nitrogen

from Wikipedia, the free encyclopedia
properties
General
Name , symbol , atomic number Nitrogen, N, 7
Element category Non-metals
Group , period , block 15 , 2 , p
Appearance Colorless gas
CAS number 7727-37-9
EC number 231-783-9
ECHA InfoCard 100,028,895
ATC code

V03 AN04

Mass fraction of the earth's envelope 0.03%
Atomic
Atomic mass 14.0067 (14.00643 - 14.00728) and
Atomic radius (calculated) 65 (56) pm
Covalent radius 71 pm
Van der Waals radius 155 pm
Electron configuration [ He ] 2 s 2 2 p 3
1. Ionization energy 14th.53413 (4) eV ≈ 1 402.33 kJ / mol
2. Ionization energy 29.60125 (9) eV ≈ 2 856.09 kJ / mol
3. Ionization energy 47.4453 (25) eV ≈ 4 577.77 kJ / mol
4. Ionization energy 77.4735 (4) eV ≈ 7 475.05 kJ / mol
5. Ionization energy 97.8901 (4) eV ≈ 9 444.96 kJ / mol
Physically
Physical state gaseous (N 2 )
Modifications 1
Crystal structure hexagonal
density 1.250 kg / m 3 at 273.15 K.
magnetism diamagnetic ( Χ m = −6.7 10 −9 )
Melting point 63.05 K (−210.1 ° C)
boiling point 77.15 K (−196 ° C)
Heat of evaporation 5.58 kJ / mol
Heat of fusion 0.36 kJ mol −1
Speed ​​of sound 333.6 m s −1 at 298.15 K.
Specific heat capacity 1040 J kg −1 K −1 at 298 K
Thermal conductivity 0.02583 W m −1 K −1
Chemically
Oxidation states −3, −2, −1, 0, 1, 2, 3 , 4, 5
Electronegativity 3.04 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
13 N. {syn.} 9.965 min ε 2.220 13 C
14 N. 99.634% Stable
15 N. 0.366% Stable
16 N. {syn.} 7.13 s β - 10.419 16 O
For other isotopes see list of isotopes
NMR properties
  Spin
quantum
number I 		γ in
rad · T −1 · s −1 		E r  ( 1 H) f L at
B = 4.7 T
in MHz
14 N. 1 +1.9338 x 10 7 1.007 · 10 −3 07.224 (2.348 T)
15 N. 1/2 −2.7126 10 7 1.043 · 10 −3 10.133 (2.348 T)
safety instructions
GHS labeling of hazardous substances
04 - gas bottle

Caution

H and P phrases H: 280
P: 403
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Nitrogen ( Latin Nitrogenium ) is a chemical element with the atomic number 7 and the element symbol N. In the periodic table it is in the fifth main group or the 15th  IUPAC group or nitrogen group and the second period . The symbol N is derived from the Latin name nitrogenium (from ancient Greek νίτρον nitron "lye salt" and γένος genos "origin"). The German term nitrogen is a reminder that molecular nitrogen extinguishes (“suffocates”) flames by displacing oxygen and that living beings suffocate in pure nitrogen because there is no oxygen. Older names are Azot, Azotum, Stickgas and Zoogenium.

Elementary nitrogen occurs only in the form of diatomic molecules (molecular nitrogen, also dinitrogen, empirical formula N 2 ); with 78% it is the main component of the air . In the earth's crust is inorganically bound nitrogen rare; it is only significant in saltpeter deposits .

In the course of evolution, a nitrogen cycle has developed in the ecosystem : As a component of proteins and many other natural substances , nitrogen is essential for living beings that bind it organically and make it bioavailable in an energy-intensive process ( nitrogen fixation ). This happens, for example, enzymatically on an iron-sulfur cluster , which is a cofactor of the enzyme nitrogenase .

