What is carbon dioxide in biology. Carbon dioxide. Carbon dioxide in nature: natural sources

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CARBON DIOXIDE(carbon(IV) monoxide, carbonic anhydride, carbon dioxide) CO 2 , a well-known bubbling ingredient in carbonated soft drinks. Man has known about the healing properties of "effervescent water" from natural sources since time immemorial, but only in the 19th century. learned to get it myself. At the same time, the substance that makes water effervescent was identified - carbon dioxide. This gas was first obtained for the purposes of carbonization in 1887 during the reaction between crushed marble and sulfuric acid; it was also isolated from natural sources. Later, CO 2 began to be obtained on an industrial scale by burning coke, calcining limestone and fermenting alcohol. For more than a quarter of a century, carbon dioxide has been stored in pressurized steel cylinders and used almost exclusively to carbonate drinks. In 1923, solid CO 2 (dry ice) was produced as a commercial product, and around 1940, liquid CO 2 was poured into special sealed tanks under high pressure.

physical properties.

At ordinary temperatures and pressures, carbon dioxide is a colorless gas with a slightly sour taste and smell. It is 50% heavier than air, so it can be poured from one container to another. CO 2 is a product of most combustion processes and, in large enough quantities, can extinguish a flame by displacing oxygen from the air. When the concentration of CO 2 increases in a poorly ventilated room, the oxygen content in the air decreases so much that a person can suffocate. CO 2 is soluble in many liquids; solubility depends on the properties of the liquid, temperature and vapor pressure of CO 2 . The ability of carbon dioxide to dissolve in water determines its widespread use in the production of soft drinks. CO 2 is highly soluble in organic solvents such as alcohol, acetone and benzene.

When pressure is increased and cooled, carbon dioxide easily liquefies and is in a liquid state at temperatures from +31 to -57 ° C (depending on pressure). Below -57°C it changes into a solid state (dry ice). The pressure required for liquefaction depends on the temperature: at +21°C it is 60 atm, and at -18°C it is only 20 atm. Liquid CO 2 is stored in sealed containers under appropriate pressure. When passing into the atmosphere, part of it turns into gas, and some - into "carbon snow", while its temperature drops to -84 ° C.

Absorbing heat from the environment, dry ice passes into a gaseous state, bypassing the liquid phase, - it sublimates. To reduce sublimation losses, it is stored and transported in airtight containers strong enough to withstand the increase in pressure as the temperature rises.

Chemical properties.

CO 2 is an inactive compound. When dissolved in water, it forms a weak carbonic acid, which turns litmus paper red. Carbonic acid improves the taste of carbonated drinks and prevents bacterial growth. Reacting with alkali and alkaline earth metals, as well as with ammonia, CO 2 forms carbonates and bicarbonates.

Distribution in nature and obtaining.

CO 2 is formed during the combustion of carbon-containing substances, alcoholic fermentation, decay of plant and animal residues; it is released during the respiration of animals, it is released by plants in the dark. In the light, on the contrary, plants absorb CO 2 and release oxygen, which maintains the natural balance of oxygen and carbon dioxide in the air we breathe. The content of CO 2 in it does not exceed 0.03% (by volume).

There are five main methods for obtaining CO 2: combustion of carbon-containing substances (coke, natural gas, liquid fuel); formation as a by-product in the synthesis of ammonia; limestone calcination; fermentation; well pumping. In the last two cases, practically pure carbon dioxide is obtained, and when carbonaceous substances are burned or limestone is calcined, a mixture of CO 2 with nitrogen and traces of other gases is formed. This mixture is passed through a solution absorbing only CO 2 . Then the solution is heated and almost pure CO 2 is obtained, which is separated from the remaining impurities. Water vapor is removed by freezing and chemical drying.

Purified CO 2 is liquefied by cooling it at high pressure and stored in large containers. To obtain dry ice, liquid CO 2 is fed into a closed chamber of a hydraulic press, where the pressure is reduced to atmospheric pressure. With a sharp decrease in pressure, loose snow and very cold gas are formed from CO 2. Snow is pressed and dry ice is obtained. Gaseous CO 2 is pumped out, liquefied and returned to the storage tank.

APPLICATION

Getting low temperatures.

In liquid and solid form, CO 2 is mainly used as a refrigerant. Dry ice is a compact material, easy to handle and allows you to create different temperature conditions. With the same mass, it exceeds ordinary ice in terms of cold capacity by more than two times, occupying half the volume. Dry ice is used in food storage. They cool champagne, soft drinks and ice cream. It is widely used in the "cold grinding" of heat-sensitive materials (meat products, resins, polymers, dyes, insecticides, paints, seasonings); when tumbling (deburring) stamped products made of rubber and plastic; during low-temperature testing of aircraft and electronic devices in special chambers; for "cold mixing" semi-finished muffins and cakes, so that they remain homogeneous during baking; for rapid cooling of containers with transported products by blowing them with a jet of crushed dry ice; when hardening alloyed and stainless steels, aluminum, etc. in order to improve their physical properties; for a tight fit of machine parts during their assembly; for cooling cutters when machining high-strength steel workpieces.

Carbonization.

The main application of gaseous CO 2 is the carbonation of water and soft drinks. First, water and syrup are mixed in the required proportions, and then the mixture is saturated with gaseous CO 2 under pressure. Carbonization of beer and wine usually occurs as a result of chemical reactions occurring in them.

Applications based on inertia.

CO 2 is used as an antioxidant in the long-term storage of many food products: cheese, meat, milk powder, nuts, instant tea, coffee, cocoa, etc. As a substance that suppresses combustion, CO 2 is used in the storage and transportation of combustible materials, such as rocket fuel, oils, gasoline, paints, varnishes, solvents. It is used as a protective medium in the electric welding of carbon steels in order to obtain a uniform, strong seam, while welding is cheaper than when using inert gases.

