Thermodynamic cycles relevant to hydrogenation of CO₂ to formic acid in water and acetonitrile

The formation of formic acid via the reduction (hydrogenation or electrochemical methods) of carbon dioxide (CO₂) has been receiving great attention, not only in the context of the production of renewable fuels or synthons, but also since it is a convenient carrier of hydrogen gas in the form of a liquid at ambient conditions. In this report, in order to help elucidate the effect of solvent on the thermodynamic parameters for the hydrogenation of CO₂ in water and acetonitrile, the acidity constant for formic acid in acetonitrile was determined experimentally, for the first time, to be 20.7 ± 0.2 at 25 °C, and also the standard Gibbs energy changes for the transfers of formic acid and formate anion from water to acetonitrile were established.