A limitation to the diagnostic use of thermodynamic measurements for the detection of dissociative equilibria

Abstract

It is shown that the qualitative features of cryoscopic and partial enthalpy curves characteristic of solvents which undergo two (practically independent) forms of dissociation may also be encountered when a solvent dissociates in only one way. In such cases even a detailed quantitative analysis based on the wrong hypothesis (of two dissociative forms) may not reveal the error, as illustrated by a hypothetical example involving sulphuric acid. Without special justification in particular cases, thermodynamic data cannot therefore be used diagnostically for the characterisation of dissociative equilibria. Corroborative evidence from, e.g., conductance or spectroscopy is generally necessary to establish the existence of the various chemical species.