Oxygen
V03AN01
{ syn. }
99.762 %
0.038%
0.2 %
Risk
Oxygen (also called Oxygenium, from Greek ὀξύς, oxys ' " sharp, pointed, sour" and γεννάω, gen- ' " produce, give birth ," together " acid producer ") is a chemical element with the chemical symbol O and atomic number 8. in the periodic table it is in the sixth main group, so one of the chalcogens. Oxygen is the most abundant element with 48.9 % of the earth's crust and about 30 % by weight of iron by the second most abundant element in the earth as a whole.
Elemental oxygen occurs predominantly in the form of a covalent homodimer on, that is, a compound of two oxygen atoms and having the empirical formula O2, referred to as molecular oxygen, dioxygen or dioxygen. It is a colorless, odorless gas that is contained in the air to 20.942 %. It is involved in many combustion and corrosion processes. Almost all living organisms need oxygen to live ( usually give plants during photosynthesis but more oxygen than they consume from ). You see him mostly by breathing from the air or by absorption from water (dissolved oxygen ). In high concentrations, however, it is toxic to most living things.
The metastable, high-energy and reactive allotrope of three oxygen atoms (O3 ) is called ozone.
Atomic oxygen, ie, oxygen in the form of free, individual oxygen atoms, is stable before only under extreme conditions, for example in the vacuum of space or in hot stellar atmospheres. However, it has a significant importance as a reactive intermediate in many reactions of atmospheric chemistry.
- 4.1 Physical Properties 4.1.1 molecular orbitals
- 4.1.2 singlet oxygen
- 5.1 ozone
- 5.2 Other allotropes
- 6.1 Indirect temperature measurement via the δ18O signal
- 7.1 Medicine 7.1.1 Emergency Medicine
- 7.1.2 Long-term oxygen therapy
- 7.1.3 Cluster Headache
- 8.1 Problematic effects
- 8.2 Hyperoxidanionen in metabolism
- 9.1 Classical Analysis
- 9.2 Instrumental quantitative analysis
History
Carl Wilhelm Scheele in 1771 and Joseph Priestley in 1774 have independently discovered in connection with the study of combustion processes the oxygen.
From the Stone Age to the Middle Ages, the fire to man was a phenomenon which has been accepted as a gift from heaven. About the nature of the fire different ideas created by the natural philosophers of antiquity to the alchemists. The fire was seen as an essential ingredient of the four- element theory. In the 17th century the idea of a "light mysterious substance " was born. This phlogiston would escape from the burning fuel, heat was understood as a substance. The German - Swedish chemist Carl Wilhelm Scheele hired attempts. When heating pyrolusite ( manganese dioxide) or potassium permanganate with concentrated sulfuric acid ( vitriol ), he gave a colorless gas. This gas encouraged combustion and Scheele called it " fire air " or the origin " Vitriolluft ". He found that air consists of oxygen and this " foul air ". Completely independent, was two years later produced by heating mercuric oxide oxygen gas, the Englishman Joseph Priestley. The Briton published his findings in 1774, Scheele published his book Chemical Observations from the air and the fire but only 1777.
With the discovery of its meaning oxygen was not released during the combustion. The Frenchman Antoine Lavoisier found in his experiments that during combustion does not escape phlogiston, but oxygen is bound. By weighing it demonstrated that a substance after combustion was not easier, but harder. This was caused by the extra weight of the recorded oxygen during the combustion process. Initially, the oxygen has been accepted as a basic component for the formation of acids. Therefore, the designation Oxygenium ( acidifier ) was proposed in 1779 by Lavoisier for oxygen. In fact, most inorganic acids in the solution of non-metal oxides in water oxygen. The halogens, such as chlorine and bromine is therefore held for a long time oxides of unknown elements. Later it was recognized that hydrogen is responsible for the acid character. 1883 succeeded Karol Olszewski and Zygmunt Wroblewski Florenty first time in producing liquid oxygen.
Occurrence
Found on Earth
Oxygen is the most common and most widely used element in the earth. It occurs both in the Earth's atmosphere and in the lithosphere, the hydrosphere and the biosphere. Oxygen has a mass fraction of 50.5 % of the earth's crust (up to 16 km depth, including hydro-and atmosphere). In air, his mass fraction is 23.16 % ( volume fraction: 20.95% ), the water 88.8 % (on the sea water, however, only 86 % because there are larger amounts of dissolved nichtsauerstoffhaltiger salts, eg, sodium chloride).