CO 2 is one of the most effective means of extinguishing fires that occur during the ignition of flammable liquids and electrical breakdowns. They produce various carbon dioxide fire extinguishers: from portable ones with a capacity of not more than 2 kg to stationary automatic supply units with a total capacity of cylinders up to 45 kg or low-pressure gas tanks with a capacity of up to 60 tons of CO 2 . Liquid CO 2 contained in such fire extinguishers under pressure, when released, forms a mixture of snow and cold gas; the latter has a greater density than air and displaces it from the combustion zone. The effect is further enhanced by the cooling effect of snow, which, evaporating, turns into gaseous CO 2 .

Chemical aspects.

Carbon dioxide is used in the production of aspirin, white lead, urea, perborates, chemically pure carbonates. Carbonic acid, formed when CO 2 is dissolved in water, is an inexpensive reagent for neutralizing alkalis. In the foundry, sand molds are cured with carbon dioxide by reacting CO 2 with sodium silicate mixed with sand. This allows you to get better quality castings. The refractory bricks used to line steel, glass and aluminum furnaces become stronger after being treated with carbon dioxide. CO 2 is also used in urban water softening systems using soda lime.

Creation of increased pressure.

CO 2 is used for pressure testing and leak testing of various containers, as well as for calibrating pressure gauges, valves, and spark plugs. It is filled in portable containers for inflating life belts and inflatable boats. A mixture of carbon dioxide and nitrous oxide has long been used to pressurize aerosol cans. CO 2 is injected under pressure into sealed containers with ether (in devices for quick engine start), solvents, paints, insecticides for subsequent spraying of these substances.

Application in medicine.

In small quantities, CO 2 is added to oxygen (to stimulate breathing) and during anesthesia. In high concentrations, it is used for the humane killing of animals.

Structural formula

True, empirical, or gross formula: CO2

Chemical composition of carbon dioxide

Molecular weight: 44.009

Carbon dioxide (carbon dioxide, carbon dioxide, carbon monoxide (IV), carbonic anhydride) is a colorless gas (under normal conditions), odorless, with a chemical formula CO2. Density under normal conditions 1.98 kg/m³ (heavier than air). At atmospheric pressure, carbon dioxide does not exist in a liquid state, passing directly from a solid to a gaseous state. Solid carbon dioxide is called dry ice. At elevated pressure and normal temperatures, carbon dioxide turns into a liquid, which is used for its storage. The concentration of carbon dioxide in the Earth's atmosphere is on average 0.04%. Carbon dioxide easily transmits ultraviolet rays and rays of the visible part of the spectrum, which come to Earth from the Sun and heat it. At the same time, it absorbs infrared rays emitted by the Earth and is one of the greenhouse gases, as a result of which it takes part in the process of global warming. A constant increase in the level of this gas in the atmosphere has been observed since the beginning of the industrial era.

Carbon monoxide (IV) - carbon dioxide, an odorless and colorless gas, heavier than air, crystallizes with strong cooling in the form of a white snow-like mass - "dry ice". At atmospheric pressure, it does not melt, but evaporates, the sublimation temperature is −78 °C. Carbon dioxide is formed during the decay and combustion of organic matter. Contained in the air and mineral springs, released during the respiration of animals and plants. Let's dissolve in water (1 volume of carbon dioxide in one volume of water at 15 °C).

According to its chemical properties, carbon dioxide belongs to acidic oxides. When dissolved in water, it forms carbonic acid. Reacts with alkalis to form carbonates and bicarbonates. It enters into reactions of electrophilic substitution (for example, with phenol) and nucleophilic addition (for example, with organomagnesium compounds). Carbon monoxide(IV) does not support combustion. Only some active metals burn in it. Interacts with oxides of active metals. When dissolved in water, it forms carbonic acid. Reacts with alkalis to form carbonates and bicarbonates.

The human body releases approximately 1 kg (2.3 lb) of carbon dioxide per day. This carbon dioxide is transported from the tissues, where it is formed as one of the end products of metabolism, through the venous system and is then excreted in the exhaled air through the lungs. Thus, the content of carbon dioxide in the blood is high in the venous system, and decreases in the capillary network of the lungs, and low in the arterial blood. The content of carbon dioxide in a blood sample is often expressed in terms of partial pressure, that is, the pressure that carbon dioxide contained in a given amount of carbon dioxide would have if only carbon dioxide occupied the entire volume of the blood sample. Carbon dioxide ( CO2) is transported in the blood in three different ways (the exact ratio of each of these three modes of transport depends on whether the blood is arterial or venous).

• Most of the carbon dioxide (from 70% to 80%) is converted by the enzyme carbonic anhydrase of erythrocytes into bicarbonate ions.
• About 5% - 10% of carbon dioxide is dissolved in blood plasma.
• About 5% - 10% of carbon dioxide is associated with hemoglobin in the form of carbamin compounds (carbohemoglobin).

