CADMIUM (CD): HISTORY, PROPERTIES, STRUCTURE, USES - CHEMISTRY - 2024

The cadmium (Cd) is a transition metal or post - transitional atomic number 48 and silver. It is malleable and ductile, with relatively low melting and boiling points. Cadmium is a rare element and only has a concentration of 0.2 g / ton of the earth's crust.

Greenockite (CdS) is the only important cadmium ore with an intense yellow coloration. Cadmium is found associated with zinc in sphalerite (ZnS), which contains between 0.1 and 03% cadmium as a Cd ²⁺ cation.

Cadmium crystals. Source: Hi-Res Images of Chemical Elements

When processing sphalerite to obtain, smelter and refine zinc, cadmium is obtained in a secondary form, this being its main source of production.

This metal was discovered in 1817, independently by Friedrich Stromayer and Karl Hermann. Stromayer baptized the new element with the name of cadmium, originating from the Latin word “cadmia”, a term as calamine (zinc carbonate) was known.

Cadmium is a chemical element with the symbol Cd and its atomic number is 48. Source: Albedo-ukr CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5/)

Cadmium is an element of great utility and numerous applications, such as anticorrosive of iron, steel and non-ferrous metals; use as a pigment; stabilization of PVC; element in alloys used in welding; rechargeable nickel cadmium batteries, etc.

However, it is a very toxic element that causes severe damage to the lungs, kidneys and bones, and it has even been reported to have a carcinogenic action, which is why its use has been limited. But despite this, it has continued to be used carefully in some applications.

History

- Double discovery

Cadmium was discovered by Friedrich Stromayer, a German chemist, in the year 1817 in a sample of zinc carbonate (calamine). That same year, KSL Hermann and JCH Roloff made the same discovery, independently, in an experiment with zinc sulphide.

Stromayer is reported to have made his discovery while fulfilling a government request to inspect pharmacies in the city of Hildesheim, Germany. Zinc oxide, as it is now, was used to treat certain skin conditions.

It appears that pharmacies did not ship zinc oxide, but instead sold zinc carbonate: a raw material for the production of zinc oxide. The manufacturers of zinc oxide argued that heating the zinc carbonate produced a yellow “zinc oxide”.

Cadmium oxide

They could not sell this “zinc oxide”, since the color of the compound was normally white; Instead, they sold zinc carbonate, also white. Faced with this situation, Stromayer decided to study the supposed yellow zinc oxide.

To do this, he heated samples of the zinc carbonate (calamine) and produced a yellow zinc oxide, as reported. After analyzing it, he concluded that the yellow color was caused by the presence of a metallic oxide of a new element.

After extracting this new metal oxide, it produced its reduction, achieving the isolation of cadmium. Stromayer determined its density and obtained a value of 8.75 g / cm ³, close to the value currently known for this parameter (8.65 g / cm ³).

Also, Stromayer pointed out that the new element had an appearance similar to platinum, and that it was also present in many zinc compounds, and even in purified zinc.

Stromayer suggested the name “cadmium” from the Latin word “cadmia”, the name given to calamine, ZnCO ₃.

Cadmium in zinc sulfide

Karl Hermann (1817) found an unexpected yellow color when processing zinc sulfide and thought it could be arsenic contamination. But once this possibility was ruled out, Hermann realized that he was in the presence of a new element.

- Applications

1840-1940

In the 1840s the use of cadmium as a pigment began to be exploited commercially. The British Pharmaceutical Codex indicates in 1907 the use of cadmium iodide as a medicine to treat "enlarged joints", scrofulous glands and chilblains.

In the 1930s and 1940s, cadmium production was aimed at plating steel and iron to protect them from corrosion. In the 1950s, cadmium compounds such as cadmium sulfide and cadmium selenide were used as sources of red, orange and yellow pigments.

1970-1990

In the 1970s and 1980s, the compounds cadmium laurate and cadmium stearate were found to be stabilizers for PVC, leading to an increase in the demand for cadmium. However, environmental regulations due to the toxicity of cadmium, caused a decrease in its consumption.

