Rubidium Hydroxide

Q: What is the molar mass of RbOH?

RbOH has a molar mass of 102.475 g/mol, from Rb (85.468) plus O (15.999) plus H (1.008). Rubidium accounts for 83% of the mass — and at roughly $40-100 per gram for analytical-grade RbOH, that means a 100 g bottle of the reagent costs as much as several kilograms of bench-grade KOH, which is the dominant reason RbOH is rare in industrial chemistry and almost exclusively a research reagent.

Q: How does RbOH compare to NaOH in practical use?

As bases in dilute aqueous solution they are indistinguishable — both ionize completely. The differences show up at higher concentration, in non-aqueous solvents, and in specific reactions where cation size matters. RbOH is roughly 1.5x more soluble than NaOH at room temperature, dissolves more exothermically, and pairs with the larger Rb+ that solvates differently in DMSO or DMF. In an industrial setting these subtle gains never justify the 100x price differential, which is why NaOH is the chloralkali-process workhorse and RbOH lives in research labs.

Q: Why do alkali-metal hydroxide base strengths increase down the group?

All the alkali-metal hydroxides are strong bases — they fully ionize in water at any practical concentration. The 'strength' trend (LiOH < NaOH < KOH < RbOH < CsOH) is more visible in non-aqueous solvents and in the heat of dissolution, and comes from the lattice-energy trend: as the M+ cation gets larger and more polarizable, M-OH lattice energy drops, the salt dissolves more readily and exothermically, and the hydration enthalpy of the bare M+ drops as well. In molten state and in aprotic solvents these effects are large enough that CsOH or RbOH will deprotonate weak C-H acids that NaOH cannot touch.

RbOH base

Molar Mass

102.475 g/mol

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Properties

State	Solid (extremely hygroscopic)
Color	Grayish-white
Solubility	Extremely soluble in water (exothermic dissolution); 180 g/100 mL at 15 °C
Melting Point	301 °C
Boiling Point	Decomposes before boiling

About Rubidium Hydroxide

Rubidium hydroxide is a strong inorganic base (RbOH, 102.475 g/mol) — a grayish-white, deeply hygroscopic solid that absorbs water and CO2 from the air so aggressively that within minutes of opening a fresh bottle the surface has crusted over with Rb2CO3. It dissolves exothermically to 180 g per 100 mL at 15 C and ionizes essentially completely to Rb+ and OH-, putting it firmly at the strong-base end of the alkali-metal hydroxide series. The textbook periodic trend that LiOH < NaOH < KOH < RbOH < CsOH in basicity comes from the same chemistry: as the cation gets larger, the lattice energy of the hydroxide drops and the salt dissociates more completely in water. RbOH and CsOH together sit at the top of the trend and are essentially indistinguishable as bases for most practical purposes — Cs gets used more in research labs simply because cesium chemistry has a longer track record. Industrial use of RbOH is sharply limited by economics: rubidium is recovered as a minor byproduct from lithium and cesium ore processing (mainly the lepidolite deposits in Bikita, Zimbabwe and Greenbushes, Australia), so global production is small and the price per gram is roughly 100x that of KOH. The compound finds use in specialty electrochemistry research (notably in low-temperature alkaline batteries where RbOH electrolyte improves performance below 0 C), as a catalyst in some siloxane polymerization reactions, in vacuum-tube cathode coatings, and as the base of choice for studies that need to isolate the effect of cation size on reaction kinetics.

Where you'll encounter it

If you have ever tried to do quantitative reactions with the heavier alkali-metal hydroxides and watched the open beaker visibly fog up with hygroscopic uptake within seconds, you have seen why RbOH (and CsOH even more so) is the kind of reagent you weigh out inside a glove box under argon and use the same day — leave it in air for an hour and the analytical concentration of your stock solution has drifted by several percent through CO2 absorption. In low-temperature alkaline batteries (the kind used in polar field instruments and military hardware deployed below -30 C), trace RbOH or CsOH added to KOH electrolyte raises the freezing point depression and ionic conductivity enough to keep the cell functional where standard KOH electrolyte would freeze and the cell would fail. In atomic physics, gram quantities of rubidium metal (often produced from RbOH plus calcium reduction at high temperature) end up in the magneto-optical traps used for laser cooling experiments — Rb-87 is the canonical alkali for Bose-Einstein condensate work, with a D2 line at 780 nm conveniently accessible to inexpensive diode lasers.

Common Uses

Specialty electrolyte additive for low-temperature alkaline batteries (below 0 C)
Strong-base reagent for kinetic studies isolating cation-size effects
Catalyst for siloxane polymerization and selected organic reactions
Cathode coating in specialty vacuum tubes and photoemissive devices
Reduction precursor for producing rubidium metal for laser-cooling experiments

Safety Information

GHS: H290 corrosive to metals, H314 causes severe skin burns and eye damage, H335 may cause respiratory irritation. Equivalent in corrosivity to NaOH or KOH but more reactive in air. The exothermic dissolution can splash on contact with water and the heat-of-hydration spike will crack glassware if the powder is added to a partially filled flask. OSHA PEL ceiling for sodium and potassium hydroxide aerosol is 2 mg/m3 and the same value should be observed for RbOH. Store under inert atmosphere or in a sealed container with molecular-sieve desiccant. PPE: full face shield, butyl rubber gloves, chemical-resistant apron, fume hood. Have a calcium gluconate gel and large-volume eyewash within reach when handling.

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.

Constituent Elements

Rb — Rubidium O — Oxygen H — Hydrogen

Frequently Asked Questions

What is the molar mass of RbOH?

RbOH has a molar mass of 102.475 g/mol, from Rb (85.468) plus O (15.999) plus H (1.008). Rubidium accounts for 83% of the mass — and at roughly $40-100 per gram for analytical-grade RbOH, that means a 100 g bottle of the reagent costs as much as several kilograms of bench-grade KOH, which is the dominant reason RbOH is rare in industrial chemistry and almost exclusively a research reagent.

How does RbOH compare to NaOH in practical use?

As bases in dilute aqueous solution they are indistinguishable — both ionize completely. The differences show up at higher concentration, in non-aqueous solvents, and in specific reactions where cation size matters. RbOH is roughly 1.5x more soluble than NaOH at room temperature, dissolves more exothermically, and pairs with the larger Rb+ that solvates differently in DMSO or DMF. In an industrial setting these subtle gains never justify the 100x price differential, which is why NaOH is the chloralkali-process workhorse and RbOH lives in research labs.

Why do alkali-metal hydroxide base strengths increase down the group?

All the alkali-metal hydroxides are strong bases — they fully ionize in water at any practical concentration. The 'strength' trend (LiOH < NaOH < KOH < RbOH < CsOH) is more visible in non-aqueous solvents and in the heat of dissolution, and comes from the lattice-energy trend: as the M+ cation gets larger and more polarizable, M-OH lattice energy drops, the salt dissolves more readily and exothermically, and the hydration enthalpy of the bare M+ drops as well. In molten state and in aprotic solvents these effects are large enough that CsOH or RbOH will deprotonate weak C-H acids that NaOH cannot touch.