Hemoglobin, the main oxygen-transporting protein in red blood cells, is capable of transporting both oxygen and carbon dioxide. However, carbon dioxide binds to hemoglobin at a different site than oxygen. It binds to the N-terminal ends of the globin chains and not to the heme. However, due to allosteric effects, which lead to a change in the configuration of the hemoglobin molecule upon binding, the binding of carbon dioxide reduces the ability of oxygen to bind to it, at a given partial pressure of oxygen, and vice versa - the binding of oxygen to hemoglobin reduces the ability of carbon dioxide to bind to it, at a given partial pressure of carbon dioxide. In addition, the ability of hemoglobin to preferentially bind to oxygen or carbon dioxide also depends on the pH of the medium. These features are very important for the successful capture and transport of oxygen from the lungs to the tissues and its successful release in the tissues, as well as for the successful capture and transport of carbon dioxide from the tissues to the lungs and its release there. Carbon dioxide is one of the most important mediators of blood flow autoregulation. It is a powerful vasodilator. Accordingly, if the level of carbon dioxide in the tissue or in the blood rises (for example, due to intensive metabolism - caused, say, by exercise, inflammation, tissue damage, or due to obstruction of blood flow, tissue ischemia), then the capillaries dilate, which leads to an increase in blood flow and respectively, to an increase in the delivery of oxygen to the tissues and the transport of accumulated carbon dioxide from the tissues. In addition, carbon dioxide in certain concentrations (increased, but not yet reaching toxic values) has a positive inotropic and chronotropic effect on the myocardium and increases its sensitivity to adrenaline, which leads to an increase in the strength and frequency of heart contractions, the magnitude of cardiac output and, as a result, , stroke and minute blood volume. It also contributes to the correction of tissue hypoxia and hypercapnia (elevated levels of carbon dioxide). Bicarbonate ions are very important for regulating blood pH and maintaining normal acid-base balance. The respiratory rate affects the amount of carbon dioxide in the blood. Weak or slow breathing causes respiratory acidosis, while rapid and excessively deep breathing leads to hyperventilation and the development of respiratory alkalosis. In addition, carbon dioxide is also important in the regulation of respiration. Although our body requires oxygen for metabolism, low oxygen levels in the blood or tissues usually do not stimulate respiration (or rather, the stimulating effect of lack of oxygen on respiration is too weak and “turns on” late, at very low blood oxygen levels, in which a person often is already losing consciousness). Normally, respiration is stimulated by an increase in the level of carbon dioxide in the blood. The respiratory center is much more sensitive to an increase in carbon dioxide than to a lack of oxygen. As a consequence, breathing highly rarefied air (with a low partial pressure of oxygen) or a gas mixture containing no oxygen at all (for example, 100% nitrogen or 100% nitrous oxide) can quickly lead to loss of consciousness without causing a feeling of lack of air (because the level of carbon dioxide does not rise in the blood, because nothing prevents its exhalation). This is especially dangerous for pilots of military aircraft flying at high altitudes (if an enemy missile hits the cockpit and depressurizes the cockpit, the pilots can quickly lose consciousness). This feature of the breathing regulation system is also the reason why on airplanes flight attendants instruct passengers in the event of a depressurization of the aircraft cabin to first put on an oxygen mask themselves before trying to help someone else - by doing this, the helper risks quickly losing consciousness himself, and even without feeling until the last moment any discomfort and the need for oxygen. The human respiratory center tries to maintain a partial pressure of carbon dioxide in the arterial blood no higher than 40 mm Hg. With conscious hyperventilation, the content of carbon dioxide in the arterial blood can decrease to 10-20 mmHg, while the oxygen content in the blood will practically not change or increase slightly, and the need to take another breath will decrease as a result of a decrease in the stimulating effect of carbon dioxide on the activity of the respiratory center. This is the reason why after a period of conscious hyperventilation it is easier to hold the breath for a long time than without prior hyperventilation. Such conscious hyperventilation followed by breath-holding can result in loss of consciousness before the person feels the need to breathe. In a safe environment, such a loss of consciousness does not threaten anything special (having lost consciousness, a person will lose control over himself, stop holding his breath and take a breath, breathing, and with it the supply of oxygen to the brain will be restored, and then consciousness will be restored). However, in other situations, such as before diving, this can be dangerous (loss of consciousness and the need to breathe will come at a depth, and in the absence of conscious control, water will enter the airways, which can lead to drowning). That is why hyperventilation before diving is dangerous and not recommended.

In industrial quantities, carbon dioxide is emitted from flue gases, or as a by-product of chemical processes, for example, during the decomposition of natural carbonates (limestone, dolomite) or in the production of alcohol (alcoholic fermentation). The mixture of gases obtained is washed with a solution of potassium carbonate, which absorb carbon dioxide, turning into hydrocarbonate. A solution of bicarbonate, when heated or under reduced pressure, decomposes, releasing carbon dioxide. In modern installations for the production of carbon dioxide, instead of bicarbonate, an aqueous solution of monoethanolamine is more often used, which, under certain conditions, is able to absorb CO2 contained in the flue gas, and when heated, give it away; thus separating the finished product from other substances. Carbon dioxide is also produced in air separation plants as a by-product of obtaining pure oxygen, nitrogen and argon. Under laboratory conditions, small amounts are obtained by reacting carbonates and bicarbonates with acids, such as marble, chalk or soda, with hydrochloric acid, using, for example, a Kipp apparatus. Using the reaction of sulfuric acid with chalk or marble results in the formation of insoluble calcium sulfate, which interferes with the reaction and is removed by a significant excess of acid. Beverages can be prepared by reacting baking soda with citric acid or acidic lemon juice. It was in this form that the first carbonated drinks appeared. Pharmacists were engaged in their manufacture and sale.

