|Name , symbol , atomic number||Cadmium, Cd, 48|
|Element category||Transition metals|
|Group , period , block||12 , 5 , d|
|Appearance||silvery gray metallic|
|Mass fraction of the earth's envelope||0.3 ppm|
|Atomic mass||112,414 (4) et al|
|Atomic radius (calculated)||155 (161) pm|
|Covalent radius||144 pm|
|Van der Waals radius||158 pm|
|Electron configuration||[ Kr ] 4 d 10 5 s 2|
|1. Ionization energy||8th.993 820 (16) eV ≈ 867.77 kJ / mol|
|2. Ionization energy||16.908 313 (12) eV ≈ 1 631.4 kJ / mol|
|3. Ionization energy||37.468 (6) eV ≈ 3 615.1 kJ / mol|
|4. Ionization energy||51.0 (1.7 eV) ≈ 4 921 kJ / mol|
|5. Ionization energy||67.9 (1.9) eV ≈ 6 551 kJ / mol|
|density||8.65 g / cm³ (25 ° C )|
|magnetism||diamagnetic ( Χ m = −1.9 10 −5 )|
|Melting point||594.22 K (321.07 ° C)|
|boiling point||1038 K (765 ° C)|
|Molar volume||13.00 10 −6 m 3 mol −1|
|Heat of evaporation||100 kJ / mol|
|Heat of fusion||6.2 kJ mol −1|
|Speed of sound||2310 m s −1 at 293.15 K.|
|Work function||4.2 eV|
|Electric conductivity||14.3 · 10 6 A · V −1 · m −1|
|Thermal conductivity||97 W m −1 K −1|
|Normal potential||−0.403 V (Cd 2+ + 2 e - → Cd)|
|Electronegativity||1.69 ( Pauling scale )|
|For other isotopes see list of isotopes|
|Authorization procedure under REACH||particularly worrying : carcinogenic ( CMR ), serious effects on human health are considered likely|
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .
Cadmium (rarely also cadmium ; of ancient Greek καδμία kadmía , latin cadmia and Cadmea " calamine ") is a chemical element with the element symbol Cd and the atomic number 48. It is mostly the transition metals counted although there is a completed d-shell has, thus rather resembles the main group elements, especially the alkaline earth metals . In the periodic table it is in the 5th period as well as the 2nd subgroup (group 12) or zinc group .
In 1817, Friedrich Stromeyer and Carl Samuel Hermann independently discovered cadmium in contaminated zinc carbonate . Stromeyer noticed that contaminated zinc carbonate discolored when heated - a behavior that pure zinc carbonate did not exhibit.
Pliny the Elder reported in its written around the year 77 Natural History Naturalis Historia of Galmeifunden in Germania: " Cadmea [...] ferunt nuper etiam in Germania provincia repertum " (German: "recently was found in the province of Germania calamine"). The term cadmium was used as early as the Middle Ages , probably for zinc or its carbonate ore. As emerges from a document issued by Emperor Friedrich II in Ravenna in April 1226, this grants the Benedictine monastery of St. Paul in Lavanttal the right “ ut Cadmiae tam argentj quam plumbi et ferri, que in territorio ipsius monasteri de cetero inveniri contigerint , ad opus suum ”(German:“ that the zinc, as well as silver, as well as lead and iron, which is found in the area of the monastery, is used for its purposes ”).
Despite the toxicity of cadmium and its compounds which recorded British Pharmaceutical Codex 1907 cadmium iodide as an agent for the treatment of swollen joints ( enlarged joints ), scrofulous glands ( scrofulous glands ) and frostbite ( chilblains ).
In 1907 the International Astronomical Union defined an Angstrom as 1 / 6438.4696 times the wavelength of a red spectral line of cadmium in dry air with a carbon dioxide content of 0.03% at a temperature of 15 ° C and a pressure of 1 atm . The General Conference on Weights and Measures in 1960 accepted 1,553,164.13 times the wavelength of a red spectral line of cadmium as the secondary definition of a meter .
In 1942 Enrico Fermi used cadmium sheets in the world's first nuclear reactor . The metal sheets could be pushed in and out of the reactor in order to control the chain reaction. Cadmium can capture moderated fission neutrons and thus influence the criticality of the reactor.