Usually, oxygen is in its compounds, and in the ground in front. In the earth's crust, almost all minerals and rocks are so oxygenated next to water. Among the most important minerals containing oxygen are silicates such as feldspars, mica, and olivine, carbonates such as calcium carbonate in the limestone and oxides such as silica as quartz.
In elemental state, oxygen is a gas in the form of O2 in the atmosphere and dissolved in waters. The amount of relatively reactive elemental oxygen only remains constant over time, because oxygen producing plants replacement delivery as much as by aerobic breathing creatures as well as other combustion processes is consumed again. Without this biological cycle oxygen would occur only in compounds, ie elemental oxygen exists in a dynamic equilibrium. The development of the oxygen concentration in the atmosphere is described in the article Development of the earth's atmosphere. Add small amounts of the oxygen allotrope is O3 ozone present in the atmosphere.
Occurrence in space
In space, oxygen is the third most abundant element after hydrogen and helium. The mass fraction of oxygen in the solar system is about 0.8 % ( this corresponds to an (atomic ) number fraction of about 500 ppm).
Oxygen is not created in the primordial nucleosynthesis, but is produced in relatively large quantities in giant stars with helium burning. This is first formed from three helium nuclei 12C ( three- alpha ) process, which is then fused with a further helium nucleus to 16O. 18O is formed by fusion of 4He with a 14N nucleus. Even in so-called main sequence stars like the sun plays a role of oxygen in energy production. In the CNO cycle ( Bethe- Weizsäcker - cycle ) represents oxygen is an intermediate of the nuclear reaction in which proton capture by a 12C nucleus, which acts as a catalyst, a 4He nucleus ( alpha particle ) arises. In extremely heavy stars occurs in the late phase of their development to the oxygen burning, in which the oxygen is used as nuclear fuel for reactions that lead to the construction of yet heavier nuclei.
Most white dwarfs, which represent the final state of 97 % of all stars in the prior theory, exist side by helium and carbon to a large extent of oxygen.
Production and representation
Technically oxygen is today almost exclusively obtained by rectification of air. The process was first developed in 1902 by Carl von Linde ( Linde process ) and designed by Georges Claude economically viable. Small amounts arising as a by-product of hydrogen production by electrolysis of water.
For oxygen recovery after Claude process air by means of compressors of 5-6 bar is compressed, cooled and then freed by first filter of carbon dioxide, humidity, and other gases. The compressed air is cooled by passing the gases flowing out of the process at a temperature near the boiling point. Then it is expanded in turbines. A part of the energy used for the compression can be recovered. This is the procedure - in contrast to the Linde process, in which no energy is recovered - much more economical.
The actual separation of nitrogen and oxygen by distillation in two distillation columns at different pressures. The distillation is carried out in counter-current principle, i.e. the heat of condensation of evaporated gas flows upward, condensed liquid drips down. Because oxygen has a higher boiling point than nitrogen, it condenses readily and collects at the bottom as nitrogen on top of the column. The separation takes place initially at 5-6 bar in the so-called medium pressure column. The resulting oxygen-enriched liquid is then further separated in the low pressure column ( pressure about 0.5 bar). By the liquid oxygen of the low pressure column, gaseous nitrogen from the high pressure column is passed. It liquefies this and heated with the votes condensation heat the liquid. The more volatile nitrogen is discharged and preferably remains a purified liquid oxygen. It still contains the noble gases krypton and xenon, which are separated in a separate column.
To produce smaller amounts of oxygen, oxygen from the air by adsorption of other gases can be separated. These air passes through molecular sieves. In this case, nitrogen and carbon dioxide are adsorbed and only oxygen and argon pass through.
One older method is based on chemical reactions barium oxide process. It is uneconomical due to the high energy costs. For barium oxide is heated in an air stream at 500 ° C to form barium peroxide forms. When heated to 700 ° C, the previously recorded oxygen is released again by thermolysis. Before development of the Linde process, this method was the only way to pure oxygen present.
Some oxygen-rich inorganic compounds such as potassium permanganate, potassium nitrate ( saltpeter ), potassium chlorate and potassium enter upon heating or reaction with reducing agents from oxygen.
A further possibility of producing oxygen in the laboratory, is the decomposition of hydrogen peroxide on platinum-plated nickel foil.
Pure oxygen can be obtained by electrolysis of 30 % potassium hydroxide to nickel electrodes. In this case, hydrogen and oxygen are evolved separately.