In the food industry, carbon dioxide is used as a preservative and leavening agent, indicated on the packaging with the code E290. Liquid carbon dioxide is widely used in fire extinguishing systems and fire extinguishers. Automatic carbon dioxide fire extinguishing systems are distinguished by starting systems, which are pneumatic, mechanical or electric. A device for supplying carbon dioxide to an aquarium may include a gas tank. The simplest and most common method for obtaining carbon dioxide is based on the design for the manufacture of an alcoholic mash drink. During fermentation, the carbon dioxide released may well provide nutrition for aquarium plants. Carbon dioxide is used to carbonate lemonade and sparkling water. Carbon dioxide is also used as a protective medium in wire welding, but at high temperatures it dissociates with the release of oxygen. The released oxygen oxidizes the metal. In this regard, it is necessary to introduce deoxidizers, such as manganese and silicon, into the welding wire. Another consequence of the influence of oxygen, also associated with oxidation, is a sharp decrease in surface tension, which leads, among other things, to more intense metal spatter than when welding in an inert atmosphere. Carbon dioxide in cartridges is used in pneumatic weapons (in gas-cylinder pneumatics) and as a source of energy for engines in aircraft modeling. Storing carbon dioxide in a steel cylinder in a liquefied state is more profitable than in the form of a gas. Carbon dioxide has a relatively low critical temperature of +31°C. About 30 kg of liquefied carbon dioxide is poured into a standard 40-liter cylinder, and at room temperature there will be a liquid phase in the cylinder, and the pressure will be approximately 6 MPa (60 kgf / cm²). If the temperature is above +31°C, then carbon dioxide will go into a supercritical state with a pressure above 7.36 MPa. The standard operating pressure for a typical 40 liter cylinder is 15 MPa (150 kgf/cm²), however, it must safely withstand pressures 1.5 times higher, i.e. 22.5 MPa - thus, work with such cylinders can be considered quite safe. Solid carbon dioxide - "dry ice" - is used as a refrigerant in laboratory research, in retail trade, in equipment repair (for example: cooling one of the mating parts during tight fitting), etc. Carbon dioxide is used to liquefy carbon dioxide and produce dry ice. installation.

Measurement of the partial pressure of carbon dioxide is required in technological processes, in medical applications - the analysis of respiratory mixtures during artificial ventilation of the lungs and in closed life support systems. Concentration analysis CO2 in the atmosphere is used for environmental and scientific research, to study the greenhouse effect. Carbon dioxide is recorded using gas analyzers based on the principle of infrared spectroscopy and other gas measuring systems. A medical gas analyzer for recording the content of carbon dioxide in exhaled air is called a capnograph. For measuring low concentrations CO2(as well as CO) in process gases or in atmospheric air, a gas chromatographic method with a methanator and registration on a flame ionization detector can be used.

Annual fluctuations in the concentration of atmospheric carbon dioxide on the planet are determined mainly by the vegetation of the middle (40-70 °) latitudes of the Northern Hemisphere. Vegetation in the tropics practically does not depend on the season, the dry belt of deserts 20-30 ° (both hemispheres) makes a small contribution to the carbon dioxide cycle, and the land strips most covered with vegetation are asymmetrically located on Earth (in the Southern Hemisphere in the middle latitudes there is an ocean). Therefore, from March to September, due to photosynthesis, the content CO2 drops in the atmosphere, and rises from October to February. Both wood oxidation (heterotrophic respiration of plants, rotting, decomposition of humus, forest fires) and the burning of fossil fuels (coal, oil, gas), which increases noticeably in the winter season, contribute to winter growth. A large amount of carbon dioxide is dissolved in the ocean. Carbon dioxide makes up a significant part of the atmospheres of some planets in the solar system: Venus, Mars.

Carbon dioxide is non-toxic, but according to the effect of its elevated concentrations in the air on air-breathing living organisms, it is classified as an asphyxiant gas (English) Russian. Slight increases in concentrations up to 2-4% in rooms lead to the development of drowsiness and weakness in people. Dangerous concentrations are considered levels of about 7-10%, at which suffocation develops, manifesting itself in headache, dizziness, hearing loss and loss of consciousness (symptoms similar to those of altitude sickness), depending on the concentration, over a period of several minutes up to one hour. When air with high concentrations of gas is inhaled, death occurs very quickly from suffocation. Although, in fact, even a concentration of 5-7% CO2 is not lethal, already at a concentration of 0.1% (such a carbon dioxide content is observed in the air of megacities), people begin to feel weak, drowsy. This shows that even at high oxygen levels, a high concentration of CO2 has a strong effect on well-being. Inhalation of air with a high concentration of this gas does not lead to long-term health disorders, and after the victim is removed from the polluted atmosphere, full recovery of health quickly occurs.

A substance with the chemical formula CO2 and a molecular weight of 44.011 g / mol, which can exist in four phase states - gaseous, liquid, solid and supercritical.

The gaseous state of CO2 is commonly known as carbon dioxide. At atmospheric pressure, it is a colorless gas without color and odor, at a temperature of +20? With a density of 1.839 kg / m? (1.52 times heavier than air), dissolves well in water (0.88 volume in 1 volume of water), partially interacting in it with the formation of carbonic acid. Included in the atmosphere on average 0.035% by volume. With a sharp cooling due to expansion (expanding), CO2 is able to desublimate - go immediately into a solid state, bypassing the liquid phase.

Gaseous carbon dioxide was previously often stored in stationary gas holders. Currently, this method of storage is not used; carbon dioxide in the required amount is obtained directly on site - by evaporating liquid carbon dioxide in the gasifier. Further, the gas can be easily pumped through any gas pipeline at a pressure of 2-6 atmospheres.

The liquid state of CO2 is technically called "liquid carbon dioxide" or simply "carbonic acid". It is a colorless, odorless liquid with an average density of 771 kg / m3, which exists only under a pressure of 3,482 ... 519 kPa at a temperature of 0 ... -56.5 degrees C (“low-temperature carbon dioxide”), or under a pressure of 3,482 ... at a temperature of 0 ... + 31.0 degrees C ("high-pressure carbon dioxide"). High-pressure carbon dioxide is most often obtained by compressing carbon dioxide to a condensation pressure, while cooling it with water. Low-temperature carbon dioxide, which is the main form of carbon dioxide for industrial consumption, is most often produced in a high-pressure cycle by three-stage cooling and throttling in special plants.

