The rubidium It is a metallic element that belongs to group 1 of the periodic table: that of the alkali metals, 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.
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 "confuses" the K ions+ by those of Rb+; however, rubidium is not an essential element to date, as it is unknown what role it plays in metabolism. 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 to 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..
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Rubidium was discovered in 1861 by 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. His object of study was the mineral lepidolite, whose sample was collected from Saxony, Germany.
They started from 150 kg of lepidolite mineral, which they treated with chloroplatinic acid, HtwoPtCl6, to precipitate potassium hexachloroplatinate, KtwoPtCl6. 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 managed to separate the RbtwoPtCl6 from KtwoPtCl6 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. It was thus, that metallic rubidium was born.
Soft, silver-gray metal. It is so smooth that it looks like butter. It is usually packaged in glass ampoules, within which an inert atmosphere predominates that protects it from reacting with air..
37
85.4678 g / mol
39 ºC
688 ºC
At room temperature: 1.532 g / cm3
At melting point: 1.46 g / cm3
The density of rubidium is higher than that of water, so it will sink while reacting violently with it.
2.19 kJ / mol
69 kJ / mol
0.82 on the Pauling scale
46.9 kJ / mol
-First: 403 kJ / mol (Rb+ gaseous)
-Second: 2632.1 kJ / mol (Rbtwo+ gaseous)
-Third: 3859.4 kJ / mol (Rb3+ gaseous)
248 pm (empirical)
58.2 W / (mK)
128 nΩm at 20 ° C
0.3. Therefore, even talc is harder than metallic rubidium..
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.+.
Reacts with oxygen to form a mixture of peroxides (RbtwoORtwo) and superoxides (RbOtwo). Although it does not react with acids and bases, it does react violently with water, generating rubidium hydroxide and hydrogen gas:
Rb (s) + HtwoO (l) => RbOH (aq) + Htwo(g)
Reacts with hydrogen to form its corresponding hydride:
Rb (s) + Htwo(g) => 2RbH (s)
And also with halogens and sulfur explosively:
2Rb (s) + Cltwo(g) => RbCl (s)
2Rb (s) + S (l) => RbtwoH.H)
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..
Rubidium atoms are arranged in such a way as to 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:
[Kr] 5s1
All 5s orbitals with their single electron overlap in all dimensions of metallic rubidium crystals. However, these interactions are weak, because as one descends through 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.
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, RbtwoCO3 (Rbtwo+CO3two-)
-Rubidium monoxide, RbtwoO (Rbtwo+ORtwo-)
-Rubidium superoxide, RbOtwo (Rb+ORtwo-)
Although very rare, rubidium could also have a negative oxidation number: -1 (Rb-). In this case, we 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..
There are compounds where individually each Rb atom presents oxidation numbers with fractional values. For example, in the Rb6O (Rb6two+ORtwo-) and Rb9ORtwo (Rb94+ORtwotwo-) the positive charge is distributed among a set of Rb atoms (clusters). Thus, in the Rb6Or the oxidation number in theory would be +1/3; while in the Rb9ORtwo, + 0.444 (4/9).
The cluster structure of Rb is shown above.9ORtwo represented by a model of spheres and bars. Note how the nine Rb atoms “enclose” the O anions.two-.
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 Otwo-, 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.+. The Rb6O and Rb9ORtwo They 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.
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 (RbtwoO), chloride (RbCl) or accompanied by other anions. Its “free” ions Rb+ 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:
-Leucite, K [AlSitwoOR6]
-Polucita, Cs (SitwoHello6NHtwoOR
-Carnalite, KMgCl36HtwoOR
-Zinnwaldite, KLiFeAl (AlSi3)OR10(OH, F)two
-Amazonite, Pb, KAlSi3OR8
-Petalite, LiAlSi4OR10
-Biotite, K (Mg, Fe)3AlSi3OR10(OH, F)two
-Rubicline, (Rb, K) AlSi3OR8
-Lepidolite, K (Li, Al)3(Yes, Al)4OR10(F, OH)two
All these minerals share one or two things in common: they are potassium, cesium or lithium silicates, 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 a 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; the Rb+ is bigger than K+, but the difference in sizes is not an impediment for the former to replace the latter in its mineral crystals.
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 those of potassium and cesium, 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 ions+ or Cs+ on its surface or inside.
More modern techniques, such as the use of an ion exchange resin, or crown ethers as complexing agents, also make it possible to isolate Rb ions.+.
Once the rubidium salt has been separated and purified, the next and last step is to reduce the Rb cations+ to 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.
Rubidium is found on Earth as two natural isotopes: the 85Rb and 87Rb. The first has an abundance of 72.17%, while the second of 27.83%.
The 87Rb is responsible for this metal being radioactive; however, its radiation is harmless and even beneficial for dating analysis. Its half-life time (t1/2) is 4.91010 years, whose time span exceeds the age of the Universe. When it decays, it becomes the stable isotope 87Mr.
Thanks to this, this isotope has been used to date the age of terrestrial minerals and rocks present since the beginning of the Earth..
In addition to isotopes 85Rb and 87Rb, there are other synthetic and radioactive ones with variable and much shorter lifetimes; For example, him 82Rb (t1/2= 76 seconds), 83Rb (t1/2= 86.2 days), 84Rb (t1/2= 32.9 days) and 86Rb (t1/2= 18.7 days). Of all of them, the 82Rb is the most used in medical studies.
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 contrary, 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..
Unlike metallic rubidium, its Rb ions+ they do not represent any apparent risk to living beings. 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 affecting the body of any animal.
In fact, it has been estimated that an adult male with a mass of 80 kg contains 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+ may end up displacing the K ions+; and consequently, the individual will suffer very strong muscle spasms until death.
Obviously, soluble rubidium salts or 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.
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.
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.
Rubidium chloride has been prescribed to combat depression, as studies determined a deficiency of this element in individuals suffering from this medical condition. It has also been used as a sedative and to treat epilepsy.
Isotope Atoms 87Rb were used to create the first Bose-Einstein condensate. This state of matter consists in that the 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..
The synthetic radioisotope 82Rb decays by emitting positrons, which is used to accumulate in potassium-rich tissues; such as those located in the brain or heart. It is thus used to analyze the functionality of the heart and the presence of possible tumors in the brain by means of a positron emission tomography..
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 also been studied..
With rubidium and cesium, the famous highly precise atomic clocks have been manufactured, used for example in GPS satellites with which the owners of their smartphones can know their location while moving on a road.
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