In the 1980s and 1990s, cadmium ceased to be used in many of its applications, but then its production increased with the creation of rechargeable nickel-cadmium batteries, which came to represent 80% of cadmium consumption in the United States..

Physical and chemical properties of cadmium

Appearance

Silvery grayish white with soft metallic luster. It becomes brittle by exposure to 80ºC, and can be cut with a knife. It is malleable and can be rolled into rolls.

Standard atomic weight

112,414 u

Atomic number (Z)

48

Item Category

Post-transitional metal, alternatively considered a transition metal. The IUPAC definition of a transition metal is one whose atoms have an incomplete d subshell or which can give rise to cations with an incomplete d subshell.

According to this definition, cadmium is not a transition metal since its Cd ²⁺ cation has its 4d orbitals completely filled with electrons (4d ¹⁰).

Odor

Toilet

Melting point

321.07 ºC

Boiling point

767 ºC

Density

Ambient temperature: 8.65 g / cm ³

At melting point (liquid): 7.996 g / cm ³

Heat of fusion

6.21 kJ / mol

Heat of vaporization

99.87 kJ / mol

Molar caloric capacity

26.020 J / (mol K)

Electronegativity

1.6 on the Pauling scale

Ionization energies

First: 867.8 kJ / mol (Cd ⁺ gas)

Second: 1631.4 kJ / mol (Cd ²⁺ gaseous)

Third: 3616 kJ / mol (Cd ³⁺ gaseous)

Thermal conductivity

96.6 W / (mK)

Resistivity

72.7 nΩ · m at 22 ºC

Hardness

2.0 on the Mohs scale. It is a metal, although dense, considerably soft.

Stability

It is slowly oxidized by humid air to form cadmium oxide, which tarnishes its metallic luster. It is not flammable, but in powder form it can burn and self-ignite.

Auto-ignition

250 ºC for cadmium is a powder form.

Refractive index

1.8 at 20 ºC

Reactivity

Cadmium can burn in air to form cadmium oxide (CaO), a brown amorphous powder, while the crystalline form is dark red.

Cadmium reacts rapidly with dilute nitric acid, and slowly with hot hydrochloric acid. It is also capable of reacting with sulfuric acid, but it does not react with alkalis. In all these reactions, cadmium salts of their corresponding anions (Cl ^-) or oxoanions (NO ₃ ^- and SO ₄ ^2-) are formed.

Structure and electronic configuration

Electron shell diagram of Cadmium, element 48 in the periodic table. Source: Pumbaa (original work by Greg Robson) CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0/)

The cadmium atoms of its crystal establish a metallic bond from their valence electrons, which are located in the 4d and 5s orbitals according to their electronic configuration:

4d ¹⁰ 5s ²

However, although the 4d orbitals are full of electrons, and it may also be thought that the "sea of ​​electrons" is abundant enough to strongly bind the Cd atoms, in reality the interactions are weak. This can be demonstrated experimentally with its low melting point (321 ° C), compared to other transition metals.

For this and other chemical reasons, cadmium is sometimes not considered a transition metal. There are so many electrons (twelve) involved in its metallic bond that they begin to greatly disturb its negative repulsions; which, together with the energetic difference between the filled 4d and 5s orbitals, weakens the Cd-Cd interaction.

The Cd atoms end up defining a compact hexagonal crystalline structure (hcp), which does not undergo phase transitions before its melting point. When hcp cadmium crystals are subjected to a pressure equivalent to 10 GPa, the structure only deforms; but without any phase change being reported.

Oxidation numbers

Cadmium cannot lose the twelve valence electrons; in fact, it cannot lose even one of its 4d orbitals, which are more stable in energy compared to the 5s orbital. Therefore it can only lose the two electrons of the 5s ² orbital, being consequently a divalent metal; as is the case with zinc, mercury, and alkaline earth metals (Mr. Becambara).

When the existence of the Cd ²⁺ cation is assumed in its compounds, then it is said that cadmium has an oxidation number or state of +2. This is your main oxidation number. For example, the following compounds contain cadmium as +2: CdO (Cd ²⁺ O ^2-), CdCl ₂ (Cd ²⁺ Cl ₂ ^-), CdSO ₄ (Cd ²⁺ SO ₄ ^2-) and Cd (NO ₃) ₂.