history

Carl Wilhelm Scheele

Naturally occurring chemical compounds of nitrogen, such as nitrates and ammonium salts , were already used in antiquity and by alchemists . Both types of compounds can be produced from excrement in addition to their occurrence as minerals . For example, the Egyptians made ammonium chloride (salmiac) from camel dung and saltpetre was extracted from the floor of stables for a long time. In 1771, Carl Wilhelm Scheele detected nitrogen as a component of the air and was confirmed by Daniel Rutherford in 1772 . Pure ammonia was first presented by Joseph Priestley in 1774 . Until the beginning of the 20th century, saltpetre was the only major source of nitrogen compounds. With the introduction of the Frank Caro process ( calcium cyanamide generation according to Adolph Frank and Nikodem Caro ), atmospheric nitrogen was made usable for the first time. The Birkeland-Eyde process , after Kristian Birkeland and Sam Eyde , was used to obtain nitric acid . These processes were soon replaced by the Haber-Bosch process , according to Fritz Haber and Carl Bosch , for the synthesis of ammonia from atmospheric nitrogen and hydrogen, and by the catalytic Ostwald process according to Wilhelm Ostwald for converting ammonia into nitric acid.

Natural occurrence and cycle of nitrogen

Nitrogen cycle

As early as the 19th century it was recognized that a large part of plant matter contains nitrogen and is an important component of all living things. It is the essential element of proteins and proteids (proteins) and DNA . Nitrogen is therefore also a component of all enzymes that control plant, animal and human metabolism. Nitrogen is essential for life on earth.

Nitrogen in the air

The earth's atmosphere consists of 78.09% by volume (75.53% by weight) of molecular nitrogen. Only a small number of microorganisms can use it, incorporate it into their body substance or give it to plants. As far as is known, plants cannot use the gaseous nitrogen in the air directly. The conversion into a form that can be used by the plants occurs through:

  • Nodule bacteria : These bacteria penetrate the roots of what are known as legumes . They feed onthe plant's assimilates . In exchange for this, they supply the host plant with ammonium . This wasreduced from the nitrogen in the air usinga special enzyme, nitrogenase , with a high expenditure of energy. This community is a symbiosis . It enables legumes to colonize even poor locations, which is why humans use these plants, especially in organic farming, to enrich the soil with nitrogen. Here legumes are the main source of nitrogen.
  • Free living microorganisms : The non-symbiotic nitrogen fixation is based on the ability of some free living microorganisms (for example Azotobacter and Cyanobacteria ) to use atmospheric nitrogen to build up the body's own protein . In the case of agricultural use, the magnitude of the binding of atmospheric nitrogen by free-living microorganisms is assumed to be 5–15 kg / ha per year.
  • Electrical discharge during thunderstorms : In areas with high levels of precipitation, 20–25 kg N per hectare can be added to the soil through rain. This happens with electrical discharges, when oxygen and nitrogen combine to form nitrogen oxides. Ultimately, these oxides react with the rainwater to form nitric acid and nitrates can form in the soil.
  • Ammonia synthesis : At the beginning of the 20th century, the chemists Fritz Haber and Carl Bosch developed a process with which ammonia can be produced from atmospheric nitrogen and hydrogen . The use of nitrogen in the atmosphere made possible by the Haber-Bosch process has contributed to a significant increase in the yield of agricultural productions. The food security could be so much improved. The plant builds up vegetable protein from the ammonia it has absorbed, which humans and animals use as food and to build up their own body protein. In the human and animal organism, the protein is largely broken down again and excreted with the faeces and urine. At the present time, every third nitrogen atom in the biosphere has already been processed by the fertilizer industry.
  • Car exhaust fumes: Burning fossil fuels ( gasoline , diesel ) releases nitrogen compounds from car traffic. The combustion process produces nitrogen oxides (NO x , primarily nitrogen dioxide NO 2 , but also nitrogen monoxide NO and other NO x compounds). In the past, these were released directly into the environment. Nowadays, cars have catalytic converters that reduce these compounds: NO x is reduced to ammonia in the catalytic converter. This is converted into ammonium in the presence of water (ammonia / ammonium equilibrium in acidified solution: NH 3 + H 3 O + ⇔ NH 4 + + H 2 O). Both the oxidized and the reduced nitrogen compounds are transported through the air and contribute significantly to the eutrophication of neighboring ecosystems .

Nitrogen in the soil

In the arable top ( A horizon ), more than 95% of the total nitrogen is usually present as organically bound nitrogen in living root matter, dead plant matter, humus matter and soil organisms. The remainder of less than 5% is inorganic nitrogen in the form of ammonium or nitrate and a very small amount in the form of nitrite . This mineral nitrogen content is determined in the spring before fertilization using the N min method. The total nitrogen content of the soils is strongly dependent on their carbon content. It is influenced by the climate , vegetation , soil type , terrain and measures taken by the farmer, such as tillage .

