Caesium hydroxide

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Caesium hydroxide or cesium hydroxide is a crystalline solid with the formula CsOH. It is notable for being the strongest normal, aqueous base; all bases stronger than it are superbases that react with water.



Caesium hydroxide is a crystalline, hygroscopic white solid resembling potassium hydroxide.


Caesium hydroxide is a very reactive alkali. It reacts with glass:

2 CsOH + CaSiO3 → Cs2SiO3 + Ca(OH)2
2 CsOH + SiO2 → Cs2SiO3 + H2O

The resulting salt is caesium silicate, or caesium waterglass, which is water-soluble and similar in properties to common waterglass. Because of this, it is best to keep this compound in polyethylene bottles.

Caesium hydroxide readily reacts with carbon dioxide in the air:

2 CsOH + CO2 → Cs2CO3 + H2O
CsOH + CO2 → CsHCO3

It is known to react with excess nitric acid forming unusual acidic adducts:

CsOH + 2HNO3 → CsHN2O6 + H2O

Availability and synthesis

Caesium hydroxide can be purchased from various reagent suppliers. It can also be synthesized from elemental caesium, which is sold to element collectors. There are two methods of doing this.

Kewl method: breaking the ampoule with caesium under water, remotely, from a safe distance. The reaction results in an explosion. Make sure that the vessel in which you perform the reaction is strong enough to contain it.

Safe method: treating caesium metal with small amounts of water vapor. This requires a glovebox with an inert atmosphere. Beware of hydrogen buildup in the glovebox and make sure no oxygen is present in it!

Another safer way is to carefully let the cesium metal oxidize in open air, in a corrosion resistant container. After the metal has completely oxidized, slowly add water vapor, as the reaction is highly exothermic and dry the resulting wet hydroxide.



Caesium hydroxide is a very corrosive substance. Caesium ions however, are not toxic.


Store this compound in closed polyethylene bottles with no access to air.


Disposing of caesium compounds is not recommended because of their rarity and price. But if you absolutely want to, you should neutralize the base with any non-toxic acid.


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