Properties
Physical Properties
Molecular oxygen is a colorless, odorless and tasteless gas which condensed at -183 ° C to a colorless liquid. In thick layers of gaseous and liquid oxygen is a blue color. Below -218.75 ° C solidifies oxygen to blue crystals. The solids are paramagnetic O2 molecules with an O- O distance of 121 pm ( double bond). The element is determined in several modifications. Between -218.75 -229.35 ° C and oxygen in the cubic γ - modification and -229.35 to -249.26 ° C before in a rhombohedral β - modification. Below -249.26 ° C is finally the monoclinic α - modification most stable. It is - in contrast to other non-metals - paramagnetic and has diradical character. The triple point is 54,36 C ( -218.79 ° C) and 0.1480 kPa. The critical point is at a pressure of 50.4 and a temperature of 154.7 K ( -118.4 ° C). The critical density is 0.436 g/cm3.
Oxygen is poorly soluble in water. The solubility is dependent on the pressure and the temperature. It increases with decreasing temperature and increasing pressure. At 0 ° C and an oxygen partial pressure of the air at 212 hPa dissolve in pure water 14.16 mg / l oxygen.
In the oxygen - gas discharge Spectrum, the molecular orbitals of oxygen are stimulated to emit light. The operating conditions are a pressure of 5-10 mbar, a high voltage of 1.8 kV, a current of 18 mA and a frequency of 35 kHz. The recombination of the ionized gas molecules, the characteristic color spectrum is emitted. Here is to a small extent, due reversibly formed by the supply of energy ozone.
Molecular orbitals
The binding and the properties of the oxygen molecule can be explained very well with the molecular orbital model. In this case, the s and p atomic orbitals of the individual atoms are assembled into bonding and antibonding molecular orbitals. The 1s and 2s orbitals of the oxygen atoms are * each to σs and σs - bonding and antibonding molecular orbitals. Since these orbitals are completely filled with electrons, they do not contribute to binding. From the 2p orbitals are a total of six molecular orbitals with different energy level. These are the binding? P, πx and πy, and the corresponding antibonding? P * -, * πx - πy and * molecular orbitals.
The π orbitals have this same energy. If electrons distributed in the molecular orbitals, it comes to following breakdown of the eight p electrons: six fill the binding and two in the antibonding π * orbitals; the bond order is thus (6-2 ) / 2 = 2 These two valence electrons determine as the properties of the O2 molecule. Oxygen has three allowed and energetically accessible quantum mechanical states for the distribution of these electrons.
In the ground state the spins of the two valence electrons of the Hund's rule are arranged parallel obeying. It is a triplet state with the term symbol 3Σg. It is the state with the lowest energy. Through the two unpaired electrons, the two π * orbitals are only half occupied. This caused some characteristic properties, such as the diradical character and the paramagnetism of the oxygen molecule.
Despite the formal bond order " two " can not specify the correct type valence-bond for O2.
Singlet Oxygen
Oxygen has two different excited states, both of which have a significantly greater energy than the ground state. In both states, the spins of the electrons are aligned antiparallel against the dog 's rule. The stable excited oxygen is named after the quantum mechanical designation for this condition also singlet oxygen ( 1O2 ). The two singlet states differ in whether the two electrons in a ( term symbol: 1? G ) or both are π * orbitals (1? G term symbol ). The 1? G - state is energetically unfavorable and changing very fast in the 1? G - state about. The 1? G - state is diamagnetic, but the energetically more stable one? G - state is due to the presence orbital moment (which is the projection of the orbital angular momentum on the internuclear connection axis corresponding quantum number - symbolized by Σ, Π, Δ, etc. - has one state? g - value of ± 2) paramagnetism comparable strength as that of triplet oxygen. Instead of using the Greek letter, the above -mentioned oxygen species as O ( 3P) will be quoted O (1D ) and O ( 1S).
