Themes

Cerium: structure, properties, obtaining, uses

Cerium is economically exploited from the minerals monazite and bastnasite, from which several of the other lanthanides are also obtained. In these minerals it is necessary to separate the Ce 4+ ions present in their oxide CeO 2 , called ceria. It is the only lanthanide that forms a very stable oxide with an oxidation state of +4 and not +3 (Ce 2 O 3 ).

Ultra-pure sample of metallic cerium sealed in a glass ampoule with argon. Source: Hi-Res Images of Chemical Elements / CC BY (https://creativecommons.org/licenses/by/3.0)

Cerium is a metal that has numerous industrial applications, as well as in improving the environment. Some of its most important uses are the following: as a flint for cigarette lighters, a catalyst for petroleum distillation , a controller for car exhaust emissions, etc.

This metal has great relevance in analytical chemistry. It is so much so that the technique has its own name: cerimetry. Ce 4+ ions , in an acid medium, are strong oxidizing agents, reducing to Ce 3+ . In the process, analytes such as: Fe 2+ , NO 2 , Sn 2+ , As 3+ , etc. are oxidized and quantified .

Discovery

Cerium was discovered by Jacob Berzelius and Wilhelm von Hisinger in Sweden in 1803, and independently by Martin Klaproth, that same year, in Germany.

Berzelius and Hisinger discovered cerium in a reddish-brown mineral known as cerite: a cerium-lanthanum silicate. They did not actually isolate the pure metal, but they did observe that the metal had two oxidation states. One of them produced colorless salts; while the other produced yellowish-red salts.

They named the newly discovered metal ‘cerium’ in honor of Ceres, an asteroid discovered by Giuseppe Piazzi in 1801. The name Ceres also corresponds to the god of agriculture in Roman mythology.

Klaproth also determined that the new element present in the wax was in the form of an oxide, which he named ockroite oxide because of its yellowish-red color.

Mossandre reacted cerium sulfide with chlorine to produce cerium chloride, reducing the latter by reacting with potassium. The result was potassium chloride and metallic cerium, observing that the metal obtained had a gray color with an opaque metallic luster.

Cerium structure

Cerium has many crystalline structures, having up to four allotropic forms only under atmospheric pressure.

When hot, cerium adopts a body-centered cubic structure (bcc), which exists only above 726 ºC, and is symbolized as δ-Ce.

Below 726 ° C to room temperature, cerium assumes a face-centered cubic structure (fcc), represented as γ-Ce.

In cold, on the other hand, cerium crystallizes with a dhcp structure, which exists in the temperature range between -150 ° C and approximately 25 ° C. This phase or allotrope is represented as β-Ce; and it is, together with γ-Ce, the most predominant phases of cerium.

And finally, we have another fcc structure, more dense, that exists below -150 ºC, and that is represented as α-Ce.

An unusual characteristic of cerium is that its crystalline phases have different transition speeds. That is, when a cerium crystal cools, not all of its structure passes to the α-Ce phase, for example, but will consist of a mixture of α-Ce and β-Ce, since the transformation of β-Ce to α- Ce, is slower than γ-Ce to α-Ce.

Electronic configuration

The abbreviated electron configuration of cerium is as follows:

[Xe] 4f 1 5d 1 6s 2

Note that three energy levels are present in their valence orbitals: 4f, 5d, and 6s. Furthermore, its four electrons have relatively similar electronic energies, which explains another structural peculiarity of cerium: it can be oxidized or reduced under high pressure or intense cooling.

The Ce 4+ cation exists and is very stable because, as mentioned above, all four electrons have similar energies; therefore, they can be “lost” without difficulty through chemical bonding . On the other hand, Ce 4+ is isoelectronic to xenon gas, thus gaining extra stability.

Cerium properties

Physical appearance

Solid silver white

Molar mass

140.116 g / mol

Atomic number

58

Melting point

795 ºC

Boiling point

3,443 ºC

Density

6,770 g / cm 3

Heat of fusion

5.46 kJ / mol

Heat of vaporization

398 kJ / mol

Molar caloric capacity

26.94 J / (mol K)

Hardness

Mohs scale: 2.5

Oxidation states

The oxidation states of cerium are +1 (Ce + ), +2 (Ce 2+ ), +3 (Ce 3+ ), +4 (Ce 4+ ), the last two being the most predominant.

