Rubidium chloride

Rubidium chloride is the alkali metal halide RbCl. This alkali halide finds diverse uses, from electrochemistry to molecular biology.

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Structure

In its gas phase, RbCl is diatomic with a bond length estimated at 2.7868 Å^[1]. This distance increases to 3.285 Å for cubic RbCl, reflecting the higher coordination number of the ions in the solid phase.^[2]

Depending on conditions, solid RbCl exists in one of three arrangements or polymorphs as determined with holographic imaging^[3]:

Sodium chloride’s octahedral 6:6

The NaCl polymorph is most common. A cubic close-packed arrangement of chloride anions with rubidium cations filling the octahedral holes describes this polymorph.^[4] Both ions are six-coordinate in this arrangement. This polymorph’s lattice energy is only 3.2 kJ/mol less than the following structure’s^[5].

Cesium chloride’s cubic 8:8

At high temperature and pressure, RbCl adopts the CsCl structure (NaCl and KCl undergo the same structural change at high pressures). Here, the chloride ions form a body-centered cubic arrangement with chloride anions occupying the vertices of a cube surrounding a central Rb⁺. This is RbCl’s densest packing motif.^[6] Because a cube has eight vertices, both ions’ coordination numbers equal eight. This is RbCl’s highest possible coordination number. Therefore, according to the radius ratio rule, cations in this polymorph will reach their largest apparent radius because the anion-cation distances are greatest^[7].

Sphalerite’s tetrahedral 4:4

The sphalerite polymorph of rubidium chloride is extremely rare, resulting in few structural studies. The lattice energy, however, for this formation is predicted to nearly 40.0 kJ/mol smaller than those of the preceding structures.^[8]

Synthesis^[9]

• The most common preparation of pure rubidium chloride involves the reaction of its hydroxide with hydrochloric acid, followed recrystallization:

RbOH_(aq) + HCl_(aq) → RbCl_(aq) + H₂O_(l)

• An equilibrium reaction involving volatile rubidium, metallic sodium, and molten rubidium chloride is another plausible pathway:

Rb_(s) + NaCl_(s) RbCl_(l) + Na_(s)

• An expensive technique requires the reaction of rubidium metal with the desired halogen:

2Rb_(s) + Cl_{2 (g)} → 2RbCl_(s)

Because RbCl is hygroscopic, it must be protected from atmospheric moisture, e.g. using a desiccator. RbCl is primarily used in laboratories. Therefore, numerous suppliers (see below) produce it in smaller quantities as needed. It is offered in a variety of forms for chemical and biomedical research.

Uses

• Generally, RbCl can be used as an electrolyte in water.
• Nanowires, an electrochemical prospect, are well oriented by dilute RbCl ions on mica^[10].
• Rubidium chloride has been shown to modify coupling between circadian oscillators via reduced photic input to the suprachiasmatic nuclei. The outcome is a more equalized circadian rhythm, even for stressed organisms^[11].
• Infusing tumor cells with rubidium chloride increases their pH. Some researchers believe that this increase prohibits the activation of enzymes such as oncogenic phosphatases and that would usually increase the cells’ malignant potential. This is thought to occur through rubidium chloride’s inactivation of essential ionic hydrogen.
• RbCl is an excellent non-invasive biomarker. The compound dissolves well in water and readily be taken up by organisms. Once broken in the body, Rb⁺ replace K⁺ in tissues because they are from the same chemical group^[12]. An example of this is the use of a radioactive isotope to evaluate perfusion of heart muscle.
• RbCl transformation for competent cells is arguably the compound’s most abundant use. Cells are treated with a hypotonic solution containing RbCl expand. Resultantly, the expulsion of membrane proteins allows negatively charged DNA to bind^[13].

References

1. ^ Lide, D.R.; Cahill, P.; Gold, JL.P. (1963) Cohesion and polymorphism in solid rubidium chloride. Journal of Chemical Physics 40 pp. 156-159
2. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, (Oxford University Press)pp. 410 & 444
3. ^ Kopecky, M.; Fábry, J.; Kub, J.; Busetto, E.; Lausi, A. (2005) X-ray diffuse scattering holography of a centrosymmetric sample. Applied Physics Letters 87 3p
4. ^ Pyper, N.C.; Kirkland, A. I.; Harding, J. H. (2006) Cohesion and polymorphism in solid rubidium chloride. Journal of Physics: Condensed Matter 18 pp. 683-702
5. ^ Shriver, D.F.; Atkins, P.W.; Cooper, H.L. (1990) Inorganic Chemistry, (Freeman), ch. 2 .
6. ^ Winter, Mark, Ph.D. (2006) Compounds of Rubidium. WebElements.
7. ^ Akutagawa, T.; Ohta, T.; Hasegawa, T.; Nakamura, T.; Christensen, C.A.; Becher, J. (2002) Formation of oriented molecular nanowires on mica surface Proceedings of the National Academy of Sciences 99 pp. 5028-33.
8. ^ Hallonquist, J.; Lindegger, M.; Mrosovsky, N. (1994) Rubidium chloride fuses split circadian activity rhythms Proceedings of the National Academy of Sciences 11 pp. 65-71.
9. ^ Hougardy, E.; Pernet, P.; Warnau, M.; Delisle, J.; Grégoire, J.-C.(2003) Marking bark beetle parasitoids within the host plant with rubidium for dispersal studies Entomologia Experimentalis et Applicata 108 pp. 107.
10. ^ New England Biolabs, Inc. (2006) RbCl Transformation Protocol

Suppliers

• GFS Chemicals
• Sigma-Aldrich
• Chem Exper