With a small and medium consumption of carbon dioxide (high pressure), tons, a variety of steel cylinders are used for its storage and transportation (from cans for household siphons to containers with a capacity of 55 liters). The most common is a 40 l cylinder with a working pressure of 15,000 kPa, containing 24 kg of carbon dioxide. Steel cylinders do not require additional care, carbon dioxide is stored without loss for a long time. High pressure carbon dioxide cylinders are painted black.

With significant consumption, for storage and transportation of low-temperature liquid carbon dioxide, isothermal tanks of the most diverse capacity, equipped with service refrigeration units, are used. There are accumulative (stationary) vertical and horizontal tanks with a capacity of 3 to 250 tons, transportable tanks with a capacity of 3 to 18 tons. Vertical tanks require the construction of a foundation and are used mainly in conditions of limited space for placement. The use of horizontal tanks makes it possible to reduce the cost of foundations, especially if there is a common frame with a carbon dioxide plant. The tanks consist of an internal welded vessel made of low-temperature steel and having polyurethane foam or vacuum thermal insulation; outer casing made of plastic, galvanized or stainless steel; pipelines, fittings and control devices. The inner and outer surfaces of the welded vessel are subjected to special treatment, due to which the probability of surface corrosion of the metal is reduced to a minimum. In expensive imported models, the outer sealed casing is made of aluminum. The use of tanks provides filling and discharge of liquid carbon dioxide; storage and transportation without loss of the product; visual control of weight and operating pressure during filling, storage and dispensing. All types of tanks are equipped with a multi-level security system. Safety valves allow inspection and repair without stopping and emptying the tank.

With an instantaneous decrease in pressure to atmospheric pressure, which occurs during injection into a special expansion chamber (throttling), liquid carbon dioxide instantly turns into a gas and a thin snow-like mass, which is pressed and carbon dioxide is obtained in a solid state, which is commonly called "dry ice". At atmospheric pressure, it is a white vitreous mass with a density of 1,562 kg / m?, with a temperature of -78.5 ° C, which sublimates in the open air - gradually evaporates, bypassing the liquid state. Dry ice can also be obtained directly at high-pressure plants used to produce low-temperature carbon dioxide from gas mixtures containing CO2 in an amount of at least 75-80%. The volumetric cooling capacity of dry ice is almost 3 times greater than that of water ice and is 573.6 kJ/kg.

Solid carbon dioxide is usually produced in briquettes 200 × 100 × 20-70 mm in size, in granules with a diameter of 3, 6, 10, 12 and 16 mm, rarely in the form of the finest powder (“dry snow”). Briquettes, pellets and snow are stored for no more than 1-2 days in stationary underground storages of the mine type, divided into small compartments; transported in special isothermal containers with a safety valve. Containers from different manufacturers with a capacity of 40 to 300 kg or more are used. Sublimation losses are, depending on the ambient temperature, 4-6% or more per day.

At a pressure of over 7.39 kPa and a temperature of more than 31.6 degrees C, carbon dioxide is in the so-called supercritical state, in which its density is like that of a liquid, and its viscosity and surface tension are like that of a gas. This unusual physical substance (fluid) is an excellent non-polar solvent. Supercritical CO2 is able to fully or selectively extract any non-polar constituents with a molecular weight of less than 2,000 daltons: terpene compounds, waxes, pigments, high molecular weight saturated and unsaturated fatty acids, alkaloids, fat-soluble vitamins and phytosterols. Insoluble substances for supercritical CO2 are cellulose, starch, high molecular weight organic and inorganic polymers, sugars, glycosidic substances, proteins, metals and many metal salts. Having similar properties, supercritical carbon dioxide is increasingly used in the processes of extraction, fractionation and impregnation of organic and inorganic substances. It is also a promising working fluid for modern heat engines.

• Specific gravity. The specific gravity of carbon dioxide depends on the pressure, temperature and state of aggregation in which it is located.
• The critical temperature of carbon dioxide is +31 degrees. The specific gravity of carbon dioxide at 0 degrees and a pressure of 760 mm Hg. is equal to 1.9769 kg/m3.
• The molecular weight of carbon dioxide is 44.0. The relative weight of carbon dioxide compared to air is 1.529.
• Liquid carbon dioxide at temperatures above 0 deg. much lighter than water and can only be stored under pressure.
• The specific gravity of solid carbon dioxide depends on the method of its production. Liquid carbon dioxide, when frozen, turns into dry ice, which is a transparent, glassy solid. In this case, solid carbon dioxide has the highest density (at normal pressure in a vessel cooled to minus 79 degrees, the density is 1.56). Industrial solid carbon dioxide is white, close to chalk in hardness,
• its specific gravity varies depending on the method of obtaining within 1.3 - 1.6.

• State equation. The relationship between the volume, temperature, and pressure of carbon dioxide is expressed by the equation
• V= R T/p - A, where
• V - volume, m3/kg;
• R - gas constant 848/44 = 19.273;
• T - temperature, K degrees;
• p pressure, kg/m2;
• A is an additional term characterizing the deviation from the equation of state for an ideal gas. It is expressed by the dependence A \u003d (0.0825 + (1.225) 10-7 p) / (T / 100) 10 / 3.

