed is limited by the fact that liquid carbon dioxide has to evaporate again in the bottle due to the absorption of heat from the environment in order to build up the pressure corresponding to the temperature.
During the sublimation of dry ice , a white mist is created from the cold carbon dioxide-air mixture and condensing humidity, which was previously used as an effect in stage technology. Today, for example, there are fog cooling attachments for normal evaporator fog machines that are operated with liquid carbon dioxide.
Increasingly, carbon dioxide is used in conjunction with an automatable blasting process in order to produce high-purity surfaces. With its combination of mechanical, thermal and chemical properties, carbon dioxide snow , for example, can loosen and remove various types of surface contamination without leaving any residue.
Supercritical carbon dioxide is used as a solvent for cleaning and degreasing, for example wafers in the semiconductor industry and textiles in dry cleaning . Supercritical carbon dioxide is used as a reaction medium for the production of fine chemicals, for example for the production of flavorings, since isolated enzymes often remain active in them and, unlike organic solvents, no solvent residues remain in the products.
In tertiary oil production , supercritical carbon dioxide is used to flood oil reservoirs in order to flush oil to the surface from greater depths.
Use as a chemical raw material
In the chemical industry, carbon dioxide is primarily used for the production of urea through conversion with ammonia . In the first step, ammonia and carbon dioxide react to form ammonium carbamate , which in the second step reacts to form urea and water.
In the Solvay process , carbon dioxide is used to produce soda (sodium carbonate). Some metal carbonates such as lead carbonate , which are obtained, for example, by reacting the metal hydroxides with carbon dioxide, are important as pigments .
With a high oil price and low electricity prices for renewable energies, for example from wind power and solar systems , it could be worthwhile in the future to use carbon dioxide for other applications such as methane production in power-to-gas systems ( Sabatier process ) and methanol production ( Power -to-Liquid ) with hydrogen from electrolysis . Further potential fields of application would be the production of formic acid and synthesis gases for the production of fuels ( power-to-fuel ) and chemical raw materials ( power-to-chemicals ). This can be done via a Fischer-Tropsch synthesis or the direct use together with ethylene oxide or propylene oxide for the production of polyols and polymers such as polyurethanes or polycarbonates. For thermodynamic reasons , however, the use of carbon dioxide is currently mostly uneconomical.
Carbon dioxide recycling
In addition to the separation and storage of carbon dioxide , research is aimed at converting the carbon dioxide that arises from the combustion of fossil fuels into usable compounds and, if possible, back into energy sources. Compounds such as methanol and formic acid can already be produced via reduction .
The synthesis of urea is also possible. A French research team is investigating the organocatalytic conversion to formamide or its derivatives. Since the process energy has to be supplied, these processes are not suitable for the economical production of energy carriers. Scientists at RWTH Aachen University developed a homogeneous catalytic process for the production of methanol from carbon dioxide and hydrogen under pressure with a special ruthenium-phosphine complex in which the catalyst and starting materials are in solution. Likewise, a continuous process for the production of formic acid with an organometallic ruthenium complex has been developed, in which carbon dioxide plays the double role both as a reactant and in supercritical form as an extractive phase for the formic acid formed. In another variant developed by a Spanish research group, carbon dioxide can be converted via an iridium-catalyzed hydrosilylation and trapped in the form of a silyl formate from which formic acid can be easily separated. This reaction, which could already be carried out on a gram scale, takes place under very mild reaction conditions, is very selective and has a high conversion.
Carbon dioxide was routinely used as an anesthetic in humans until the 1950s, particularly in the United States, and was found to be very satisfactory. This method is no longer used in traditional anesthesia for humans, as more effective, inhalable anesthetics have been introduced.
This method is still used today for stunning animals for slaughter . Pigs are lowered into a pit in groups with an elevator system, the atmosphere of which contains at least 80% carbon dioxide, and lose consciousness in it. This process is controversial and is subject to intensive efforts to improve animal welfare . Fish are anesthetized by introducing gaseous carbon dioxide or adding carbonated water. In Germany, stunning slaughter animals with carbon dioxide is only permitted for pigs, turkeys , day-old chicks and salmon fish .
In the context of animal euthanasia , carbon dioxide is used for killing. In Germany, use is limited to small laboratory animals , also for purposes such as the procurement of feed animals in animal husbandry. However, the legality of such animal kills without prior stunning is questioned. For officially ordered killing of livestock, the club , carbon dioxide may also be used to kill other animals if a special permit is available. The Veterinary Association for Animal Welfare (DVT) describes this method as suitable for poultry.
