Ionic medium effect on the rate of hydrolysis of pyrophosphate ions at neutral pH and 70–85°C

The rates of hydrolysis of pyrophosphate (diphosphate) ions with the P2O74- skeleton have been studied in aqueous solutions containing various alkali-metal and tetraalkylammonium halides at neutral pH and 70, 80 and 85°C. The apparent rate constant of hydrolysis at a given pH and temperature increases in the order LiClacceleration effect upon formation of the metal–polyphosphate complexes. Furthermore, the activation enthalpy is practically unchanged, while the activation entropy varies depending on the ionic medium. It is thus supposed that the change in the hydration structure around the HP2O73- ion owing to the overlap with that of medium cations plays an essential role in the ionic medium effect.