The formation of singlet oxygen is possible in several ways: either photochemically from triplet oxygen, and chemically from other oxygen compounds. A direct extraction from triplet oxygen by irradiating with electromagnetic radiation (e.g., light) is, however, excluded from the quantum-mechanical reasons. The reason for this is that, should not change by the quantum mechanics of the overall system in a spinning irradiation bosons to which a part of the photon. One way to circumvent this prohibition, is the simultaneous irradiation with photons and collision of two molecules. By this unlikely event, which is likely to occur in the liquid phase, the blue color of the liquid oxygen is formed ( absorption in the red spectral range ). Also with the help of suitable dyes such as methylene blue or eosin, can be represented by the photochemical route singlet oxygen. Chemically it is derived from peroxides. In the reaction of hydrogen peroxide with sodium hypochlorite initially formed unstable Peroxohypochlorige acid, which decomposes rapidly in hydrogen chloride or sodium chloride, and singlet oxygen. Experimentally, one can also introduce chlorine in an alkaline hydrogen peroxide solution, then being first formed hypochlorite, which then reacts further. The singlet oxygen reacts rapidly with emission in the red region at 633.4 nm and 703.2 nm to triplet oxygen.
This form of oxygen is a strong and selective oxidizing agent and is widely used in organic chemistry. He is thus in contrast to normal oxygen with 1,3-dienes in a [ 4 2 ] cycloaddition to peroxides. With alkenes and alkynes singlet oxygen reacts in a [2 2 ] cycloaddition.
Chemical Properties
Oxygen reacts with most elements of the periodic table directly. There are some exceptions, especially among the non-metals and precious metals. With nitrogen reactions are possible only under special conditions, such as lightning, but also in the internal combustion engine. Fluorine is only at low temperatures at which electrical discharges compound Disauerstoffdifluorid ( O2F2 ). The noble metal is gold, the halogens chlorine, bromine and iodine, as well as the noble gases do not react directly with oxygen. Some other precious metals such as platinum and silver react poorly with oxygen.
Elemental, gaseous oxygen is relatively unreactive, many reactions do not or only slowly take place under normal conditions. The reason for this is that oxygen is metastable, and the reactions are kinetically inhibited with other substances. Either a high activation energy or very reactive free radicals are needed for reaction. This barrier can be ( for example, platinum ) can be exceeded by increasing the temperature, light, or catalysts. In addition, for many metals, the reaction is prevented in that the material is coated with a thin metal oxide layer and is passivated thereby. In some reactions, such as the oxyhydrogen reaction ranging from a few radicals for reaction, as these react further by a chain reaction mechanism. Significantly more oxidizing than gaseous oxygen is liquid oxygen despite the low temperatures. In this, the reactive singlet oxygen readily forms. Even in the presence of water or water vapor run many oxidations with oxygen more easily.
Reaction with oxygen are almost always redox reactions in which oxygen is usually absorbs two electrons, and is reduced to the oxide. The element is therefore one of the oxidants. Often these reactions are due to the large vacant binding or lattice energy under strong heat. There is also explosively extending reactions such as the hydrogen-oxygen reaction or dust explosions from finely divided substances in air or pure oxygen.
Allotropes
In addition to the dioxygen O2 described in this article makes several allotropes of oxygen which can be distinguished according to the number of oxygen atoms. The most important allotrope is ozone O3, along with the rare Tetra allotropes of oxygen (O4 ) and Oktasauerstoff ( O8 ) are known.
Ozone
Ozone (O3) is a blue characteristic smell gas, which consists of three oxygen atoms. It is unstable, highly reactive, and a strong oxidizing agent. It is formed from molecular oxygen, and oxygen atoms, but also for example by the reaction of nitrogen dioxide with oxygen under UV radiation.
Due to its high reactivity, it is close to the ground would be detrimental to human health - in the ozone layer of the atmosphere, however, the ozone plays a major role in the absorption of striking the ground UV radiation.
Other allotropes
A high-pressure phase of the oxygen arises at pressures greater 10 GPa as a red solid. According to crystallographic studies is assumed that it concerns Oktasauerstoff O8 -rings. In addition, Tetra oxygen exists as a very rare and unstable allotrope of the oxygen. It could be proved in 2001 in the mass spectrometer. In low concentration it comes ago in liquid oxygen.
Isotopes
The most common stable oxygen isotope 16O is ( 99.76 %), next to it still comes 18O (0.20 %) and 17O (0.037 %) before. In addition to the stable oxygen isotopes are still a total of 13 unstable, radioactive nuclides from 12O to 28O known which are artificially produced. Their half lives often are only milliseconds to seconds, with two minutes 15O case has the longest half-life and is commonly used in positron emission tomography.
The only stable isotope of the rare 17O has a nuclear spin of 5/2 and thus can be used for NMR studies. The other stable isotopes possess the nuclear spin 0 and are thus NMR inactive.