Carbon dioxide is used as a laxative in suppositories . The reaction of sodium dihydrogen phosphate and sodium hydrogen carbonate during the dissolution of the suppository releases carbon dioxide and stretches the intestine, which in turn triggers the stool reflex.
In carbon dioxide fertilization , it is used as fertilizer in greenhouses . The reason is the carbon dioxide deficiency caused by photosynthetic consumption when there is insufficient fresh air, especially in winter when the ventilation is closed. The carbon dioxide is introduced either directly as a pure gas or as a combustion product from propane or natural gas. This achieves a coupling of fertilization and heating. The possible increase in yield depends on how strong the lack of carbon dioxide is and how strong the light is for the plants. Carbon dioxide is used in aquaristics as a fertilizer for aquatic plants (CO 2 diffuser). By adding organic matter, the carbon dioxide content in the water can be increased by breathing at the expense of the oxygen content.
The gas is used to catch blood-sucking insects and vectors that use the carbon dioxide found in the breath to find hosts , such as mosquitoes . It is released from dry ice , from gas cylinders or from the combustion of propane or butane and lures insects close to the intake opening of special traps. The gas is also used in the cultivation of microorganisms , especially for obligate (strictly) anaerobic bacteria that can only grow under anoxic conditions. They can be incubated in a CO 2 incubator that is supplied by a gas bottle. In addition to strictly anaerobic bacteria, there are also so-called capnophilic bacteria, which require a proportion of 5–10 percent by volume of carbon dioxide in the surrounding atmosphere for growth. They are often cultivated in a closable anaerobic pot into which a commercially available reagent carrier is placed, the chambers of which are filled with sodium hydrogen carbonate and tartaric acid or citric acid . Moistening releases CO 2 - similar to the principle of baking powder .
Physiological effects and hazards
Effect on animals and people
Too high a proportion of carbon dioxide in the air we breathe has harmful effects on animals and humans. These are not only based on the displacement of oxygen in the air. DIN EN 13779 divides the room air into four quality levels depending on the carbon dioxide concentration. At values below 800 ppm, the indoor air quality is considered good, values between 800 and 1000 ppm (0.08 to 0.1% by volume) are considered medium, values from 1000 to 1400 ppm are considered moderate quality. At values above 1400 ppm, the indoor air quality is considered to be low. For comparison: the global mean the CO 2 content of the air is around 400 ppm by volume; however, it fluctuates strongly regionally, depending on the time of day and the season. The maximum workplace concentration for a daily exposure of eight hours per day is 5000 ppm. At a concentration of 1.5% (15,000 ppm), the respiratory time volume increases by more than 40%.
Carbon dioxide dissolved in the blood activates the breathing center of the brain in a physiological and slightly increased concentration .
In significantly higher concentrations, it leads to a reduction or elimination of the reflex breathing stimulus, initially to respiratory depression and finally to respiratory arrest . From around 5% carbon dioxide in the inhaled air headaches and dizziness occur, with higher concentrations accelerated heartbeat ( tachycardia ), rise in blood pressure, shortness of breath and unconsciousness , the so-called carbon dioxide anesthesia . Carbon dioxide concentrations of 8% lead to death within 30 to 60 minutes. An accumulation of carbon dioxide in the blood is called hypercapnia .
Accidents occur repeatedly in wine cellars, feed silos, wells and cesspools due to high carbon dioxide concentrations. Fermentation processes there produce considerable amounts of carbon dioxide; when fermenting one liter of must, for example, around 50 liters of fermentation gas. Often several people fall victim to fermentation gas poisoning because the helpers inhale carbon dioxide themselves during the rescue attempt and become unconscious. The rescue of an injured person from suspicious situations with carbon dioxide is only possible by professional emergency responders with self- contained breathing apparatus .
If adequate ventilation is not provided, natural carbon dioxide sources in caves and mine tunnels can create high concentrations of the gas. These are then close to the ground, so that especially smaller animals can suffocate. The dog grotto in Italy , for example, has a carbon dioxide concentration of around 70%.
The carbon dioxide concentration in the blood influences its pH value and thus has an indirect effect on the oxygen balance. The carbonic acid-bicarbonate system , a carbonic acid - bicarbonate buffer, represents around 50% of the total buffer capacity of the blood, which is catalyzed by the enzyme carbonic anhydratase.
