RUBIDIUM: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, USES - CHEMISTRY - 2024

The rubidium is a metal element belonging to group 1 of the periodic table: the alkali metal, being represented by the chemical symbol Rb. Its name sounds similar to ruby, and it is because when it was discovered its emission spectrum showed characteristic lines of deep red color.

It is one of the most reactive metals that exist. It is the first of the alkali metals that, despite being not very dense, sinks in water. It also reacts with it more explosively compared to lithium, sodium and potassium. There have been experiments in which blisters burst where it is stored (bottom image) to fall and explode in bathtubs.

Ampoule with one gram of rubidium stored under an inert atmosphere. Source: Hi-Res Images of Chemical Elements

Rubidium is distinguished by being a more expensive metal than gold itself; not so much because of its scarcity, but because of its wide mineralogical distribution in the earth's crust and the difficulties that arise when isolating it from potassium and cesium compounds.

It shows a clear tendency to associate with potassium in its minerals, being found as impurities. Not only in geochemical matters it forms a duo with potassium, but also in the field of biochemistry.

The organism "mistakes" the K ⁺ ions for those of Rb ⁺; however, rubidium is not an essential element to date, as its role in metabolism is unknown. Even so, rubidium supplements have been used to alleviate certain medical conditions such as depression and epilepsy. On the other hand, both ions give off a violet flame in the heat of the lighter.

Due to its high cost, its applications are not based too much on the synthesis of catalysts or materials, but as a component for various devices with theoretical physical bases. One of them is the atomic clock, solar cells and magnetometers. This is why rubidium is sometimes regarded as an underrated or under-studied metal.

History

Rubidium was discovered in 1861 by the German chemists Robert Bunsen and Gustav Kirchhoff, using spectroscopy. To do this, they used the Bunsen burner and the spectroscope, invented two years earlier, as well as analytical precipitation techniques. Their object of study was the mineral lepidolite, whose sample was collected from Saxony, Germany.

They started with 150 kg of lepidolite mineral, which they treated with chloroplatinic acid, H ₂ PtCl ₆, to precipitate potassium hexachloroplatinate, K ₂ PtCl ₆. However, when they studied its spectrum by burning it in the Bunsen burner, they realized that it exhibited emission lines that did not coincide with any other element at that time.

The emission spectrum of this new element is characterized by having two well-defined lines in the red region. That is why they baptized it with the name 'rubidus' which means 'dark red'. Later, Bunsen and Kirchhoff succeeded in separating Rb ₂ PtCl ₆ from K ₂ PtCl ₆ by fractional crystallization; to finally reduce it to its chloride salt using hydrogen.

Identifying and isolating a salt of the new element rubidium, the German chemists only needed to reduce it to its metallic state. To achieve this they tried in two ways: applying electrolysis to rubidium chloride, or heating a salt that is easier to reduce, such as its tartrate. Thus, the metallic rubidium was born.

Physical and chemical properties

Appearance

Soft, silver-gray metal. It is so smooth it looks like butter. It is usually packaged in glass ampoules, inside which an inert atmosphere predominates that protects it from reacting with air.

Atomic number (Z)

37

Molar mass

85.4678 g / mol

Melting point

39 ºC

Boiling point

688 ºC

Density

At room temperature: 1.532 g / cm ³

At melting point: 1.46 g / cm ³

The density of rubidium is higher than that of water, so it will sink while reacting violently with it.

Heat of fusion

2.19 kJ / mol

Heat of vaporization

69 kJ / mol

Electronegativity

0.82 on the Pauling scale

Electronic affinity

46.9 kJ / mol

Ionization energies

-First: 403 kJ / mol (Rb ⁺ gaseous)

-Second: 2632.1 kJ / mol (Rb ²⁺ gaseous)

-Third: 3859.4 kJ / mol (Rb ³⁺ gaseous)

Atomic radio

248 pm (empirical)

Thermal conductivity

58.2 W / (m K)

Electrical resistivity

128 nΩ m at 20 ° C

Mohs hardness

0.3. Therefore, even talc is harder than metallic rubidium.

Reactivity

Flame test for rubidium. When it reacts, it gives off a violet flame. Source: Didaktische.Medien

Rubidium is one of the most reactive alkali metals, after cesium and francium. As soon as it is exposed to the air, it begins to burn, and if it is struck, it shoots light sparks. If heated, it also emits a violet flame (top image), which is a positive test for Rb ⁺ ions.