• Triple point of carbon dioxide. The triple point is characterized by a pressure of 5.28 ata (kg/cm2) and a temperature of minus 56.6 degrees.
• Carbon dioxide can exist in all three states (solid, liquid and gaseous) only at the triple point. At pressures below 5.28 ata (kg/cm2) (or at temperatures below minus 56.6 degrees), carbon dioxide can exist only in solid and gaseous states.
• In the vapor-liquid region, i.e. above the triple point, the following relations hold
• i "x + i" "y \u003d i,
• x + y = 1, where,
• x and y - the proportion of the substance in liquid and vapor form;
• i" is the enthalpy of the liquid;
• i"" - steam enthalpy;
• i is the enthalpy of the mixture.
• From these values ​​it is easy to determine the values ​​of x and y. Accordingly, for the region below the triple point, the following equations will be valid:
• i"" y + i"" z \u003d i,
• y + z = 1, where,
• i"" - enthalpy of solid carbon dioxide;
• z is the proportion of the substance in the solid state.
• At the triple point for three phases, there are also only two equations
• i"x + i""y + i"""z = i,
• x + y + z = 1.
• Knowing the values ​​of i," i"," i""" for the triple point and using the above equations, you can determine the enthalpy of the mixture for any point.

• Heat capacity. The heat capacity of carbon dioxide at a temperature of 20 degrees. and 1 ata is
• Ср = 0.202 and Сv = 0.156 kcal/kg*deg. Adiabatic exponent k = 1.30.
• The heat capacity of liquid carbon dioxide in the temperature range from -50 to +20 deg. characterized by the following values, kcal / kg * deg. :
• Deg.С -50 -40 -30 -20 -10 0 10 20
• Wed, 0.47 0.49 0.515 0.514 0.517 0.6 0.64 0.68

• Melting point. The melting of solid carbon dioxide occurs at temperatures and pressures corresponding to the triple point (t = -56.6 degrees and p = 5.28 atm) or above it.
• Below the triple point, solid carbon dioxide sublimates. The sublimation temperature is a function of pressure: at normal pressure it is -78.5 degrees, in vacuum it can be -100 degrees. and below.

• Enthalpy. The enthalpy of carbon dioxide vapor in a wide range of temperatures and pressures is determined by the Planck and Kupriyanov equation.
• i = 169.34 + (0.1955 + 0.000115t)t - 8.3724p(1 + 0.007424p)/0.01T(10/3), where
• I - kcal / kg, p - kg / cm2, T - deg. K, t - deg. C.
• The enthalpy of liquid carbon dioxide at any point can be easily determined by subtracting the latent heat of vaporization from the enthalpy of saturated steam. Similarly, by subtracting the latent heat of sublimation, one can determine the enthalpy of solid carbon dioxide.

• Thermal conductivity. Thermal conductivity of carbon dioxide at 0 deg. is 0.012 kcal / m * hour * deg. C, and at a temperature of -78 deg. it drops to 0.008 kcal/m*hour*deg.C.
• Data on the thermal conductivity of carbon dioxide in 10 4 tbsp. kcal/m*h*deg.С at above-zero temperatures are given in the table.
• Pressure, kg/cm2 10 deg. 20 deg. 30 deg. 40 deg.
• gaseous carbon dioxide
• 1 130 136 142 148
• 20 - 147 152 157
• 40 - 173 174 175
• 60 - - 228 213
• 80 - - - 325
• liquid carbonic acid
• 50 848 - - -
• 60 870 753 - -
• 70 888 776 - -
• 80 906 795 670
  The thermal conductivity of solid carbon dioxide can be calculated by the formula:
  236.5 / T1.216 st., kcal / m * hour * deg. C.

• Thermal expansion coefficient. The volume expansion coefficient a of solid carbon dioxide is calculated depending on the change in specific gravity and temperature. The linear expansion coefficient is determined by the expression b = a/3. In the temperature range from -56 to -80 degrees. the coefficients have the following values: a * 10 * 5st. \u003d 185.5-117.0, b * 10 * 5 st. = 61.8-39.0.

• Viscosity. Viscosity of carbon dioxide 10 * 6st. depending on pressure and temperature (kg*sec/m2)
• Pressure, ata -15 degrees. 0 deg. 20 deg. 40 deg.
• 5 1,38 1,42 1,49 1,60
• 30 12,04 1,63 1,61 1,72
• 75 13,13 12,01 8,32 2,30

• Dielectric constant. The dielectric constant of liquid carbon dioxide at 50 - 125 ati is in the range of 1.6016 - 1.6425.
• Dielectric constant of carbon dioxide at 15 deg. and pressure 9.4 - 39 atm 1.009 - 1.060.

• Moisture content of carbon dioxide. The content of water vapor in moist carbon dioxide is determined using the equation,
• X = 18/44 * p'/p - p' = 0.41 p'/p - p' kg/kg, where
• p' - partial pressure of water vapor at 100% saturation;
• p is the total pressure of the vapor-gas mixture.

• Solubility of carbon dioxide in water. The solubility of gases is measured by volumes of gas reduced to normal conditions (0 degrees, C and 760 mm Hg) per volume of solvent.
• The solubility of carbon dioxide in water at moderate temperatures and pressures up to 4 - 5 atm obeys Henry's law, which is expressed by the equation
• P \u003d H X, where
• P is the partial pressure of the gas above the liquid;
• X is the amount of gas in moles;
• H is Henry's coefficient.

• Liquid carbon dioxide as a solvent. The solubility of lubricating oil in liquid carbon dioxide at a temperature of -20 deg. up to +25 deg. is 0.388 g in 100 CO2,
• and increases to 0.718 g in 100 g of CO2 at a temperature of +25 degrees. WITH.
• The solubility of water in liquid carbon dioxide in the temperature range from -5.8 to +22.9 degrees. is not more than 0.05% by weight.

Safety

According to the degree of impact on the human body, gaseous carbon dioxide belongs to the 4th hazard class according to GOST 12.1.007-76 “Harmful substances. Classification and general safety requirements”. The maximum permissible concentration in the air of the working area has not been established; when assessing this concentration, one should be guided by the standards for coal and ozocerite mines, set within 0.5%.

