Rubidium oxide

Rubidium oxide is the chemical compound with the formula Rb₂O. Rubidium oxide is highly reactive towards water, and therefore it would not be expected to occur naturally. The rubidium content in minerals is often calculated and quoted in terms of Rb₂O. In reality, the rubidium is typically present as a component of (actually, an impurity in) silicate or aluminosilicate. A major source of rubidium is lepidolite, KLi₂Al(Al,Si)₃O₁₀(F,OH)₂ wherein Rb sometimes replaces K.

Rb₂O is a yellow colored solid. The related species Na₂O, K₂O, and Cs₂O are colorless, pale-yellow, and orange, respectively. Like most other alkali metal oxides, Rb₂O, adopts the anti-fluorite structure (Cs₂O, uniquely for an oxide, adopts the anti-CdCl₂ structure).

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Properties

Like other alkali metal oxides, Rb₂O is a strong base. Thus, Rb₂O reacts exothermically with water to form rubidium hydroxide.

Rb₂O + H₂O → 2 RbOH

So reactive is Rb₂O toward water that it is considered hygroscopic. Upon heating, Rb₂O reacts with hydrogen to rubidium hydroxide and rubidium hydride:^[4]

Rb₂O + H₂ → RbOH + RbH

Syntheses

For laboratory use, RbOH is usually used in place of the oxide. RbOH can be purchased for ca. US$5/g (2006). The hydroxide is more useful, less reactive toward atmospheric moisture, and less expensive than the oxide.

As for most alkali metal oxides,^[5] the best synthesis of Rb₂O does not entail oxidation of the metal but reduction of the anhydrous nitrate:

10Rb + 2 RbNO₃ → 6 Rb₂O + N₂

Typical for alkali metal hydroxides, RbOH cannot be dehydrated to the oxide. Instead, the hydroxide can be reduced to the oxide using Rb metal:

2 Rb + 2 RbOH → 2 Rb₂O + H₂

Metallic Rb reacts with O₂, as indicated by its tendency to rapidly tarnish in air. The tarnishing process is relatively colorful as it proceeds via bronze-colored Rb₆O and copper-colored Rb₉O₂.^[5] The suboxides of rubidium that have been characterized by X-ray crystallography include Rb₉O₂ and Rb₆O, as well as the mixed Cs-Rb suboxides Cs₁₁O₃Rb_n (n = 1, 2, 3).^[1]

The final product of oygenation of Rb is principally RbO₂, i.e. rubidium superoxide.

Rb + O₂ → RbO₂

This superoxide can then be reduced to Rb₂O using excess rubidium metal:

3 Rb + RbO₂ → 2 Rb₂O

Safety considerations

Rubidium oxide, like other strongly alkaline compounds, can cause skin burns upon contact See MSDS.

References

1. ^ Simon, A. ”Group 1 and 2 Suboxides and Subnitrides — Metals with Atomic Size Holes and Tunnels” Coordination Chemistry Reviews 1997, volume 163, Pages 253-270.doi:10.1016/S0010-8545(97)00013-1

1. Rubidium oxide at Engineering Database, accessed in August 2005.
2. Rubidium oxide at WebElements, accessed in December 2005.
3. Rubidium oxide at Fisher Scientific, accessed in August 2005.
4. H. Nechamkin, The Chemistry of the Elements, p 34; McGraw-Hill, New York, 1968.
5. A. F. Holleman, E. Wiberg, "Inorganic Chemistry" Academic Press: San Diego, 2001.