Rubidium

Typical wastewaters

• municipal sewage
  Rb⁺, conservative and enriched relative to Sr²⁺ because blood and urine are rubidium rich; the dissolved Rb to Sr ratio traces the effluent plume
• geothermal brine and lithium mica processing
  Rb⁺, the hydrated ion, no complexation or hydrolysis
  named as industrial sources by the element chapter

1 · Identity

Symbol, number

Rb, 37

Oxidation states in water

+1 only, the hydrated Rb⁺ ion; no hydrolysis, no complexation of note, no redox chemistry

Note

The element chapter covers the metal and its violent reaction with water; dissolved rubidium is simply the ion left behind.

2 · Occurrence in water

Natural sources

Weathering of potassium feldspars and micas, in which rubidium substitutes for potassium; potash brines and evaporites (element chapter). It stays in solution like potassium and is taken up by plants and animals with it.

Anthropogenic sources

Sewage effluent carries rubidium enriched relative to strontium because blood and urine are Rb enriched; the dissolved Rb to Sr ratio traced the Blue Plains outfall plume in the tidal Potomac. Geothermal brines and lithium mica processing are the industrial sources (element chapter).

3 · Speciation

Rb⁺ throughout; no hydrolysis, no significant ion pairing, no redox change. It follows potassium through water treatment and the environment and is taken up by ion exchange onto clays somewhat more strongly than potassium, by analogy with caesium.

Solubility

All common salts are freely soluble.

Hydrolysis

None.

Complexation

Negligible.

Precipitates

None.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Rb is conservative and easy; only the Rb to Sr ratio, not the concentration, carries the sewage signal, so measure both on the same filtered, acidified sample.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

No drinking water guideline. The US EPA provisional (PPRTV, 2016) assessment derived only a screening subchronic reference dose of 5 x 10⁻3 mg/kg per day for rubidium chloride (human LOAEL 5.3 mg/kg per day in small antidepressant trials: weight gain, diarrhoea, nausea, polyuria, confusion, agitation; uncertainty factor 1000), no chronic value, and found no IRIS, HEAST or drinking water health advisory value. Rats given 1,200 mg/L rubidium chloride in drinking water (167 mg/kg per day) for 10 days showed altered saliva flow and composition; mice at 299 to 896 mg/kg per day in drinking water for 3 weeks developed sound-induced convulsive seizures and deaths, worse when dietary potassium and magnesium were deficient. Rat studies indicated that lower doses take longer to produce toxicity, so no chronic value was derived.

Bioaccumulation

Taken up with potassium by all organisms; blood and urine are Rb enriched relative to Sr, which is the basis of the effluent tracer.

Identity

Name and symbol

Rubidium, Rb

Atomic number

37 protons

Position

group 1 · period 5 · s-block · alkali metal

CAS number

7440-17-7

Atomic structure

Atomic mass

85.4678 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹
[Kr] 5s¹

Electrons per shell

2, 8, 18, 8, 1

Valence electrons

1 outer shell

Physical properties

State at room temperature

Solid

Melting point

312.46 K (39.31 °C)

Boiling point

961 K (687.85 °C)

Density

1.53 g/cm3

Appearance

grey white

Thermal conductivity

58.2 W/(m·K)

Electrical resistivity

128 nΩ·m at 20 °C

Electrical conductivity

7.81 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

31.06 J/(mol·K)

Chemical properties

Oxidation states

+1

Electronegativity

0.82 (Pauling Scale)

Ionisation energy

4.177 eV
1st 403, 2nd 2,633, 3rd 3,860 kJ/mol

Electron affinity

0.468 eV

Atomic radius

empirical 220, covalent 220, van der Waals 303 pm

Ionic radius

Rb⁺ 152 pm

Reactivity

A soft alkali metal, the second most electropositive element after caesium: one loosely held 5s electron makes it ignite spontaneously in air and react violently with water, so it is stored under dry mineral oil or in an inert atmosphere.

with water

Reacts violently with water, sinking as it goes, to rubidium hydroxide and hydrogen, which the heat of reaction ignites: $\ce{2 Rb + 2 H2O -> 2 RbOH + H2}$.

with oxygen, air

Ignites spontaneously in air; in excess oxygen it burns to the superoxide: $\ce{Rb + O2 -> RbO2}$, with Rb2O and suboxides formed in limited air.

with acids

Reacts explosively with dilute acids to the rubidium salt and hydrogen: $\ce{2 Rb + 2 HCl -> 2 RbCl + H2}$.

with halogens

Burns in the halogens to the rubidium halides RbF, RbCl, RbBr and $\ce{RbI: 2 Rb + Cl2 -> 2 RbCl}$.