When using dry ice, when using vessels with liquid low-temperature carbon dioxide, safety measures must be observed to prevent frostbite of the hands and other parts of the worker's body.

Food additive E290 (carbon dioxide) is used in the food industry as a preservative, acidity regulator and antioxidant. In everyday life, the additive E290 is better known as carbon dioxide.

According to its physical properties, carbon dioxide is a colorless, odorless gas with a slightly sour taste. Additive E290 can dissolve in water with the formation of weak carbonic acid. Chemical formula of carbon dioxide: CO 2 .

On an industrial scale, carbon dioxide is obtained from flue gases by absorption with potassium carbonate or monoethanolamine. To do this, a mixture of industrial gases is passed through a solution of potassium carbonate. Carbon dioxide is absorbed by this solution, forming bicarbonate. Next, the bicarbonate solution is heated or subjected to reduced pressure, as a result of which pure carbon dioxide is released from it.

In addition, carbon dioxide can be obtained in special air separation plants as a by-product in the extraction of pure oxygen, argon and nitrogen.

In small quantities in the laboratory, carbon dioxide is obtained by reacting carbonates with acids. For example, during the reaction of chalk with hydrochloric acid, unstable carbonic acid is formed, followed by its decomposition into carbon dioxide and water:

• CaCO 3 + 2HCl = CaCl 2 + CO 2 + H 2 O

Carbon dioxide is part of the atmosphere and many living cells of our body. For this reason, the E290 additive can be classified as a relatively harmless food additive.

However, it should be remembered that carbon dioxide contributes to the accelerated absorption of various substances into the gastric mucosa. It is this effect that manifests itself with rapid intoxication as a result of the use of carbonated alcoholic beverages.

In addition, carbonated drinks are nothing more than a weak carbonic acid. Therefore, excessive consumption of drinks with the addition of E290 is contraindicated for people with diseases of the stomach and gastrointestinal tract (ulcers, gastritis).

There are also more harmless "side effects" of exposure to carbon dioxide on the body. So, when drinking carbonated drinks, most people experience belching and "bloating".

There is another opinion regarding the harm of the food additive E290. Highly carbonated drinks can help flush out calcium from the body's bones.

In the food industry, carbon dioxide is used as an E290 preservative additive in the production of alcoholic and non-alcoholic beverages. Carbonic acid, formed by the reaction of carbon dioxide with water, has a disinfecting and antimicrobial effect.

In the bakery business, the E290 additive can be used as a baking powder, giving splendor to bakery products.

Carbon dioxide is widely used in the production of wine products. By adjusting the amount of carbon dioxide in wine must, fermentation can be controlled.

Carbon monoxide can also be used as a protective gas during storage and transportation of various food products.

Other uses for carbon dioxide:

• in welding production as a protective atmosphere;
• in refrigeration units in the form of "dry ice";
• in fire extinguishing systems
• in gas cylinder pneumatics

The E290 additive is approved for use in the food industry in almost all countries of the world, including Ukraine and the Russian Federation.

Carbon dioxide is a colorless gas with a barely perceptible odor, non-toxic, heavier than air. Carbon dioxide is widely distributed in nature. It dissolves in water, forming carbonic acid H 2 CO 3, giving it a sour taste. The air contains about 0.03% carbon dioxide. The density is 1.524 times greater than the density of air and is equal to 0.001976 g / cm 3 (at zero temperature and a pressure of 101.3 kPa). Ionization potential 14.3V. The chemical formula is CO 2 .

In welding production, the term is used "carbon dioxide" cm. . The "Rules for the Design and Safe Operation of Pressure Vessels" adopted the term "carbon dioxide", and in - term "carbon dioxide".

There are many ways to produce carbon dioxide, the main ones are discussed in the article.

The density of carbon dioxide depends on the pressure, temperature and state of aggregation in which it is located. At atmospheric pressure and a temperature of -78.5 ° C, carbon dioxide, bypassing the liquid state, turns into a white snow-like mass. "dry ice".

Under a pressure of 528 kPa and at a temperature of -56.6 ° C, carbon dioxide can be in all three states (the so-called triple point).

Carbon dioxide is thermally stable, dissociates into carbon monoxide and only at temperatures above 2000°C.

Carbon dioxide is first gas to be described as a discrete substance. In the seventeenth century, a Flemish chemist Jan Baptist van Helmont (Jan Baptist van Helmont) noticed that after burning coal in a closed vessel, the mass of ash was much less than the mass of the burned coal. He explained this by the fact that coal is transformed into an invisible mass, which he called "gas".

The properties of carbon dioxide were studied much later in 1750. Scottish physicist Joseph Black (joseph black.

He discovered that limestone (calcium carbonate CaCO 3 ) when heated or reacted with acids, releases a gas, which he called "bound air". It turned out that "bound air" is denser than air and does not support combustion.

CaCO 3 + 2HCl \u003d CO 2 + CaCl 2 + H 2 O

Passing "bound air" i.e. carbon dioxide CO 2 through an aqueous solution of lime Ca (OH) 2 calcium carbonate CaCO 3 is deposited on the bottom. Joseph Black used this experience to prove that carbon dioxide is released as a result of animal respiration.

CaO + H 2 O \u003d Ca (OH) 2

Ca(OH) 2 + CO 2 = CaCO 3 + H 2 O

Liquid carbon dioxide is a colorless, odorless liquid whose density varies greatly with temperature. It exists at room temperature only at a pressure of more than 5.85 MPa. The density of liquid carbon dioxide is 0.771 g/cm 3 (20°C). At temperatures below +11°C it is heavier than water, and above +11°C it is lighter.