Nitrogen in plants

Tasks in the plant

Nitrogen is built into photosynthesis products to produce proteins, among other things, and thus promotes growth. Nitrogen is of great importance as an essential component of deoxyribonucleic acid and chlorophyll . Depending on the species, the proportion of dry matter is 2–6%, or an average of 1.5%. The nitrogen is mostly absorbed in the form of ammonium or nitrate salts.

Deficiency symptoms

  • poor stature
  • pale green color of the leaves. Older people become chlorotic and fall off prematurely.
  • blooming too early (emergency bloom)
  • Yellowing

Excess symptoms

  • Mastig growth
  • Leaves dark green
  • Delayed flowering
  • Plant susceptible to frost and disease
  • Leaf tissue appears spongy and soft

Extraction and presentation

Today, nitrogen is primarily obtained by fractional distillation of liquefied air in air separation plants using the Linde process with a purity of up to 99.99999%. Nitrogen with impurities below 1 ppb requires additional purification steps. A biological method using rice seedlings exists to remove the remaining oxygen.

Schematic structure of the membrane process

Nitrogen with a degree of purity of approx. 99% is obtained much more cost-effectively through multi-stage adsorption / desorption on zeolites . Another method for the decentralized production of nitrogen is the membrane process. Compressed air is pressed through a plastic membrane at a pressure of 5 to 13 bar. The rate of diffusion of nitrogen and argon through this membrane is significantly slower than that of oxygen, water and carbon dioxide, which means that the gas flow on the inside of the membrane is enriched with nitrogen. The purity of the nitrogen can be controlled by adjusting the flow rate (up to 99.995% for small quantities, 99% for industrial standards.)

A somewhat old-fashioned method is to bind the oxygen in the air to charcoal while heating, and then wash out the carbon dioxide that has formed . The atmospheric oxygen can also be removed by passing the air over glowing copper or an alkaline pyrogallol or sodium dithionite solution.

In the laboratory, pure nitrogen can be produced by heating an aqueous ammonium nitrite solution or a solution of the ammonium chloride / sodium nitrite mixture to around 70  ° C :

Alternatively, thermolysis of sodium azide , which is used to produce spectroscopically pure nitrogen, is possible.

properties

Physical Properties

Valence line formula of the N 2 molecule
Oxidation states of nitrogen
−3 NH 3 , NaNH 2 , Li 3 N , HCN , methylamine
−2 N 2 H 4
−1 N 2 H 2 , hydroxylamine
0 N 2
+3 HNO 2 , NaNO 2 , nitromethane
+5 ENT 3 , NaNO 3

Molecular nitrogen is a colorless, odorless and tasteless gas that condenses to a colorless liquid at low temperatures (−196 ° C). Nitrogen is sparingly soluble in water (23.2 ml nitrogen in 1 l water at 0 ° C) and is not flammable. Nitrogen is the only element of the nitrogen group that forms (pp) π-bonds with itself. The atomic distance of this triple bond is 109.8 pm.

Spectrum of a nitrogen gas discharge
Spectrum of a nitrogen gas discharge

In a gas discharge spectral tube, the molecular orbitals of nitrogen are excited to glow at a residual pressure of approx. 5–10 mbar when operated with 1.8 kV high voltage , 18 mA current and a frequency of 35 kHz. When the ionized gas molecules are recombined , the characteristic color spectrum is emitted.

The critical point is: temperature −146.95 ° C, pressure 33.9 bar, density 0.314 g / cm 3 .

Nitrogen preferably forms covalent bonds in its compounds . In the 2s 2 p 3 electronic configuration , the formation of three covalences leads to octet completion; Examples for this are:

All of these compounds have a trigonal pyramidal structure and a lone pair of electrons. They can react as nucleophiles and as bases via this lone pair of electrons .

The molecular dinitrogen N 2 is very inert due to the stable triple bond present in the nitrogen molecule and the associated high bond dissociation energy of 942 kJ / mol. Therefore, it usually takes a lot of energy to break this connection and to bind nitrogen to other elements. The activation energy required is also high, and it can be reduced if necessary using suitable catalysts .