Indirect temperature measurement via the δ18O signal
Water molecules with the lighter by 12% 16O evaporate faster. Therefore ice must come with a higher relative proportion of 18O from warmer times when 18O contribute only increases in the strong evaporation of warmer periods for cloud formation. The higher the global temperature is, the more can penetrate into the polar regions laden with heavy oxygen isotopes clouds, without as rain before.
In colder periods, more 18O is in marine sediments. Sea ice consists mainly of the lighter water molecules from 16O. If it comes in a cold phase to a strong formation of sea ice, increased remains seawater 18O, which in sedimentary layers of this time is increasingly detectable by the permanent storage of oxygen in the calcite shells of marine animals (calcium carbonate). Also, there are regional differences in the 18O enrichment in organisms by nature of their drinking water source.
By an isotope study of ice cores or sediment samples and the determination of 18O-/16O-Verhältnisses with the help of a mass spectrometer can be information about the average temperature and thus the warming and cooling down in former times to win. In addition, the age of the core sample can be accurately determined by determining the number of oscillations between warm ( summer) and cold ( winter).
Use
Oxygen is used for industrial combustion, oxidation and heating processes, in medicine and in aerospace.
Medicine
Oxygen for use in human medicine under applicable law, rules of strict control. The bottled in bottles marked white medical oxygen is considered in Germany as a finished product in terms of the Medicines Act ( AMG).
Caution is advised in the administration of oxygen, when patients with chronic lung disease (see COPD) with elevated CO2 partial pressure. In them can lead to oxygen to CO2 narcosis with respiratory arrest the sudden " glut ".
Emergency Medicine
Injuries and many diseases of the lung and heart disease and some particular states of shock can lead to a lack of oxygen (hypoxia) in the arteries (arteries) and in the tissues of vital organs. For this reason, patients administered in the emergency and intensive care medicine very often additional oxygen. Wherein the self- breathing patient, ambient air is enriched by means of various probes, and oxygen masks, for mechanically ventilated patients, the oxygen is mixed in the ventilator. The effect of the oxygen concentration in the blood can be measured by means of pulse oximetry, or based on blood gas analysis.
Long-term oxygen therapy
In diseases with a severe chronic oxygen deficiency in the blood of both the quality of life and the survival time can be improved by a long-term and daily supply of oxygen for several hours ( long-term oxygen therapy). The pure oxygen can result in breathing problems due to repression of carbon dioxide from the blood vessels as well as the undesirable increase in brain activity in the hypothalamus, insula, and hippocampus. These negative consequences can be avoided by the addition of carbon dioxide.
Cluster Headache
According to the recommendations of the World Health Organization, the inhalation of oxygen for treatment of cluster headache attacks is suitable. The use of highly concentrated oxygen using special mask systems relieves the discomfort usually effective within minutes.
Technology
Oxygen is mainly used industrially in metallurgy for the production of pig iron and steel, as well as for copper refining. Pure oxygen or oxygen enriched air is used here on the one hand to achieve high temperatures, on the other hand, for refining of the raw steel, i.e. for removing undesirable impurities from carbon, silicon, manganese and phosphorus, which is oxidized and separated. Pure oxygen has the advantage that no nitrogen is introduced into the melt in comparison with air. Nitrogen has a detrimental effect on the mechanical properties of steel (see also Thomas method). Oxygen in chemical processes is generally used for the oxidation of various raw materials, as in the olefin oxidation of ethylene to ethylene oxide and the partial (partial) oxidation of heavy oil and coal. Also, oxygen required for the production of hydrogen and synthesis gas, and the manufacture of sulfuric and nitric acid. Further prepared by oxidation with oxygen major products are acetylene, acetaldehyde, acetic acid, vinyl acetate and chlorine.
Various fuel gases (propane, hydrogen, acetylene, and others) achieve only by mixing with oxygen sufficiently hot and soot-free flame for welding and brazing or melting and Formbarmachen of glass. After heating and igniting the cutting of concrete with a (self- combustive ) oxygen lance or flame cutting of iron is done alone by a sharp jet of oxygen.
Oxygen is also used for the preparation of ozone as an oxidant in the fuel cell and in the semiconductor technology. In the missile art, liquid oxygen is used as oxidizing agent, and abbreviated with LOX (liquid oxygen ).
In environmental effluents are released by the introduction of oxygen gas faster by bacteria of organic pollutants and toxins.