Typical compounds

• RbOH rubidium hydroxide
  strong base, starting point for rubidium chemistry
• RbCl rubidium chloride
  the metal is made by reducing it with calcium
• RbO₂ rubidium superoxide
  product of burning the metal in oxygen
• Rb₂CO₃ rubidium carbonate
  used in optical glasses
• RbNO₃ rubidium nitrate
  purple colour in fireworks
• RbAg₄I₅ rubidium silver iodide
  highest room-temperature conductivity of any ionic crystal

Occurrence, production and use

Crustal abundance

9.0×101 milligrams per kilogram

Oceanic abundance

1.2×10-1 milligrams per liter

Occurrence and sources

The element is much more abundant than was thought several years ago. It is now considered to be the 16th most abundant element in the earth's crust. Rubidium occurs in pollucite, leucite, and zinnwaldite, which contains traces up to 1%, in the form of the oxide. It is found in lepidolite to the extent of about 1.5%, and is recovered commercially from this source. Potassium minerals, such as those found at Searles Lake, California, and potassium chloride recovered from the brines in Michigan also contain the element and are commercial sources. It is also found along with cesium in the extensive deposits of pollucite at Bernic Lake, Manitoba.

• lepidolite (lithium mica, up to 3.5 percent Rb2O)
  zoned granite pegmatites; the commercial source as a by-product of lithium extraction
• pollucite (caesium aluminosilicate, up to 1.5 percent Rb2O)
  pegmatites; by-product of caesium mining (Bikita Zimbabwe depleted 2018, Sinclair Australia mined out 2019)
• carnallite, leucite, potash brines and evaporites
  brines in northern Chile and China; evaporites in New Mexico, Utah, France, Germany
• crustal and oceanic abundance
  about 90 ppm ¹; 90 mg/kg crust and 0.12 mg/L seawater ²

Extraction, production

By-product concentrate from lepidolite and pollucite processing

Rubidium concentrate is recovered while lithium or caesium is extracted from the ore and imported for chemical conversion; no official world production figure exists for 2024 and neither USGS nor RSC states a reaction for the separation or for making the metal

Uses

Rubidium is used in vacuum tubes as a getter, a material that combines with and removes trace gases from vacuum tubes. It is also used in the manufacture of photocells and in special glasses. Since it is easily ionized, it might be used as a propellant in ion engines on spacecraft. Recent discoveries of large deposits of rubidium suggest that its usefulness will increase as its properties become better understood.

Rubidium forms a large number of compounds, although none of them has any significant commercial application. Some of the common rubidium compounds are: rubidium chloride (RbCl), rubidium monoxide (Rb2O) and rubidium copper sulfate Rb2SO4·CuSO4·6H20). A compound of rubidium, silver and iodine, RbAg4I5, has interesting electrical characteristics and might be useful in thin film batteries.

Because rubidium can be easily ionized, it has been considered for use in "ion engines" for space vehicles; however, cesium is somewhat more efficient for this purpose. It is also proposed for use as a working fluid for vapor turbines and for use in a thermoelectric generator using the magnetohydrodynamic principle where rubidium ions are formed by heat at high temperature and passed through a magnetic field. These conduct electricity and act like an amature of a generator thereby generating an electric current. Rubidium is used as a getter in vacuum tubes and as a photocell component. It has been used in making special glasses. RbAg4I5 is important, as it has the highest room conductivity of any known ionic crystal. At 20°C its conductivity is about the same as dilute sulfuric acid. This suggests use in thin film batteries and other applications.

• Glass and fibre optics: rubidium carbonate added to speciality glass to lower electrical conductivity and improve durability in fibre-optic networks; the leading market
• Electronics and timekeeping
  • photoemissive cathodes in photocells, photomultipliers, night-vision and motion sensors
  • rubidium atomic frequency standards for telecom synchronisation and GPS; rubidium fountain clocks in the US Naval Observatory timescale
  • ultracold rubidium atoms in quantum-computing research
  • oxygen getter in vacuum tubes
• Chemicals: catalyst or promoter in ammonia synthesis, sulfuric acid synthesis, hydrogenation, oxidation and polymerisation reactions
• Medicine
  • rubidium-82 as a blood-flow tracer in PET imaging
  • rubidium salts in antishock agents and research on epilepsy, thyroid disorder and depression
• Pyrotechnics: rubidium nitrate and hydroxide for violet colour and as oxidiser

Safety, toxicity

GHS classification, signal word Danger

• H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases
• H314 Causes severe skin burns and eye damage Skin corrosion/irritation

Discovery and name

Discovered by

Gustav Kirchhoff and Robert Bunsen

Discovered

1861

First isolated

George de Hevesy

Named by

not in sources

Origin of the name

from Latin rubidus, 'deep red', for the color of its emission spectrum

Rubidium can be liquid at room temperature. It is a soft, silvery-white metallic element of the alkali group and is the second most electropositive and alkaline element. It ignites spontaneously in air and reacts violently in water, setting fire to the liberated hydrogen. As with other alkali metals, it forms amalgams with mercury and it alloys with gold, cesium, sodium, and potassium. It colors a flame yellowish violet. Rubidium metal can be prepared by reducing rubidium chloride with calcium, and by a number of other methods. It must be kept under a dry mineral oil or in a vacuum or inert atmosphere.