Polymer nitrogen

In a publication in August 2004, researchers from the Max Planck Institute for Chemistry in Mainz announced that they had generated a new crystalline form, so-called polymeric nitrogen with single bonds, at pressures of over 110  G Pa at a temperature of over 2000  K. This modification has a unique cubic structure called the “cubic gauche” structure. Due to the high instability, the possible uses are limited, but one could imagine polymeric nitrogen as an explosive or energy storage device, for example. Poly nitrogen would then be by far the most powerful non-nuclear explosive.

A polymeric form of nitrogen analogous to black phosphorus , known as black nitrogen , was described in 2020. The structure contains two-dimensional layers in which the nitrogen atoms are networked in a uniform zigzag pattern. These 2D layers are similar in their electronic properties to graphene , so that the material could be of interest for many technical applications. It was produced by laser heating of nitrogen at a pressure of 140 GPa in a diamond cell.

Isotopes

A total of 16 isotopes between 10 N and 25 N and other isomers of nitrogen are known. Of these, two, the isotopes 14 N and 15 N, are stable and occur in nature. The isotope with the greater proportion of the natural isotope composition is 14 N with 99.636%, 15 N has a proportion of 0.364%. The longest-lived unstable isotopes are 13 N, which changes to 13 C with a half-life of 9.965 minutes under β + radiation , and 16 N, which decays to 16 O with a half-life of 7.13 seconds under beta decay. All other isotopes only have short half-lives of seconds or milliseconds.

The 15 N isotope was discovered by Naude (1929) and used a few years later by Norman and Werkman (1943) in the first field tests. Even today, this isotope is used in a similar way for biochemical studies of nitrogen metabolism in arable land or in plants, but also as an indicator for the conversion of proteins. The proportion of 15 N in nitrogen in the atmosphere is 0.3663%.

15 N can be enriched like other isotopes of gaseous substances, for example by thermal diffusion separation .

use

Nitrogen compounds

Since the beginning of the 20th century atmospheric nitrogen can technically be fixed: The large-scale lime nitrogen - synthesis began around 1901 that the nitric acid after the Birkeland-Eyde process around 1905, and that of ammonia ( Haber process ) from the 1908th

Nitrogen compounds are used in a wide variety of applications in organic chemistry and are used as fertilizers .

Many explosives are nitrogen compounds. These are nitro compounds or nitric acid esters . If there are enough nitro groups in the molecule , e.g. B. in picric acid , the oxygen atoms of the nitro groups can react exothermic when excited by impact or temperature increase with the carbon or hydrogen atoms in the same molecule . Thus, in a very short time, the solid turns into a high-temperature gas that expands with great force. So explosives are in a metastable state. With a few nitro groups, there is only a rapid and incomplete combustion, e.g. B. with nitrocellulose (including celluloid ).

Nitrogen gas

Nitrogen in a discharge tube

Nitrogen is used to fill aircraft tires on large aircraft . Avoiding oxygen (about 21% in air) at about 10 bar pressure prevents aircraft tires from catching fire from the inside due to the great heat generated (from friction and walking) when landing or during take-off . A small positive side effect is that nitrogen is about 2.5% lighter than air.

Nitrogen is used as a protective gas during welding and as an incandescent filler gas. The inert properties of nitrogen are important here. It is approved as a food additive E 941 as propellant gas, packing gas, gas for whipping cream and the like .

Nitrogen is used in beverage dispensing systems when, due to structural conditions (long pipeline, large height difference), a high dispensing pressure is necessary. Nitrogen is used here together with carbon dioxide as a mixed gas. Since nitrogen does not dissolve in the drink, you can also tap at higher pressures without too much foam formation or carbonization .

At the same increased pressure, nitrogen has a lower solubility in - always water-based - beverages than carbon dioxide. By relaxing while tapping, smaller foam bubbles are achieved. Since nitrogen, unlike CO 2 , does not reduce the pH value , i.e. it does not have an acidic effect, mixed milk and coffee beverages can also be foamed in the direction of sour without changing their taste.

In April 2015 , the US state of Oklahoma legally approved the use of nitrogen gas for asphyxiation as a means of executing the death penalty .

Liquid nitrogen

Boiling nitrogen in a metal cup (−196 ° C), leather gloves as protection against the cold

Due to its low boiling point , liquid nitrogen is used as a cooling medium in cryogenics . The nitrogen removes its heat of evaporation from the goods to be cooled and keeps them cold until it has evaporated. Compared to liquid oxygen , which boils at −183 ° C (90 K), the boiling point of liquid nitrogen is another 13 K lower, it boils at −196 ° C (77 K) and causes atmospheric oxygen and other gases to condense this way can be separated.

