Cesium hydroxide monohydrate [35103-79-8]

Caesium hydroxide or cesium hydroxide (CsOH) is a chemical compound consisting of caesium ions and hydroxide ions. It is a strong base (pK_b=-1.76), much like the other alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. In fact, caesium hydroxide is corrosive enough to corrode through glass quickly.

Due to its high reactivity, caesium hydroxide is extremely hygroscopic. Laboratory caesium hydroxide is typically a hydrate.

It is an anisotropic etchant of silicon, exposing octahedral planes. This technique can form pyramids and regularly shaped etch pits for uses such as Microelectromechanical systems. It is known to have a higher selectivity to etch highly p-doped silicon than the more commonly used potassium hydroxide.

This compound is not commonly used in experiments due to the high extraction cost of caesium and its reactive behaviour. It acts in similar fashion to the compounds rubidium hydroxide and potassium hydroxide, although more reactive.