Cadmium is a very rare element. Its share in the earth's crust is only about 3 · 10 −5 %. Cadmium is extremely rare in its natural form, i.e. in its elementary form. So far, only five localities in three countries are known: The river Khann'ya in Vilyuy River basin, the Jana near -Flussbecken Verkhoyansk and Billeekh intrusion in the Russian Republic of Sakha (Yakutia, East Siberia); the Goldstrike pits near Lynn in Eureka County in the US state of Nevada ; and the Burabaiskii massif in the Aqmola area of Kazakhstan.
The ores containing cadmium are primarily the cadmium blende greenockite (CdS) with up to 77.81% Cd and the cadmium spar otavite (CdCO 3 ) with up to 65.20% Cd, which, however, are too rare for commercial mining. Both are almost always with different zinc ores such as sphalerite (ZnS) and Smithsonite (ZnCO 3 ) associated .
In total, a little more than 20 cadmium minerals are known to date (as of 2018). The very rare cadmium oxide monteponite has the highest Cd content with up to 87.54%. Other minerals include hawleyite (77.81% Cd), cadmoselite (58.74% Cd) and drobecite (IMA 2002-034, 40.07% Cd).
Cadmium as a mineral
Naturally occurring cadmium in its elemental form was first described in 1979 by BV Oleinikov, AV Okrugin and NV Leskova and recognized by the International Mineralogical Association (IMA) as an independent mineral type (IMA's internal entry number: 1980-086a ).
According to the systematics of minerals according to Strunz (9th edition) , cadmium is classified under system no. 1.AB.05 (elements - metals and intermetallic compounds - zinc-brass family - zinc group). In the outdated 8th edition of Strunz's mineral systematics, however, cadmium is not yet listed. Only in the "Lapis mineral directory", which was last updated in 2018, which is still based on this form of system numbering out of consideration for private collectors and institutional collections, the mineral was given the system and mineral number. I / A.04-40 . The systematics of minerals according to Dana , which is mainly used in English-speaking countries , lists the element mineral under the system no. 01.01.05.02 .
Extraction and presentation
The extraction of cadmium depends on the method in which the zinc is extracted. In dry zinc extraction, the cadmium is first reduced with the zinc. Since cadmium has a lower boiling point than zinc, it evaporates more easily. As a result, a cadmium-zinc mixture evaporates from the reduction vessel and reacts at another point with oxygen to form cadmium and zinc oxide . This mixture is then mixed with coke in a distillation vessel and the cadmium is distilled off from the zinc. By fractional distillation , higher purities of cadmium can be achieved.
In wet zinc extraction, the dissolved cadmium ions are reduced with zinc dust and precipitated. The resulting cadmium is oxidized to cadmium oxide with oxygen and dissolved in sulfuric acid. From the thus resulting cadmium sulfate solution is prepared by electrolysis with aluminum anodes and lead cathode particularly pure electrolyte cadmium recovered.
Cadmium is a shiny silver metal with a density of 8.65 g / cm³. It is soft ( Mohs hardness 2), plastically deformable and can be cut with a knife as well as drawn into wires and hammered out into leaves.
Cadmium solidifies exclusively in the hexagonal crystal system in the space group P 6 3 / mmc (space group no. 194) . The lattice parameters of pure cadmium are a = 0.2979 nm (corresponds to 2.98 Å ) and c = 0.5617 nm (corresponds to 5.62 Å) with 2 formula units per unit cell . Similar to tin , typical noises occur when bending cadmium of medium purity ( called tin screaming for tin ). Polished cadmium loses its shine after a few days in air, even if it is more corrosion-resistant than zinc. In carbonated air, it forms a gray-white, carbon dioxide-containing coating. When heated up, it burns with a reddish to yellow flame to give a brownish, steaming cadmium oxide CdO.
In chemical compounds it is usually bivalent. Chemically it is similar to zinc , but it tends to form complex compounds with the coordination number 4. In air , cadmium darkens the surface through oxidation . In an alkaline medium , the surface is insoluble in sulfuric acid and hydrochloric acid difficult and nitric acid soluble.