Liquid nitrogen ( density 0.8085 kg / L at −195.8 ° C) is used, among other things, to create the superconducting state in high-temperature superconductors. It is also used for the storage of biological and medical samples, egg cells and sperm , as well as for the shock freezing of biological material. An example of the cooling is infrared - photo receivers to reduce the thermal noise or even made a half-conductive state to bring about in them.

In civil engineering it is used to freeze the ground . In the field of materials technology , liquid nitrogen is used to remove residual austenite in certain hardened steels or to artificially age the materials by “freezing”. Liquid nitrogen is also used, for example, to shrink gear shafts to such an extent that attached gears hold on to the shaft through an interference fit. When recycling cables, the insulating material becomes brittle when it is cooled with liquid nitrogen and can be knocked off by the metal ( aluminum or copper).

In the “nitrogen burial” ( promession ) (which is forbidden in Germany ), the corpse is frozen in a bath of liquid nitrogen and then ground to a powder.

Nitrogen consumers are often provided with nitrogen instead of in pressurized gas bottles as liquid nitrogen in thermo containers similar to a thermos bottle . These containers are known as dewars . For this purpose, liquid nitrogen is filled from double-walled tank trucks.

Chemical reaction

As a nitrification refers to a chemical reaction in which accommodates a reactant nitrogen.

A typical example of azotization is the appearance of calcium cyanamide :

safety

Although air consists of over 78% nitrogen and nitrogen is an inert gas and therefore non-toxic, safety measures must be taken when handling gaseous nitrogen in large quantities. For example, if due to the amount of nitrogen there is a risk that people will enter work areas such as machine rooms, which are filled with nitrogen gas for fire protection reasons , and there is no or only an insufficient amount of oxygen for breathing due to displacement. If a person enters such an area, an insidious, since the person concerned is not consciously perceivable, normobaric hypoxia due to nitrogen, which after a few seconds leads to slight disturbances of consciousness followed by unconsciousness and after a few minutes to death from suffocation . For example, in the run-up to the first mission of the space shuttle in March 1981, two technicians died who had entered an area in the Mobile Launcher Platform filled with nitrogen for fire safety reasons .

The reason for this danger is that the human body does not have a sufficiently fast sensor system in the carotid body to detect the lack of oxygen. The feeling of suffocation occurs with an increase in the level of carbon dioxide in the blood, followed by violent breathing reflexes and panic . If, on the other hand, the carbon dioxide can be exhaled without any problems in a pure nitrogen atmosphere, which is the case in larger rooms filled with nitrogen, there is no perceptible feeling of suffocation and the lack of oxygen leads to hypoxia, which is not consciously perceptible by the person concerned.

Therefore, in areas where larger amounts of nitrogen are handled and there is a potential risk of suffocation, for example in the event of malfunctions, special warning devices must be available in addition to the necessary ventilation, which visually or acoustically indicate an oxygen deficiency. In addition, it may be necessary to wear personal protective equipment , which indicates in good time that the oxygen content has fallen below a limit value.

proof

Nitrogen, which is present in organically bound form, can be determined qualitatively by means of Lassaigne's sample and quantitatively by means of nitrogen determination according to Will-Varrentrapp , Kjeldahl's nitrogen determination , an azotometer or elemental analysis. For inorganically bound nitrogen, the cross-match for ammonium ions or the ring test for nitrate ions are carried out as detection reactions. To carry out the ring test, the sample solution (sulfuric acid, heavy metal-free) is mixed with fresh iron (II) sulfate solution and underlaid with concentrated sulfuric acid . At the interface between the two liquids, the nitrate ions are reduced to nitrogen monoxide (NO). In aqueous solution, this radical forms a brown complex with other iron ions , which becomes visible as a "ring" at the phase boundary in the test tube:

Step 1:

Redox reaction

as well as step 2:

Complex formation reaction

links

Drills with coating made of titanium nitride
Tripeptide made from valine , glycine and alanine
Pieces of trinitrotoluene
Structural formula of trinitrotoluene
Structural formula of nitroglycerin

ammonia

Ammonia is a colorless, water-soluble and poisonous gas with a strong pungent smell that causes tears and is suffocating. It is the basic material for the production of other nitrogen compounds, for example urea , ammonium salts such as ammonium nitrate and ammonium chloride , amides and imides . Ammonia is produced almost exclusively using the Haber-Bosch process .