It reacts with oxygen to form a mixture of peroxides (Rb ₂ O ₂) and superoxides (RbO ₂). Although it does not react with acids and bases, it does react violently with water, generating rubidium hydroxide and hydrogen gas:

Rb (s) + H ₂ O (l) => RbOH (aq) + H ₂ (g)

Reacts with hydrogen to form its corresponding hydride:

Rb (s) + H ₂ (g) => 2RbH (s)

And also with halogens and sulfur explosively:

2Rb (s) + Cl ₂ (g) => RbCl (s)

2Rb (s) + S (l) => Rb ₂ S (s)

Although rubidium is not considered a toxic element, it is potentially dangerous and poses fire hazards when it comes in contact with water and oxygen.

Structure and electronic configuration

Rubidium atoms are arranged in such a way that they establish a crystal with a body centered cubic structure (bcc). This structure is characteristic for alkali metals, which are light and tend to float on water; except from rubidium down (cesium and francium).

In rubidium bcc crystals, their Rb atoms interact with each other thanks to the metallic bond. This is governed by a "sea of ​​electrons" from its valence shell, from the 5s orbital according to its electronic configuration:

5s ¹

All 5s orbitals with their single electron overlap in all dimensions of metallic rubidium crystals. However, these interactions are weak, since as one moves down the alkali metal group, the orbitals become more diffuse and, therefore, the metallic bond weakens.

That is why the melting point of rubidium is 39ºC. Likewise, its weak metallic bond explains the softness of its solid; so soft it looks like silver butter.

There is not enough bibliographic information regarding the behavior of its crystals under high pressure; if there are denser phases with unique properties such as sodium.

Oxidation numbers

Its electronic configuration indicates at once that rubidium strongly tends to lose its single electron to become isoelectronic to the noble gas krypton. When it does, the monovalent cation Rb ^{+ is formed}. It is then said that in its compounds it has oxidation number +1 when the existence of this cation is assumed.

Due to the tendency of rubidium to oxidize, the assumption that Rb ⁺ ions exist in its compounds is correct, which in turn indicates the ionic character of these compounds.

In almost all rubidium compounds it exhibits an oxidation number of +1. Examples of them are the following:

-Rubidium chloride, RbCl (Rb ⁺ Cl ^-)

-Rubidium hydroxide, RbOH (Rb ⁺ OH ^-)

-Rubidium carbonate, Rb ₂ CO ₃ (Rb ₂ ⁺ CO ₃ ^2-)

-Rubidium monoxide, Rb ₂ O (Rb ₂ ⁺ O ^2-)

-Rubidium superoxide, RbO ₂ (Rb ⁺ O ₂ ^-)

Although very rare, rubidium could also have a negative oxidation number: -1 (Rb ^-). In this case, one would speak of a “rubidide” if it formed a compound with an element less electronegative than it, or if it was subjected under special and rigorous conditions.

Clusters

There are compounds where individually each Rb atom presents oxidation numbers with fractional values. For example, in Rb ₆ O (Rb ₆ ²⁺ O ^2-) and Rb ₉ O ₂ (Rb ₉ ⁴⁺ O ₂ ^2-) the positive charge is distributed among a set of Rb atoms (clusters). Thus, in Rb ₆ O the oxidation number in theory would be +1/3; while in Rb ₉ O ₂, + 0.444 (4/9).

Cluster structure of Rb9O2. Source: Axiosaurus

Above is the cluster structure of Rb ₉ O ₂ represented by a spheres and bars model. Note how the nine Rb atoms "enclose" the O ^2- anions.

By way of elucubration, it is as if part of the original metallic rubidium crystals remained unchanged while they were separated from the mother crystal. They lose electrons in the process; those necessary to attract the O ^2-, and the resulting positive charge is distributed among all the atoms of said cluster (set or aggregates of Rb atoms).

Thus, in these rubidium clusters the existence of Rb ⁺ cannot be formally assumed. Rb ₆ O and Rb ₉ O _{2 are} classified as rubidium suboxides, in which this apparent anomaly of having an excess of metal atoms in relation to the oxide anions is fulfilled.

