cadmium


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properties
General
Name , symbol , atomic number Cadmium, Cd, 48
Element category Transition metals
Group , period , block 12 , 5 , d
Appearance silvery gray metallic
CAS number 7440-43-9
EC number 231-152-8
ECHA InfoCard 100.028.320
ATC code

D11 AC02

Mass fraction of the earth's envelope 0.3 ppm
Atomic
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) eV3 615.1 kJ / mol
4. Ionization energy 51.0 (1.7 eV)4 921 kJ / mol
5. Ionization energy 67.9 (1.9) eV6 551 kJ / mol
Physically
Physical state firmly
Crystal structure hexagonal
density 8.65 g / cm³ (25 ° C )
Mohs hardness 2.0
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
Chemically
Oxidation states 2
Normal potential −0.403 V (Cd 2+ + 2 e - → Cd)
Electronegativity 1.69 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
106 Cd 1.25% Stable
107 Cd {syn.} 6.50 h ε 1.417 107 Ag
108 Cd 0.89% Stable
109 Cd {syn.} 462.6 d ε 0.214 109 Ag
110 Cd 12.49% Stable
111 Cd 12.8% Stable
112 Cd 24.13% Stable
113 Cd 12.22% 7.7 x 10 15 a β - 0.316 113 in
113 m Cd {syn.} 14.1 a β - 0.580 113 in
114 Cd 28.73  % Stable
115 Cd {syn.} 53.46 h β - 1.446 115 in
116 Cd 7.49% (3.0 ± 0.3) 10 19 a β - β - 116 Sn
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
111 Cd 1/2 5.698 · 10 7 0.00124 21.2
113 Cd 1/2 5.961 · 10 7 0.00135 22.2
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
02 - Highly / extremely flammable 06 - Toxic or very toxic 08 - Dangerous to health 09 - Dangerous for the environment

(Flame only applies to the non-stabilized form)

danger

H and P phrases H: 250-350-330-361fd-341-372-410
P: 273-391-210-260-281-308 + 313-405-501
Authorization procedure under REACH particularly worrying : carcinogenic ( CMR ), serious effects on human health are considered likely
Toxicological data
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 .

history

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.

Occurrence

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

Development of cadmium production over time

Cadmium is obtained exclusively as a by-product in zinc smelting, and to a small extent also in lead and copper smelting . Smaller amounts are also produced when recycling iron and steel .

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.

properties

A bar of crystalline cadmium
Spectrum of a cadmium gas discharge

Physical Properties

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. Template: room group / 194

Because of its high toxicity, CdO was examined by the USA as a chemical warfare agent during World War II .

Chemical properties

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.

use

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:

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 .

proof

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.

safety instructions

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.

toxicology

In the chemical industry, cadmium is an unavoidable by-product of zinc , lead and copper extraction . Cadmium can also be found in fertilizers and pesticides .

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

Due to industrial or environmental factors, cadmium can gradually accumulate in the body and cause chronic poisoning that is difficult to detect .

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.

In acute cadmium poisoning, biliary excretion can be supported by administering penicillamine or dimercaprol . There is no known effective therapy for acute cadmium poisoning beyond this.

Symptoms

links

Category: Cadmium compound

Oxides and hydroxides

Halides

Chalcogenides

Other connections

literature

Web links

Wiktionary: Cadmium  - explanations of meanings, word origins, synonyms, translations
Commons : Cadmium  - collection of pictures, videos and audio files

Individual evidence

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