Nitrides

Nitrogen forms different types of nitrides with other elements :

Titanium nitride has a sodium chloride - crystal structure . It is used for coatings that extend the service life of products such as tools . These gold-colored layers are usually very thin.

Silicon nitride occurs in three modifications (α-Si 3 N 4 , β-Si 3 N 4 and γ-Si 3 N 4 ), which differ in their crystal structure . It is used in semiconductor technology and for measuring tips of atomic force microscopes .

Nitrogen oxides

In oxidation processes , for example in the combustion of gasoline , oil and coal produced nitrogen oxides . The nitrogen source can be air or fuel . Some nitrogen oxides, such as nitrogen dioxide , nitrogen monoxide and nitrous tetroxide , are poisonous , corrosive and harmful. Nitrogen dioxide and nitrous tetroxide are partly responsible for acid rain . Nitrous oxide (laughing gas) is used as an anesthetic used. It damages the ozone layer and is a very powerful greenhouse gas .

Halides

Nitrogen trifluoride is a powerful oxidizing agent . It is used for the production of semiconductors , flat panel displays and solar cells used. Iodine nitrogen (nitrogen triiodide) is a very unstable compound that is highly explosive and reacts strongly exothermically in the event of friction, impact or vibration .

Acids and their salts

Nitric acid is a strong acid that is used, among other things, in the manufacture of fertilizers , dyes and explosives . The salts of nitric acid are the nitrates . Ammonium nitrate , sodium nitrate , potassium nitrate and calcium nitrate are important fertilizers.

Nitric acid is a moderately strong, unstable acid which, when heated , breaks down in a disproportionation reaction to form nitric acid , nitrogen monoxide and water :

The salts of nitrous acid are the nitrites . Sodium nitrite and potassium nitrite are used as preservatives .

Hypo-nitrous acid is a weak, unstable acid that breaks down to nitrous oxide and water at room temperature :

Hydrazoic acid is an unstable , highly explosive, irritating, pungent smelling liquid to the mucous membranes . The salts of hydrazoic acid are called azides . Lead azide is used as an initial explosive .

Other inorganic compounds

Hydrogen cyanide ( hydrogen cyanide ) is a colorless to slightly yellowish, flammable, very volatile and water-soluble liquid . Hydrogen cyanide and its salts , the cyanides , for example potassium cyanide and sodium cyanide , are highly toxic.

Other nitrogen-containing acids are cyanic acid , isocyanic acid and popic acid .

Organic compounds

Nitrogen is found in numerous organic compounds , for example

literature

Web links

Wiktionary: nitrogen  - explanations of meanings, word origins, synonyms, translations
Commons : nitrogen  album with pictures, videos and audio files
Wikibooks: Inorganic Chemistry / Nitrogen Internship  - Learning and Teaching Materials

Individual evidence

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  2. ↑ The values ​​for the properties (info box) are taken from www.webelements.com (nitrogen) , unless otherwise stated .
  3. ↑ The standard value recommended by IUPAC is given; Since the isotopic composition of this element can vary locally, the mass range given in brackets results for the mean atomic mass. See: Michael E. Wieser, Tyler B. Coplen: Atomic weights of the elements 2009 (IUPAC Technical Report). In: Pure and Applied Chemistry. 2010, p. 1, doi: 10.1351 / PAC-REP-10-09-14 .
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  19. ↑ Press release of the Max Planck Society of August 3, 2004 .
  20. ↑ Dominique Laniel, Bjoern Winkler, Timofey Fedotenko, Anna Pakhomova, Stella Chariton, Victor Milman, Vitali Prakapenka, Leonid Dubrovinsky, Natalia Dubrovinskaia: High-Pressure Polymeric Nitrogen Allotrope with the Black Phosphorus Structure in Phys. Rev. Lett. 124 (2020) 216001, doi : 10.1103 / PhysRevLett.124.216001 .
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  25. ↑ Additive Admissions Ordinance : Annex 3 (to Section 5, Paragraph 1 and Section 7) Generally permitted additives .
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