The specific gravity of liquid carbon dioxide varies significantly with temperature, so the amount of carbon dioxide is determined and sold by weight. The solubility of water in liquid carbon dioxide in the temperature range of 5.8-22.9°C is not more than 0.05%.

Liquid carbon dioxide turns into a gas when heat is applied to it. Under normal conditions (20°C and 101.3 kPa) when 1 kg of liquid carbon dioxide evaporates, 509 liters of carbon dioxide are formed. With excessively rapid gas extraction, a decrease in pressure in the cylinder and insufficient heat supply, carbon dioxide cools, its evaporation rate decreases, and when the “triple point” is reached, it turns into dry ice, which clogs the hole in the reduction gear, and further gas extraction stops. When heated, dry ice directly turns into carbon dioxide, bypassing the liquid state. Much more heat is required to vaporize dry ice than to vaporize liquid carbon dioxide - so if dry ice has formed in a cylinder, it evaporates slowly.

Liquid carbon dioxide was first obtained in 1823. Humphrey Davy(Humphry Davy) and Michael Faraday(Michael Faraday).

Solid carbon dioxide is "dry ice", similar in appearance to snow and ice. The content of carbon dioxide obtained from dry ice briquettes is high - 99.93-99.99%. Moisture content in the range of 0.06-0.13%. Dry ice, being in the open air, evaporates intensively, therefore, containers are used for its storage and transportation. Carbon dioxide is produced from dry ice in special evaporators. Solid carbon dioxide (dry ice) supplied in accordance with GOST 12162.

Carbon dioxide is the most commonly used:

• to create a protective environment for metals;
• in the production of carbonated drinks;
• cooling, freezing and food storage;
• for fire extinguishing systems;
• for cleaning surfaces with dry ice.

The density of carbon dioxide is quite high, which makes it possible to protect the reaction space of the arc from contact with air gases and prevents nitriding at relatively low carbon dioxide flow rates in the jet. Carbon dioxide is, during the welding process, it interacts with the weld metal and has an oxidizing and carburizing effect on the metal of the weld pool.

Previously an obstacle to the use of carbon dioxide as a protective medium were at the seams. The pores were caused by boiling of the hardening metal of the weld pool from the release of carbon monoxide (CO) due to its insufficient deoxidation.

At high temperatures, carbon dioxide dissociates to form highly active free, monatomic oxygen:

Oxidation of the weld metal released during welding from carbon dioxide free is neutralized by the content of an additional amount of alloying elements with a high affinity for oxygen, most often silicon and manganese (in excess of the amount required to alloy the weld metal) or fluxes introduced into the welding zone (welding).

Both carbon dioxide and carbon monoxide are practically insoluble in solid and molten metal. Free active oxidizes the elements present in the weld pool, depending on their affinity for oxygen and concentration according to the equation:

Me + O = MeO

where Me is a metal (manganese, aluminum, etc.).

In addition, carbon dioxide itself reacts with these elements.

As a result of these reactions, when welding in carbon dioxide, a significant burnout of aluminum, titanium and zirconium is observed, and less intense - silicon, manganese, chromium, vanadium, etc.

The oxidation of impurities occurs especially vigorously at . This is due to the fact that when welding with a consumable electrode, the interaction of molten metal with gas occurs when the drop is at the end of the electrode and in the weld pool, and when welding with a non-consumable electrode, only in the bath. As is known, the interaction of gas with metal in the arc gap is much more intense due to the high temperature and the larger contact surface of the metal with the gas.

Due to the chemical activity of carbon dioxide with respect to tungsten, welding in this gas is carried out only with a consumable electrode.

Carbon dioxide is non-toxic and non-explosive. At concentrations of more than 5% (92 g/m 3 ) carbon dioxide has a harmful effect on human health, as it is heavier than air and can accumulate in poorly ventilated rooms near the floor. This reduces the volume fraction of oxygen in the air, which can cause the phenomenon of oxygen deficiency and suffocation. Premises where welding is carried out using carbon dioxide must be equipped with general-exchange supply and exhaust ventilation. The maximum allowable concentration of carbon dioxide in the air of the working area is 9.2 g/m 3 (0.5%).

Carbon dioxide is supplied by . To obtain high-quality seams, gaseous and liquefied carbon dioxide of the highest and first grades are used.

Carbon dioxide is transported and stored in steel cylinders or large-capacity tanks in a liquid state, followed by gasification at the plant, with a centralized supply of welding stations through ramps. 25 kg of liquid carbon dioxide is poured into a standard one with a water capacity of 40 liters, which at normal pressure occupies 67.5% of the volume of the cylinder and gives 12.5 m 3 of carbon dioxide upon evaporation. Air accumulates in the upper part of the cylinder along with gaseous carbon dioxide. Water, being heavier than liquid carbon dioxide, collects at the bottom of the cylinder.

To reduce the humidity of carbon dioxide, it is recommended to install the cylinder with the valve down and, after settling for 10 ... 15 minutes, carefully open the valve and release moisture from the cylinder. Before welding, it is necessary to release a small amount of gas from a normally installed cylinder in order to remove air trapped in the cylinder. Part of the moisture is retained in carbon dioxide in the form of water vapor, worsening when welding a seam.

When the gas is released from the cylinder, due to the effect of throttling and absorption of heat during the evaporation of liquid carbon dioxide, the gas is significantly cooled. With intensive gas extraction, the reducer can be blocked by frozen moisture contained in carbon dioxide, as well as dry ice. To avoid this, when taking carbon dioxide, a gas heater is installed in front of the reducer. The final removal of moisture after the reducer is carried out with a special desiccant filled with glass wool and calcium chloride, silica helium, copper sulphate or other moisture absorbers.

The carbon dioxide cylinder is painted black, with the inscription in yellow letters "CARBON DIOXIDE".