Because of the high toxicity of cadmium, its importance is decreasing. Since December 2011 it has been banned in jewelry, alloys for soldering and in PVC in the European Union . Cadmium is or was used:
- as corrosion protection for ferrous materials (cadmium plating and massive loss anodes in shipbuilding)
- as a surface coating for aluminum materials in defense technology (e.g. for rocket launchers )
- for nickel-cadmium batteries
- for yellow to deep red color pigments made from cadmium sulfide and cadmium selenide for paints and plastics (meanwhile of little practical importance due to possible health hazards, especially when the corresponding articles are burned)
- as alloy metal in low-melting alloys , for example bearing materials or Wood's metal
- earlier than lubricant in disc brakes
- as a component of soldering materials ( tin solder ), also for hard solders
- for the manufacture of semiconductors
- Cadmium oxide as a luminescent substance in black and white television tubes and an additive in blue and green phosphorus in color tubes
- Cadmium oxide as an admixture to silver in switch contacts
- as a shielding material against thermal neutrons and for control rods in nuclear technology due to the particularly high effective cross-section of the isotope 113 for neutron capture
- as a source of high-energy gamma radiation (around 7 MeV ) from thermal neutrons for the later generation of positrons by pair generation
- Cadmium sulfide in light meters , the spectral sensitivity of which is similar to that of the human eye
- Cadmium telluride as an infrared-sensitive sensor for cameras ( focal plane arrays )
- in thin-film solar cells as cadmium telluride or cadmium sulfide to generate electricity
- Cd stearate as a stabilizer in plastics, for example in PVC (insensitive to light, but now of little practical importance due to possible health risks)
- formerly in the Weston standard elements for determining the unit of measurement of electrical voltage , 1 volt
- Cadmium bismuth alloys for fuses
- Silver- cadmium alloys as a deoxidizer in the manufacture of sterling silver
- for jewelry: gold-green gold- cadmium alloys
- Cadmium lamp
- Helium-cadmium laser
- Cadmium ions for blocking voltage-activated calcium channels in electrophysiology
- for coloring glass in yellow, orange and red by adding cadmium sulfide , selenide and telluride or mixtures thereof.
The cadmium chalcogenides cadmium sulfide (yellow), cadmium selenide (red) and cadmium telluride (black) are important II-VI semiconductors . They are, for example, nanoparticulate as quantum dots (Engl. Quantum dots ) produced and u. a. used in biochemistry in vitro .
The so-called glow tube test can serve as a preliminary test for cadmium. For this purpose, some of the original substance is heated in a high-melting incandescent tube and the resulting sulphide-oxide mixture is reduced to the metals with sodium oxalate . Cadmium evaporates as a highly volatile component and is deposited as a metal mirror on the upper part of the tube.
Subsequent addition of sulfur and renewed glowing forms cadmium sulfide from the metal level and sulfur vapor, which is red when heated and yellow at room temperature. This color change can be repeated a few times.
The detection reaction for cadmium cations is the precipitation with sulphide solution or hydrogen sulphide water as yellow cadmium sulphide . Other heavy metal ions interfere with this detection, so that a cation separation process must be carried out beforehand .
Polarography can be used for the quantitative determination of traces of cadmium . Cadmium (II) ions in 1 M KCl give a step at −0.64 V (against SCE ). In the ultra- trace range , inverse voltammetry can be used on mercury electrodes. The graphite furnace AAS from cadmium is also very sensitive . Here, 0.003 µg / l can still be detected . The relatively volatile element does not tolerate high pyrolysis temperatures . A matrix modifier such as palladium - magnesium nitrate can help.
Cadmium is classified as very toxic and its compounds from harmful (such as cadmium telluride) to toxic (e.g. cadmium sulfide) to very toxic (as with cadmium oxide); there is also a justified suspicion of carcinogenic effects in humans. Inhaled dust containing cadmium causes damage to the lungs, liver and kidneys.
In work areas in which heated cadmium compounds are used (soldering stations and cadmium baths), good ventilation or extraction must be ensured.
