Nitrogen dioxide at low concentration in air is very poorly absorbed by water in simple bubblers, unless certain reactive solutes are present. These seem capable of undergoing a fast surface reaction, possibly of the charge-transfer type, with nitrogen dioxide acting as the electron-acceptor. By choosing suitable concentrations, the relative reactivities of different compounds can be estimated fairly accurately; phenoxide ions are highly reactive while aliphatic amines, including sulphamic acid, are inactive.