Methanesulfonic acid (MsOH) is a colorless liquid with the chemical formula CH3SO3H. It is the simplest of the alkylsulfonic acids. Salts and esters of methanesulfonic acid are known as mesylates (or methanesulfonates, as in ethyl methanesulfonate). It is hygroscopic in its concentrated form. Methanesulfonic acid may be considered an intermediate compound between sulfuric acid (H2SO4), and methylsulfonylmethane ((CH3)2SO2), effectively replacing an OH group with a CH3 group at each step. This pattern can extend no further in either direction without breaking down the SO2 group. Methanesulfonic acid can dissolve a wide range of metal salts, many of them in significantly higher concentrations than in hydrochloric or sulfuric acid.
Methanesulfonic acid is used as an acid catalyst in organic reactions because it is a non-volatile, strong acid that is soluble in organic solvents. Methanesulfonic acid is convenient for industrial applications because it is liquid at ambient temperature, while the closely related p-toluenesulfonic acid (PTSA) is solid. However, in a laboratory setting, solid PTSA is more convenient.
Methanesulfonic acid can be used in the generation of borane (BH3) by reacting methanesulfonic acid with NaBH4 in an aprotic solvent such as THF or DMS, the complex of BH3 and the solvent is formed.
Methanesulfonic acid is considered a particularly suitable supporting electrolyte for electrochemical applications, where it stands as an environmentally friendly alternative to other acid electrolytes used in plating processes. Methanesulfonic acid is also the electrolyte of choice in zinc-cerium (see cerium(III) methanesulfonate) and lead-acid (methanesulfonate) flow batteries.
Methanesulfonic acid is also a primary ingredient in rust and scale removers. It is used to clean off surface rust from ceramic, tiles and porcelain which are usually susceptible to acid attack.