In the European Union , since December 10, 2011, cadmium has been banned from being used and placed on the market in many plastics, paints, stabilizers, solders and certain metal products, especially consumer goods such as jewelry. Before that, silver hard solder typically contained 10% to 25% cadmium, jewelry for children up to 30%, PVC 0.2%. A limit value of 0.01 percent by weight (100 mg / kg) is often set for placing on the market, as it is assumed that a content below this is an unintentional, i.e. unavoidable, contamination. With Regulation (EU) 2016/217 of February 16, 2016, the ban was extended to the placing on the market of cadmium in certain paints and varnishes - including those with a higher zinc content - and in products coated with such agents. There are still exceptions, for example for certain building materials such as fences made from hard recycled PVC, provided the cadmium content in the plastic does not exceed 0.1 percent by mass and the product is labeled as recycled PVC, for special applications such as aviation or the military or because of the high power density for Ni-Cd batteries in cordless electronic devices.
Admission and dangers
The World Health Organization has adjusted its statement on the tolerable intake level for cadmium downwards several times in recent years, most recently in 2013 to a tolerable monthly intake (TMI) of 25 µg per kilogram of body weight. In 2009, the European Food Safety Authority issued a significantly lower value of 2.5 µg per kilogram of body weight tolerable weekly intake (TWI).
Cadmium is mainly consumed by humans through food. Foods rich in cadmium include: liver , mushrooms , clams and other shellfish , cocoa powder and dried seaweed . In addition, flaxseed contains a lot of cadmium, which is why it is recommended not to consume more than 20 g of flaxseed per day. In addition, since the introduction of artificial fertilizers , cadmium has accumulated on agricultural land and thus in almost all foods. The resources of phosphates are limited and most of them are contaminated with cadmium or radioactive heavy metals . The cadmium content of the phosphate deposits varies greatly. Many industrialized countries have already introduced a limit value for cadmium in fertilizers. For example, a limit value of 1.5 mg / kg applies to the placing on the market of fertilizers in Germany and 50 mg / kg for fertilizers with more than 5% phosphate, while these limit values in Austria are 3 mg / kg and 75 mg / kg P 2 O 5 lie. Also tobacco smoke transported relatively large amounts of cadmium into the lungs, is distributed from where it with the blood in the body.
People who work in factories with high cadmium emissions are particularly exposed to increased risks. Wild garbage dumps , metalworks or fires are also dangerous. Inhaling cadmium can cause serious lung damage and even death. Documented consequences after accidents in industry - such as in the Chinese province of Guangdong - or after decades of emissions - as in the case of Itai-Itai disease (in humans) and Gressenich disease (in grazing cattle) - make the real dangers clear.
Damage in humans
About 5% of cadmium is absorbed from food in the intestine. If there is a lack of iron and calcium , the resorption rate increases, which suggests that all three metals use the same transport route. Cadmium first stimulates the synthesis of metallothioneins in the liver , with which it forms a complex and is transported via the bloodstream to the renal glomeruli, where it is filtered and then absorbed from the renal tubules. The metallothionein-cadmium complex is metabolized in the tubular cells and Cd is released. In turn, Cd activates an increased synthesis of metal thione, whereby even more cadmium is bound. The accumulation in the kidneys causes damage to this organ, resulting in proteinuria . Due to this protein binding, cadmium is only excreted extremely slowly, the half-life for it remaining in the body is up to 30 years. The cadmium content therefore rises from birth and only falls again at the age of 50–60 years.
Cadmium also damages the bones as it ultimately leads to the mobilization of calcium. In the intestine, Cd competes with calcium for the binding sites on the Ca-binding protein in the intestinal mucosa. In addition, Cd blocks the new synthesis of 1,25-dihydroxycholecalciferol ( calcitriol ) in the renal tubular cells. 1,25-dihydroxycholecalciferol is necessary to activate the synthesis of calcium-binding protein in the intestinal mucosal cell. All in all, cadmium causes a reduced reabsorption of calcium in the intestine and kidneys as well as an increased excretion with the urine with the consequence of a release of calcium from the bones and thus the breakdown of the same.
- Diarrhea , stomach pain and vomiting violently
- Kidney damage
- Broken bones
- Damage to the central nervous system
- Damage to the immune system
- Reproductive disorders and possibly even infertility
- Mental disorders
- Possible DNA damage and cancer development
- Loss of smell
Oxides and hydroxides
- Cadmium sulfate CdSO 4
- Cadmium nitrate Cd (NO 3 ) 2
- Cadmium cyanide Cd (CN) 2
- Cadmium stearate Cd (C 17 H 35 COO) 2
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