Effect on plants
On plants, a slightly increased carbon dioxide concentration has the effect of carbon dioxide fertilization , since the plants need CO 2 for carbon dioxide assimilation during photosynthesis . However, excessively high concentrations are also harmful to plants. For C3 plants , the optimum is usually between 800 and 1000 ppm, for C4 plants , however, it is only just over 400 ppm. The C4 plant maize as indicator plant showed streaks on its leaves at 10,000 ppm CO 2 after a six-day exposure period. In rice changes were noted in the nutrient composition (proteins, micronutrients and vitamins). Protein, iron, zinc, vitamins B1, B2, B5 and B9 decrease with excessively increasing CO 2 concentration, while vitamin E increases. Such a reduction in the quality of plant foods would exacerbate the problem of global malnutrition .
Plants and photosynthetic bacteria absorb carbon dioxide from the atmosphere and convert it into carbohydrates such as glucose through photosynthesis under the action of light and absorption of water .
- simplified net reaction equation for oxygenic photosynthesis
This process simultaneously releases oxygen from the decomposition of water. The resulting carbohydrates serve as an energy source and building material for all other biochemical substances such as polysaccharides , nucleic acids and proteins . Carbon dioxide thus provides the raw material for the formation of all biomass in the primary production of ecosystems .
The breakdown of biomass through aerobic respiration is, in reverse to the process of photosynthesis, connected again with the formation of carbon dioxide and the consumption of oxygen.
- simplified net reaction equation for aerobic respiration
All organisms in an ecosystem breathe continuously, while photosynthesis is tied to the availability of light. This leads to a cyclical increase and decrease of carbon dioxide in the daily and seasonal rhythm depending on the different light intensities.
The carbon dioxide concentration in water bodies also fluctuates according to the daily and seasonal rhythms mentioned. Carbon dioxide is in a chemical equilibrium with the other dissolved carbonic acid species, which essentially determines the pH value in the water. The chemical equilibrium of the dissociations of ammonium / ammonia , nitrite / nitrous acid , sulphide / hydrogen sulphide and other acid-base pairs, which are noticeable by the toxicity for the organisms in the water, depend on the pH value .
If the supply of carbon dioxide in a body of water is exhausted through photosynthesis, which is noticeable by a pH value close to 8.3, some types of algae and aquatic plants are able to obtain the required carbon dioxide from the dissolved hydrogen carbonate, whereby they release hydroxide ions , so that the pH value becomes more and more alkaline. In nutrient-rich waters such as carp ponds, the pH value can then rise to 12, with the corresponding health consequences for the fish, for example carp gill necrosis .
In 2012, scientists at the Biodiversity and Climate Research Center calculated for the first time in a joint study with other institutions that cryptogamous layers of lichens, algae and moss bind around 14 billion tons of carbon dioxide annually in addition to nitrogen. They bind as much carbon dioxide as is released each year by forest fires and the burning of biomass worldwide. Fighting climate change with the help of the cryptogamous layers is not possible, however, because the extensive vegetation stores the greenhouse gas carbon dioxide for only a few years.
The storage and release of carbon dioxide in soils is important . The extent to which the release of soil organic carbon is influenced by the respective environmental conditions and other factors is currently largely unknown. However, the release is accelerated by warming, which has been shown in recent studies, and could have an impact on the climate. A study published in 2019 shows that if the CO 2 concentration exceeds 1,200 ppm, stratocumulus clouds break down into scattered clouds, which could further fuel global warming.
By specifying the CO 2 emissions, various processes can be compared in terms of energy and ecology. For this purpose, the release of carbon dioxide when burning fossil fuels is converted.
Detection and quantitative determination
A simple detection of carbon dioxide is possible with an aqueous calcium hydroxide solution , the so-called lime water sample . For this purpose, the gas to be examined is introduced into the solution. If the gas contains carbon dioxide, it reacts with calcium hydroxide to form water and calcium carbonate (lime), which precipitates as a whitish solid and makes the solution cloudy.
In an aqueous solution, carbon dioxide is determined by titration with 0.1 N sodium hydroxide solution up to pH 8.3, the color change of the phenolphthalein indicator . The measurement of the acid binding capacity (SBV) , the pH value and the electrical conductivity or the ionic strength enables the calculation of the carbon dioxide content from these parameters according to the dissociation equilibrium of the carbonic acid. The Severinghaus electrode , a pH electrode with a buffer solution made from sodium hydrogen carbonate, determines the carbon dioxide concentration of a solution by measuring the change in pH value.
Carbon dioxide can be detected using infrared or Raman spectroscopy , whereby the asymmetrical stretching vibrations and tilting vibrations are infrared-active, while the symmetrical stretching vibrations at a wave number of 1480 cm −1 are Raman-active. The measuring device used for this is called a non-dispersive infrared sensor .
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