Rubidium Carbonate
Properties
| State | Solid (hygroscopic) |
| Color | White |
| Solubility | Very soluble in water (450 g/L at 20 °C); slightly soluble in ethanol and methanol |
| Melting Point | 837 °C (decomposes) |
About Rubidium Carbonate
Rubidium carbonate is a white, deeply hygroscopic alkali-metal carbonate (Rb2CO3, 230.944 g/mol) prepared either by neutralizing RbOH with CO2 or by metathesizing Rb2SO4 with BaCO3 and filtering off the insoluble BaSO4. It dissolves to about 450 g/L at 20 C, gives a strongly basic solution (pH around 12 for a 1% solution from carbonate hydrolysis), and unusually for an alkali-metal carbonate it shows measurable solubility in polar organic solvents like methanol and ethanol — a property it shares with cesium carbonate that makes both compounds useful as mild, soluble bases in synthetic organic chemistry. The dominant industrial use is as a refractive-index modifier in specialty optical glass, where 1-3 mol% Rb2CO3 raises the refractive index, lowers the glass transition temperature, and improves the thermal-shock resistance of crown-glass formulations used in camera lenses and fiber-optic preforms. The compound also appears as a flux in ceramic glazes, as the standard precursor for synthesizing other rubidium salts in the lab, and increasingly in perovskite-photovoltaic research, where adding 5-10 mol% RbI to mixed-cation FAPbI3-MAPbI3 perovskite ink stabilizes the photoactive black phase against the parasitic yellow delta-phase that otherwise destroys efficiency.
Where you'll encounter it
If you've ever tried Cs2CO3 as the base in an O-alkylation or N-alkylation reaction and it worked beautifully where K2CO3 failed, Rb2CO3 is the cheaper and slightly less basic cousin that often does the same job — solubility in DMF and acetone is what makes the alkali-metal carbonates effective for these reactions, and the rubidium salt sits between potassium and cesium on both solubility and basicity. The reason you have not heard of it as much is purely economic: rubidium is recovered as a byproduct of lithium and cesium processing from lepidolite mines in Australia and Zimbabwe, and global production is on the order of a few tonnes per year versus thousands of tonnes for Cs2CO3. In perovskite solar cell research, the four-cation 'CsFAMARb' formulation that hit certified efficiency above 23% in 2017 owes its phase stability to that 5 mol% Rb addition stabilizing the cubic black perovskite phase against the room-temperature delta-yellow transition.
Common Uses
- Refractive-index modifier in specialty optical glass and fiber-optic preforms
- Mild soluble base for O- and N-alkylation in synthetic organic chemistry
- Phase-stabilizing dopant in mixed-cation perovskite solar cells (5-10 mol%)
- Flux in ceramic glaze formulations and specialty enamels
- Standard lab precursor for synthesizing other rubidium compounds
Safety Information
GHS: Skin corrosion/irritation Category 2, Serious eye damage Category 1. Strongly basic in solution; reacts with acids releasing CO2 effervescence. Dust is a respiratory irritant and the powder is deeply hygroscopic — store in a tightly sealed container with desiccant. OSHA has no specific PEL for rubidium compounds, but the alkali-carbonate dust limit of 5 mg/m3 (respirable nuisance dust) is the practical guideline. Use chemical-splash goggles, nitrile gloves, and a P100 respirator when transferring the dry powder. Soluble rubidium has low acute toxicity but chronic intake disturbs potassium homeostasis; oral LD50 in rats is around 1.6 g/kg.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.