For the food industry oxygen as a food additive is permitted as an e 948 and is - like in addition to nitrogen, carbon dioxide and nitrous oxide as a propellant, packaging gases, gas for whipping cream ( whipped cream ) be used.
Wellness
In the wellness and food industry is sometimes advertised for products that are enriched with oxygen. For example, bottled water is sold, which is to have an increased oxygen content. A positive effect on health and well -being is not to be expected, since oxygen dissolves only in small quantity in water and is used in many orders of magnitude more - namely, with every breath - absorbed through the lungs than via the stomach.
Biological Significance
Oxygen is in the nature in a constant cycle. He is constantly by autotrophic organisms such as cyanobacteria ( obsolete: blue-green algae ), algae and plants released by photolysis of water in oxygenic photosynthesis. It is a final product of this biochemical reaction, and is discharged to the environment. Cyanobacteria were probably the first organisms, who enriched molecular oxygen as a waste product in the atmosphere. Previously existed a virtually oxygen-free anaerobic atmosphere of the earth.
Most aerobic organisms, including most eukaryotes, including humans and plants, and many bacteria, these require oxygen to live. Eukaryotes need it to obtain energy by oxidation in the mitochondria. The oxygen is thereby reduced in the respiratory chain to form water again. The oxygenation of metabolites by enzymes ( oxygenases ) is often used in the degradation of substances; the reaction requires oxygen and takes place in all aerobic organisms.
Since oxygen and some of its compounds are highly reactive and can damage cell structures, organisms have protective enzymes such as catalase and peroxidase. For organisms lacking these enzymes, oxygen is toxic. When removing the oxygen, reactive oxygen species such as free radicals, which can also destroy biological molecules. If they are not caught quickly enough, creates so-called oxidative stress, which is responsible for the aging process.
In the phagocytes (scavenger cells ) of the immune system, these reactive oxygen species can serve (hydrogen peroxide and Hyperoxidionen ) next enzymes to destroy captured pathogens.
Problematic effects
If pure oxygen or air is inhaled with a higher oxygen percentage over time, it can lead to poisoning of the lungs, are the so-called Lorrain Smith effect. The air sacs ( alveoli ) are prevented by swelling in their normal function.
The Paul Bert effect refers to an oxygen toxicity of the central nervous system. This can occur at high pressure breathing any oxygen-nitrogen mixtures, however, the risk increases with increasing the oxygen content and the total pressure. For oxygen partial pressures above 1.6 bar occurs within a relatively short time to poisoning. This plays a role, for example, when diving, as it limits the maximum depth pending the oxygen partial pressure.
In the space of pure oxygen, for example, in space suits breathed, but under greatly reduced pressure in order to minimize health problems, and because of the space suit under normal pressure would be too stiff.
Hyperoxidanionen in metabolism
Hyperoxidanionen (old name: superoxide anions ) are singly negatively charged and radical oxygen ions ( O2 - ), caused by electron transfer to molecular oxygen. These are extremely reactive. Sometimes they are formed as a by- product of metabolism (metabolism ), such as by secondary reactions in some oxidases ( xanthine oxidase). Hyperoxidanionen also arise during photosynthesis complex I and as a byproduct of the respiratory chain ( mitochondrial respiration ). Xenobiotics and cytostatic antibiotics, thereby promoting their development. When inflammation occurrence Hyperoxidanionen is released into the extracellular milieu by a membrane-bound NADPH-dependent oxidase. They lead to oxidative stress. For example, it is the breakdown of fatty acids into peroxisomes of transferring electrons from FADH2 to molecular oxygen. The resulting Hyperoxidanionen can react further hydrogen peroxide to the cell poison. At the expiration of the respiratory chain, this radical oxygen species produced in small quantities. There are suspicions that the genetic lesions that produce such oxygen species are involved in the aging process. It is therefore for the organism essential to reduce these Hyperoxidanionen quickly. This is done by means of the superoxide dismutase.
Analysis
Classical analysis
Dissolved oxygen oxidizes divalent manganese in higher oxidation states. This again is completely reduced by the method of Winkler iodide. The molar amount of iodine thus formed is in a stoichiometric ratio of 1:2 to the amount of substance of the originally dissolved oxygen and can be iodometric back-titrated with thiosulfate.