Electronegativity

1.2 on the Pauling scale

Ionization energy

First: 534 kJ / mol

Second: 1,050 kJ / mol

Third: 1,949 kJ / mol

Reactivity

Cerium oxidizes in air to form an oxide layer. This process is accelerated by heating, forming the yellow cerium dioxide, CeO 2 , also known as ceria:

Ce + O 2  → CeO 2

Cerium is a pyrophoric metal, that is, when the chips that originate are scraped off they immediately ignite. It is also an electropositive metal, which reacts weakly with water, a reaction that increases with temperature, producing cerium (III) hydroxide and hydrogen gas:

2 Ce + 6 H 2 O → 2 Ce (OH) 3  + 3 H 2

Cerium is attacked by acids and bases, strong or weak, with the exception of hydrofluoric acid, with which it forms a protective layer of cerium fluoride on the surface of the metal.

On the other hand, cerium is a strong reducing agent, capable of reacting violently with zinc, antimony and phosphorus at 400ºC.

Obtaining

Cerium is present in several minerals, including: monazite, bastnäsite, allanite, cerite and samarskite, the most economically important minerals being monazite and bastnäsite.

Bastnäsite, for example, after being collected receives a treatment with hydrochloric acid to clean it of impurities, such as calcium carbonate. Later, it is calcined in the open air to oxidize it to rust.

Most lanthanides are oxidized to form sesquioxides (Ln 2 O 3 ). Sesquioxides correspond to oxides formed by three atoms of oxygen and two atoms of another element. However, cerium is oxidized to cerium dioxide, which is insoluble in water, and can be leached or extracted with 0.5 M hydrochloric acid, thus separating it from the other lanthanides.

Metallic cerium can be obtained by electrolysis of molten cerium (III) chloride, or by reduction of cerium (III) fluoride with the use of calcium or magnesium. It is also produced by the nuclear fission of uranium, plutonium, and thorium.

Uses / applications

Lighters

Cerium is used in combination with various chemical elements, such as lanthanum, neomidium, and praseomidium, in addition to iron and magnesium oxides, to act as flint in cigarette and gas lighters.

illumination

Cerium is used in carbon arc lighting, used in the motion picture industry, and also as a phosphor in fluorescent lighting and color television.

Metallurgy

Cerium is used in metallurgy as a stabilizer for alloys and welding electrodes.

Glasses

Cerium oxide is used as a polishing compound that produces high-quality optical surfaces, and is also used as a bleaching agent for glass, rendering it opaque to near-ultraviolet radiation.

Cerium is used in the light blanket invented by Austrian chemist Carl Auer von Welsbach, with cerium dioxide mixed with thorium oxide being used to produce brilliant white light. Cerium oxide prevents television glass plates from darkening from electron bombardment.

Oil industry

Cerium is used as a catalyst in the fractional distillation of petroleum.

Environment

Cerium oxide is used as a catalytic converter to reduce the emissions of carbon monoxide and nitrogen oxides in the exhaust gases of motor vehicles. These oxides are very toxic to humans.

Cerium oxide, added to diesel fuel, serves as a catalyst for combustion and removal of carbon particles, thus preventing their emission into the atmosphere in the form of soot.

Medicine

Cerium oxalate has been used to treat nausea and vomiting, especially those that occur during pregnancy.

Cerium is used in the treatment of wounds produced in third degree burns, not only for its antiseptic effect, but also helps prevent septic and systemic complications, which occur after burns by fixing toxins released.

Flammacerium (cerium nitrate – silver sulfadiazine) is used as a cream to prevent infections of wounds due to major burns, with cerium nitrate reducing the onset of immunosuppression.

Cerium was used as an antineoplastic, a discarded practice. However, studies for its use have been restarted.

Small amounts of cerium are found in humans, primarily in bones due to its similarity to calcium.

It has been suggested that cerium could intervene in metabolism, with some positive effects. For example, cerium would act on metabolism, causing a decrease in blood pressure, cholesterol levels, appetite and the risk of blood clotting.

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Check Also
Close
Back to top button