Where to find and obtaining

Earth crust

Lepidolite mineral sample. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

Rubidium is the 23rd most abundant element in the earth's crust, with an abundance comparable to that of the metals zinc, lead, cesium, and copper. The detail is that its ions are widely diffused, so it does not predominate in any mineral as the main metallic element, and its ores are also scarce.

It is for this reason that rubidium is a very expensive metal, even more than gold itself, since its process of obtaining from its ores is complex due to the difficulty of its exploitation.

In nature, given its reactivity, rubidium is not found in its native state, but as an oxide (Rb ₂ O), chloride (RbCl) or accompanied by other anions. Its “free” Rb ⁺ ions are found in the seas with a concentration of 125 µg / L, as well as in hot springs and rivers.

Among the minerals of the earth's crust that contain it in a concentration of less than 1% we have:

-Leucita, K

-Polucite, Cs (Si ₂ Al) O ₆ nH ₂ O

-Carnalite, KMgCl ₃ · 6H ₂ O

-Zinnwaldite, KLiFeAl (AlSi ₃) O ₁₀ (OH, F) ₂

-Amazonite, Pb, KAlSi ₃ O ₈

-Petalite, LiAlSi ₄ O ₁₀

-Biotite, K (Mg, Fe) ₃ AlSi ₃ O ₁₀ (OH, F) ₂

-Rubiclin, (Rb, K) AlSi ₃ O ₈

-Lepidolite, K (Li, Al) ₃ (Si, Al) ₄ O ₁₀ (F, OH) ₂

Geochemical Association

All of these minerals share one or two things in common: they are silicates of potassium, cesium or lithium, or they are mineral salts of these metals.

This means that rubidium has a strong tendency to associate with potassium and cesium; It can even substitute for potassium during the crystallization of minerals or rocks, as happens in the deposits of pegmatites when magma crystallizes. Thus, rubidium is a by-product of the exploitation and refining of these rocks and their minerals.

Rubidium can also be found in common rocks such as granite, clays and basalt, and even in carboniferous deposits. Of all natural sources, lepidolite represents its main ore and from which it is exploited commercially.

In carnalite, on the other hand, rubidium can be found as RbCl impurities with a content of 0.035%. And in higher concentration there are polucite and rubicline deposits, which can have up to 17% rubidium.

Its geochemical association with potassium is due to the similarity of their ionic radii; Rb ⁺ is larger than K ⁺, but the difference in sizes is not an impediment for the former to be able to replace the latter in its mineral crystals.

Fractional crystallization

Whether starting with lepidolite or polucite, or with any of the minerals mentioned above, the challenge remains the same to a greater or lesser degree: separate rubidium from potassium and cesium; that is, apply mixture separation techniques that allow having rubidium compounds or salts on the one hand, and potassium and cesium salts, on the other.

This is difficult since these ions (K ⁺, Rb ⁺ and Cs ⁺) share a great chemical similarity; They react in the same way to form the same salts, which hardly differ from each other thanks to their densities and solubilities. That is why fractional crystallization is used, so that they can crystallize slowly and in a controlled manner.

For example, this technique is used to separate a mixture of carbonates and alum from these metals. The recrystallization processes must be repeated several times to guarantee crystals of greater purity and free of co-precipitated ions; a rubidium salt that crystallizes with K ⁺ or Cs ⁺ ions on its surface or inside.

More modern techniques, such as the use of an ion exchange resin, or crown ethers as complexing agents, also allow the Rb ⁺ ions to be isolated.

Electrolysis or reduction

Once the rubidium salt has been separated and purified, the next and last step is to reduce the Rb ⁺ cations to the solid metal. To do this, the salt is melted and subjected to electrolysis so that rubidium precipitates on the cathode; or a strong reducing agent is used, such as calcium and sodium, capable of rapidly losing electrons and thus reducing rubidium.

Isotopes

Rubidium is found on Earth as two natural isotopes: ⁸⁵ Rb and ⁸⁷ Rb. The first has an abundance of 72.17%, while the second of 27.83%.

The ⁸⁷ Rb is responsible for this metal being radioactive; however, its radiation is harmless and even beneficial for dating analysis. Its half-life time (t _1/2) is 4.9 · 10 ¹⁰ years, the time span of which exceeds the age of the Universe. When it decays, it becomes the stable isotope ⁸⁷ Mr.