Instrumental quantitative analysis
The oxygen sensor used for controlling combustion of gasoline engines, measures the oxygen content in the automobile exhaust gas in relation to the O2 content in the ambient air. To the waste gas stream is passed through a yttrium -doped Zirconiumdioxidröhrchen which is provided inside and outside with platinum electrodes. In this case, the outer electrode is in contact with the ambient air. Different O2 partial pressures at the electrodes lead to an electrical potential difference is measured. The advantages of this measurement are located in the low detection limit of a few ppm, and the wide operating temperature range (300 ° C to 1500 ° C).
The Clark electrode is an amperometric sensor for electro-chemical determination of dissolved gaseous oxygen. Platinum and Ag / AgCl reference electrode are located in an electrolyte system, which is separated by a gas-permeable teflon membrane of the sample solution. Dissolved oxygen can diffuse through the membrane into the electrolyte solution and is reduced in a cathodic potential range of -600 mV to -800 mV. The measured current is proportional to the oxygen concentration in the sample solution.
In the optical methods one uses the fact that oxygen is able to quench the fluorescence of the excited molecules. On the basis of fluorescent transition metal complexes known as optrodes have been developed which determine the oxygen content of the fluorescence quenching probe molecules. Probe molecules are often used as the metal - ligand complexes for use. As metal ions, Ru ( II), Ir (II ), Pt (II) and Pd ( II) have proven different as ligands dipyridines, Phenanthroline and ( fluorinated ) porphyrins. The probes are embedded in polymer matrices. The excitation is usually with LEDs or laser diodes. A distinction is made between point measurements, for example by means of optical fibers and optical fiber imaging measurement method using a planar sensor films. Optrodes with detection limits of 0.02 hPa ( O2) was achieved, which corresponds to a concentration of 1 ppb.
Compounds
Oxygen forms compounds with almost all elements - except the noble gases helium, neon, argon and krypton. Since oxygen is very electronegative, it happens in almost all its compounds in the oxidation states II, only in peroxides -I. These ions are also referred to as a closed- shell ions. Peroxides are usually unstable and easily go into oxides over.
Positive oxidation states has oxygen only in connections with the more electronegative fluorine element, with which it forms compounds with the oxidation state I ( Disauerstoffdifluorid O2F2 ) and II ( oxygen difluoride OF2 ). Since them the negative polarization when fluorine is present, they are not referred to as oxides, but as fluorides.
In addition to the compound oxide of oxygen still occurs in ionic compounds and radicals as peroxide ( O22 - ), superoxide (O2 - (oxidation state of -1 / 2)) and Ozonidanion ( O3 (oxidation state of -1 / 3) ) and as Dioxygenylkation ( O2 ) on.
Oxygen forms depending on the binding partner of both ionically compounds covalently constructed.
Inorganic oxygen compounds
The inorganic oxygen compounds include the compounds of oxygen and metals, semi- metals, non- metals, such as hydrogen, carbon, nitrogen, sulfur and the halogens. They are among the most important compounds in general.
Oxides
Most oxygen compounds are oxides. In them the oxygen occurs ionically or covalently bound, in the oxidation state - II. Many naturally occurring salts, which are often important sources for the production of metals are oxides.
With the metals forming oxygen in low oxidation states and ionically constructed generally basic oxides.
With increasing oxidation level of the oxides have increasingly amphoteric (zinc (II ) oxide, aluminum (III ) oxide ), and finally acidic character ( chromium ( VI) oxide ).
With non-metals oxygen forms only covalent oxides. The oxides of non-metals in low oxidation states react mostly neutral ( nitrous oxide ), with increasing oxidation state increasingly acidic.
Among the oxygen compounds of non-metals with hydrogen play a special role. Oxygen with hydrogen forming two compounds. In the first place, the water has to be mentioned, without which there would be no life on Earth. The second compound is hydrogen peroxide (H2O2), a thermodynamically unstable compound which is used as an oxidizing and bleaching agent.
Although most of the oxygen-containing carbon compounds are classified in the field of organic chemistry, there are some important exceptions. The simple oxides of carbon monoxide (CO) and carbon dioxide (CO2 ) and carbonic acid and salts thereof, carbonates, are considered to be inorganic compounds.
In a salt-like compound are lower amounts of oxide ions known as to be expected from the stoichiometry and valence of oxygen, it is called suboxides. In these element - element bonds occur, the formal oxidation state of the element is less than 1. Elements forming suboxides, are the alkali metals rubidium and cesium, and also boron, or carbon.