In addition to this oxidation number, there are also +1 (Cd ⁺) and -2 (Cd ^2-). The oxidation number +1 is observed in the Cd ₂ ²⁺ dication, in which each cadmium atom has a positive charge. Meanwhile, the -2 is quite strange, and would apply to the "cadmide" anion.

Where to find and obtaining

Greenockite crystals. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

Cadmium is a rare element that has a concentration of 0.2 g / ton in the earth's crust. The only important cadmium mineral is greenockite (CdS), which is not mineable from a mining and commercial point of view.

Cadmium is found associated with zinc in the mineral sphalerite (ZnS), which usually contains it at a concentration ranging between 0.1% and 0.3%; but in some cases the cadmium concentration in sphalerite can reach 1.4%.

The rocks processed to obtain phosphorous fertilizers can have a cadmium concentration of 300 mg / kg of fertilizer. Also coal can contain small but significant amounts of cadmium.

A major source of cadmium is volcanic emissions that cadmium can carry into surface waters. The use of phosphorous fertilizers in agricultural soils has led to its contamination with cadmium.

The cadmium present in acid soils is absorbable by plants. Some of the vegetables are used by man as food, which explains how the intake of water and food is the main source of cadmium entry in unexposed persons or smokers.

Treatment of sphalerite

During the mining, smelting and refining of the zinc present in sphalerite, cadmium is usually obtained as a by-product. A similar event also occurs, although to a much lesser degree, during the processing of copper and lead.

Similarly, small amounts of cadmium can be obtained from recycling iron and steel scrap.

The sphalerite is roasted so that the zinc sulphide transforms into its oxide, ZnO. The same reaction is suffered by cadmium sulfide:

2 ZnS + 3 O ₂ → 2 ZnO + 2 SO ₂

If this oxide mixture is heated with charcoal, they will be reduced to their respective metals:

ZnO + CO → Zn + CO ₂

Also, zinc and cadmium can be produced by electrolysis as the oxides dissolve in sulfuric acid.

Either method generates a cadmium-contaminated zinc. Upon melting, cadmium can be vacuum distilled due to its lower melting point (321 ° C) compared to zinc (420 ° C).

Isotopes

Among the natural and stable isotopes of cadmium we have, with their respective abundances here on Earth:

- ¹⁰⁶ Cd (1.25%)

- ¹⁰⁸ Cd (0.89%)

- ¹¹⁰ Cd (12.47%)

- ¹¹¹ Cd (12.8%)

- ¹¹² Cd (24.11%)

- ¹¹⁴ Cd (28.75%)

- ¹¹³ Cd (12.23%)

The ¹¹³ Cd is radioactive, but because of such great value half - life (t _1/2 = 7.7 × 10 ¹⁵ years), can be considered stable. And then there is ¹¹⁶ Cd, also radioactive, with a half-life of 3.1 · 10 ¹⁹ years, so it can be considered a stable isotope, representing 7.51% of cadmium.

Note that the average atomic mass is 112.414 u, closer to 112 than 114. The existence of a predominant isotope above the others is not observed in cadmium.

Risks

General

Cadmium absorption occurs mainly from food, especially liver, mushrooms, shellfish, cocoa powder, and dried seaweed.

An emblematic case occurred in China in the last century, where there was significant cadmium contamination in the population. The cadmium contamination was due to its high concentration in rice, caused by the presence of cadmium in the soils of cereal crops.

A smoker has an average intake of 60 µg / day. The maximum concentration of cadmium allowed in the blood is 15 µg / day. Non-smokers have a cadmium concentration in their blood around 0.5 µg / L.

The lungs absorb between 40 and 60% of the cadmium in tobacco smoke. Cadmium absorbed in the lungs is transported in the blood, forming complexes with proteins, cysteine ​​and glutathione, which then end up in the liver, kidneys, etc.

