Chemical Properties
Soft, silvery-white solid. Easily oxidized
in air.High heat capacity and heat transfer
coefficient. Soluble in acids and alcohol.
General Description
A soft silvery metal. Shipped in very limited quantities sealed in a copper tube and over packed in a wooden box. Used in electronics.
Reactivity Profile
RUBIDIUM METAL is a strong reducing agent. Burns spontaneously in dry oxygen [Mellor 2:468 1946-47]. Readily catches fire in air when molten or with a sulfur vapor [Mellor 2: 469 1946-47]. Causes explosive decomposition of maleic anhydride. [Chem Safety Data Sheet SD-88 1962; Chem. Haz. Info. Series C-71 1960] Burns in chlorine [Mellor 2, Supp. 1:380 1956]. Interaction with mercury is exothermic and may be violent, [Mellor, 1941, Vol. 2, 469].
Air & Water Reactions
Tarnishes rapidly upon exposure to air. Reacts violently with water to form corrosive RUBIDIUM(7440-17-7) hydroxide and hydrogen, a flammable gas. The heat of the reaction usually ignites the hydrogen.
Hazard
Reacts vigorously with air and water, must
be stored under kerosene or similar liquid, danger-
ous fire and explosion risk. Metal causes serious
skin burns.
Health Hazard
Inhalation or contact with vapors, substance or decomposition products may cause severe injury or death. May produce corrosive solutions on contact with water. Fire will produce irritating, corrosive and/or toxic gases. Runoff from fire control may cause pollution.
Fire Hazard
Produce flammable gases on contact with water. May ignite on contact with water or moist air. Some react vigorously or explosively on contact with water. May be ignited by heat, sparks or flames. May re-ignite after fire is extinguished. Some are transported in highly flammable liquids. Runoff may create fire or explosion hazard.
Isotopes
There are 30 isotopes of rubidium, ranging from Rb-75 to Rb-98. Rb-85 is theonly stable form of rubidium and constitutes 72.17% of all rubidium isotopes found inthe Earth’s crust. Rb-87 is radioactive (a half-life of 4.9×1010 years) and makes up about27.83% of the remainder of rubidium found in the Earth’s crust. All the other 28 isotopes make up a tiny fraction of all the rubidium found on Earth and are radioactive withvery short half-lives.
Origin of Name
Rubidium is named for the Latin word rubidus, meaning “reddish.
Occurrence
Rubidium does not exist in its elemental metallic form in nature. However, in compoundforms it is the 22nd most abundant element on Earth and, widespread over most land areasin mineral forms, is found in 310 ppm. Seawater contains only about 0.2 ppm of rubidium,which is a similar concentration to lithium. Rubidium is found in complex minerals and untilrecently was thought to be a rare metal. Rubidium is usually found combined with other Earthmetals in several ores. The lepidolite (an ore of potassium-lithium-aluminum, with traces ofrubidium) is treated with hydrochloric acid (HCl) at a high temperature, resulting in lithiumchloride that is removed, leaving a residue containing about 25% rubidium. Another processuses thermochemical reductions of lithium and cesium ores that contain small amounts ofrubidium chloride and then separate the metals by fractional distillation.
Characteristics
Rubidium is located between potassium and cesium in the first group in the periodic table.It is the second most electropositive alkali element and reacts vigorously and explosively in airor water. If placed on concrete on a sunny day, it would melt and then react violently withmoist air to release hydrogen with enough heat to burn the hydrogen. If a chunk of rubidiummetal is left on a table exposed to the air, it combusts spontaneously. Rubidium must be storedin oil, such as kerosene.
Preparation
Although rubidium metals have been prepared by fused salt electrolysis, the highly reactive nature of the metals complicates the collection step and favors the use of other preparative methods where the metals can be removed in vapor form from the reaction mixture. The oxides, hydroxides, carbonates, halides, sulphates, chromates and nitrates of rubidium have been reduced to the metals by strong reducing metals such as sodium, calcium, magnesium, barium, iron, zirconium, aluminum or silicon at moderately high temperatures. The preferred method, however, involves the reduction of the anhydrous metal chlorides with calcium metal under vacuum. Anhydrous rubidium chloride is mixed with a large excess of calcium chips and heated under vacuum at 700- 800°C. As the chloride is reduced, metal vapors issue from the reaction mixture and are led under the vacuum to a cooler portion of the vessel where they condense and drop into a collection vessel.
Production Methods
Rubidium is recovered from its ore lepidolite or pollucite. Mineral lepidolite is a lithium mica having a composition: KRbLi(OH,F)Al2Si3O10. The ore is opened by fusion with gypsum (potassium sulfate) or with a mixture of barium sulfate and barium carbonate. The fused mass is extracted with hot water to leach out water-soluble alums of cesium, rubidium, and potassium. The solution is filtered to remove insoluble residues. Alums of alkali metals are separated from solution by fractional crystallization. Solubility of rubidium alum or rubidium aluminum sulfate dodecahydrate, RbAl(SO4)2?12H2O falls between potassium and cesium alum.
Alternatively, the mineral is opened by prolonged heating with sulfuric acid. Often calcium fluoride (fluorspar) is added for removal of silicon. Alkali metals are converted into water-soluble sulfates. After filtering residual solid, the solution is treated with ammonium or potassium carbonate or carbon dioxide. Lithium precipitates as lithium carbonate. Alkali metal carbonates are converted back to alums and separated by fractional crystallization.
Rubidium alum obtained by either method above is decomposed by treatment with alkali solutions for removal of aluminum and sulfate. Aluminum is precipitated as aluminum hydroxide. Addition of barium hydroxide to the filtrate removes sulfate, precipitating barium sulfate. Evaporation of the solution crystallizes rubidium as hydroxide.
Rubidium also may be recovered by the chlorostannate method. In this method the alkali metal carbonate solution obtained from the mixed alum is treated with carbon dioxide. Most potassium is precipitated as bicarbonate, KHCO3. Addition of hydrochloric acid converts the carbonates to chlorides. The chlorides are converted to chlorostannates by carefully adding stoichiometric quantities of stannic chloride at pH just below 7:
2RbCl + SnCl4 → Rb2SnCl6
Cesium chlorostannate, Cs2SnCl6, more insoluble than the rubidium salt, precipitates before any rubidium starts to precipitate. Under such controlled addition of stannic chloride, potassium chloride remains in solution in chloride form. Rubidium chlorostannate complex, on thermal decomposition, forms rubidium chloride, RbCl.
Rubidium metal may be obtained from its carbonate, hydroxide or chloride by reduction with magnesium or calcium at high temperatures in the presence of hydrogen:
Rb2CO3 + 3Mg → 2Rb + 3MgO +C
2RbOH + Mg → 2Rb + Mg(OH)2
2RbCl + Ca → 2Rb + CaCl2
Rubidium is a flammable solid. It is stored in dry hexane, isooctane or other saturated hydrocarbon liquids. Alternatively, the metal may be packaged and stored in well-sealed borosilicate glass ampules or stainless-steel containers under vacuum or an inert atmosphere.
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