Tetramethylammonium hydroxide pentahydrate

Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with the molecular formula N(CH3)4+ OH−, and is one of the simplest members of this class of organic compounds. This substance is known in a (relatively) stable solid form only as the pentahydrate. Commercially, the forms in which TMAH is most commonly encountered are as concentrated solutions in water or methanol. The solid and solutions are colorless, or yellowish if impure. Although TMAH has virtually no odor when pure, samples often have a strongly fishy smell from the trimethylamine which is commonly present as an impurity. TMAH has numerous and diverse industrial and research applications (see below).