An acute inhalation of cadmium can produce symptoms similar to those observed in a flu-like process; such as colds, fever and muscle aches, which can cause lung damage. Meanwhile, chronic exposure to cadmium can cause lung, kidney, and bone disease.

Effect on the kidneys

In the kidneys, cadmium usually causes an alteration in the metabolism of phosphorus and calcium, evidenced by an increase in the production of kidney stones. In addition, it causes kidney damage manifested in the appearance in the urine of retinol transporter protein and β-2-microglobulin.

Effect on reproduction

Maternal exposure to cadmium is associated with a low birth weight of the child and an increase in the rate of spontaneous abortions.

Bone damage

Cadmium is related in Japan to the presence of Itai-Itai disease in the last century. This disease is characterized by low bone mineralization, bone fragility with a high rate of fractures, increased osteoporosis, and bone pain.

Carcinogenesis

Although experiments in rats established a relationship between cadmium and prostate cancer, this has not been demonstrated in humans. An association between cadmium and kidney cancer has been shown, and it has also been linked to lung cancer.

Applications

Nickel cadmium rechargeable bacteria

Different cells or Ni-Cd batteries. Source: Boffy b via Wikipedia.

Cadmium hydroxide was used as a cathode in Ni-Cd batteries. These were used in the railway and aeronautical industry, as well as in instruments for collective use including cell phones, video cameras, laptops, etc.

The cadmium consumption for the manufacture of Ni-Cd batteries represented 80% of the cadmium production. However, due to the toxicity of this element, Ni-Cd batteries have been gradually replaced by nickel-metal hydride batteries.

Pigments

Cadmium red. Source: Marco Almbauer

Cadmium sulfide is used as a yellow pigment and cadmium selenide as a red pigment, known as cadmium red. These pigments are characterized by their brilliance and intensity, which is why they have been used in plastics, ceramics, glass, enamels and artistic colors.

It has been noted that the painter Vincent Van Gogh used cadmium pigments in his paintings, which allowed him to achieve a variety of bright reds, oranges and yellows.

The coloration of cadmium pigments must be attenuated before being ground with oils or mixed in watercolors and acrylics.

TV

Cadmium-containing components were used in the phosphor of black and white televisions, as well as in the blue and green phosphors for color television picture tubes.

The phosphor was part of the screen that was irradiated by cathode rays, being responsible for the formation of the image. Cadmium, despite its toxicity, has begun to be used in recently created QLED televisions.

PVC stabilization

Cadmium compounds formed with carboxylate, laurate and stearate were used as stabilizers for polyvinyl chloride, since they delay the degradation produced by exposure to heat and ultraviolet light that decompose PVC during its manufacturing process.

Due to cadmium toxicity, again, cadmium-bound PVC stabilizers have been replaced by other stabilizers, such as barium-zinc, calcium-zinc, and organotin.

Alloys

Cadmium has been used in bearing alloys due to its high fatigue resistance and low coefficient of friction. Cadmium has a relatively low melting point, which is why it is used in low melting point alloys, and is a component in addition to many types of welds.

Cadmium can also be used in electrically conductive, thermally conductive and electrical contact alloys.

Covering

Cadmium is used to protect steel, aluminum, and other non-ferrous metal fasteners, as well as moving parts. Cadmium coating provides corrosion protection in saline and alkaline media. In addition, it serves as a lubricant.

Cadmium is also used in many electrical and electronic applications that require corrosion resistance and low electrical resistivity.

Nuclear reactors

Cadmium is used in nuclear reactors for its ability to capture neutrons, which makes it possible to control excess neutrons from nuclear fission, avoiding additional nuclear fissions.

Semiconductors

Cadmium selenide and telluride are compounds that act as semiconductors in light detection and in solar cells. The HgCdTe is sensitive to infrared light and is used as a motion detector as well as a switch for remote control devices.

biology

He-Cd laser light. Source: Rather Anonymous (https://www.flickr.com/photos//35766549)

Helium-Cd is involved in the formation of a laser beam of blue-violet light with a wavelength ranging between 325 and 422 nm, usable in fluorescence microscopes.

Cadmium is used in molecular biology to block calcium channels, dependent on the membrane potential.

References

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