Thanks to this, this isotope has been used to date the age of earth minerals and rocks present since the beginning of the Earth.

In addition to the ⁸⁵ Rb and ⁸⁷ Rb isotopes, there are other synthetic and radioactive ones with variable and much shorter lifetimes; for example, the ⁸² Rb (t _1/2 = 76 seconds), ⁸³ Rb (t _1/2 = 86.2 days), ⁸⁴ Rb (t _1/2 = 32.9 days) and ⁸⁶ Rb (t _{1 / 2} = 18.7 days). Of all of them, ⁸² Rb is the most used in medical studies.

Risks

Metal

Rubidium is such a reactive metal that it must be stored in glass ampoules under an inert atmosphere so that it does not react with oxygen in the air. If the blister breaks, the metal can be placed in kerosene or mineral oil to protect it; however, it will end up being oxidized by the oxygen dissolved in them, giving rise to rubidium peroxides.

If, on the other hand, it is decided to place it on wood, for example, it will end up burning with a violet flame. If there is a lot of humidity, it will burn just by being exposed to the air. When a large chunk of rubidium is thrown into a volume of water, it explodes vigorously, even igniting the hydrogen gas produced.

Therefore, rubidium is a metal that not everyone should handle, since practically all its reactions are explosive.

Ion

Unlike metallic rubidium, its Rb ⁺ ions do not pose any apparent risk to living things. These dissolved in water interact with cells in the same way as K ⁺ ions do.

Therefore, rubidium and potassium have similar biochemical behaviors; however, rubidium is not an essential element, while potassium is. In this way appreciable amounts of Rb ⁺ can accumulate inside cells, red blood cells and viscera without negatively impacting the body of any animal.

In fact, an adult male with a mass of 80 kg has been estimated to contain about 37 mg of rubidium; and that in addition, an increase in this concentration in the order of 50 to 100 times does not lead to undesirable symptoms.

However, an excess of Rb ⁺ ions can end up displacing the K ⁺ ions; and consequently, the individual will suffer very strong muscle spasms until death.

Naturally, rubidium salts or soluble compounds can trigger this immediately, so none of them should be ingested. In addition, it can cause burns by simple contact, and among the most toxic we can mention the fluoride (RbF), hydroxide (RbOH) and cyanide (RbCN) of rubidium.

Applications

Gas collector

Rubidium has been used to capture or remove traces of gases that may be in vacuum-sealed tubes. Precisely due to their high tendency to capture oxygen and moisture in them, they eliminate them on their surface as peroxides.

Pyrotechnics

When rubidium salts burn, they give off a characteristic reddish-violet flame. Some fireworks have these salts in their composition so that they explode with these colors.

Supplement

Rubidium chloride has been prescribed to combat depression, as studies determined a deficit of this element in individuals suffering from this medical condition. It has also been used as a sedative and to treat epilepsy.

Bose-Einstein condensate

Atoms of the ⁸⁷ Rb isotope were used to create the first Bose-Einstein condensate. This state of matter consists in that atoms at a temperature quite close to absolute zero (0 K), are grouped or "condensed", behaving as if they were one.

Thus, rubidium was the protagonist of this triumph in the field of physics, and it was Eric Cornell, Carl Wieman and Wolfgang Ketterle who received the Nobel Prize in 2001 thanks to this work.

Tumor diagnosis

The synthetic radioisotope ⁸² Rb decays by emitting positrons, which is used to accumulate in potassium-rich tissues; such as those located in the brain or heart. It is therefore used to analyze the functionality of the heart and the presence of possible tumors in the brain by means of a positron emission tomography.

Component

Rubidium ions have found a place in different types of materials or mixtures. For example, his alloys have been made with gold, cesium, mercury, sodium, and potassium. It has been added to glasses and ceramics probably to increase their melting point.

In solar cells perovskites have been added as an important component. Likewise, its possible use as a thermoelectric generator, heat transfer material in space, fuel in ion propulsion engines, electrolytic medium for alkaline batteries and in atomic magnetometers has been studied.

Atomic clocks

With rubidium and cesium, the famous, highly precise atomic clocks have been made, used for example in GPS satellites with which the owners of their smartphones can know their location while moving on a road.

References

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