Oxygen compounds having oxygen -oxygen bonds
In particular with alkali metals forming oxygen compounds having oxygen -oxygen bonds. These include the peroxides, the Hyper oxides and ozonides. Peroxides such as hydrogen peroxide have the O22 - ion and a formal oxidation state of the oxygen of -1. Due to the easy cleavage of oxygen -oxygen bond to form easily radicals, which act on organic bleaching agents and are accordingly used as a bleaching agent. Also known are organic peroxides.
In the radical hyperoxides dioxide (1 -) is anion O2 before, the formal oxidation state is - ½ for each oxygen atom. Superoxide ions are formed in metabolism and are among the reactive oxygen species, salt-like hyper- oxides are known only from the alkali metals other than lithium. Ozonides derived from ozone and accordingly have the O3 - anion. Saltlike ozonides are like Hyperoxide known by all alkali metals except lithium, plus there is also organic ozonides incurred to alkenes by the addition of ozone.
Hydroxides
Another large group of oxygen compounds are the hydroxides, with the participation of hydrogen dar. These are predominantly ionic compounds, which is the hydroxide ion in common. To the hydroxides of alkali metals such as sodium hydroxide ( NaOH) or potassium hydroxide ( KOH), they are generally not very soluble in water.
Oxygen acids
In the reaction of non-metal oxides, and metal oxides in high oxidation states of metals with water to form the so-called oxy-acids, which are responsible for the naming of the oxygen.
The strongest inorganic oxygen acids are derived from the non-metals nitrogen ( nitric acid ) and sulfur ( sulfuric acid ) and the halogens from (halogen oxyacids ). The rule is that the acid strength ( pKa ) increases with increasing number of oxygen atoms:
Organic oxygen compounds
Oxygen - in addition to carbon, hydrogen and nitrogen - one of the most important elements of organic chemistry. It forms a number of important functional groups which both carbon-oxygen single bonds, and - containing carbon-oxygen double bonds, - having the carbonyl group.
The simplest organic compounds containing oxygen, part of methanal ( H2CO ), which are formally derived from carbon dioxide ( CO2) differs only in that instead of the second oxygen atom, two hydrogen atoms are bonded to the carbon. Important for the classification of organic chemistry, however, is that methanal from the organic alcohol methanol ( CH3OH ) is derived, which in turn is a derivative of the simplest alkane, methane ( CH4).
The main classes of compounds:
- Alcohols: From the carbon-oxygen single bond some important classes of compounds derived. The first are the alcohols, in which a carbon and a hydrogen atom ( hydroxy group ) are bonded to the oxygen atom. The most famous and at the same time simplest members of this group are methanol CH3OH and ethanol C2H5OH.
- Phenols: These molecules contain at least one hydroxy group, which is connected to an aromatic ring.
- Ether: Are the oxygen atom bonded two carbon atoms, the group is called the ether group and the class of corresponding ether. A well-known representatives of ether is the major solvent diethyl ether ( C2H5 ) 2O.
- Aldehydes The carbonyl group is a very versatile functional group that is present in many classes of materials. These differ in the additional groups are bonded to the carbon atom. The aldehyde R -CHO, in which the carbon atom of the carbonyl group is a hydrogen atom is bonded, is present in aldehydes, such as acetaldehyde.
- Ketones: They contain the keto group, R -CO -R, in which two hydrocarbon groups are bonded to the carbon atom of the carbonyl group. One example is acetone.
- Carboxylic acids: the carboxyl or carboxylic acid group of the R-COOH carboxylic acid having a carbon atom at both a carbonyl and a hydroxyl group. The most important carboxylic acids are formic and acetic acid.
- Esters: Similarly, the carboxy group, the ester group R-CO -O- R 'is established. Her the proton of the carboxylic acid is replaced with another hydrocarbon group. The esters formed from carboxylic acids and alcohols are named accordingly. An example is ethyl acetate from acetic acid and ethanol ( ethyl alcohol).
- Carboxamides: In them, the hydroxyl group of the carboxyl group is replaced by an amino group.
Another important group of organic oxygen- compounds are the carbohydrates or saccharides. Chemically, this polyhydroxycarbonyl compounds ( hydroxy aldehydes or hydroxy ketones ). Thus combining the characteristics with those of the alcohols, aldehydes and ketones.
In addition, there are also a number of additional compounds having functional groups, in which the oxygen to a further hetero atom such as nitrogen, sulfur or phosphorus, such as organic phosphates (such as ATP or within the DNA molecules) is attached.