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

Abstract

The rates of hydrolysis of pyrophosphate (diphosphate) ions with the P₂O₇^4- 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 LiCl<NaCl<KCl<(CH₃)₄NBr<(n-C₃H₇)₄NBr. As the P₂O₇^4- ion is protonated to form HP₂O₇^3- and H₂P₂O₇^2-, the protonation constants at elevated temperature were evaluated on the basis of the van't Hoff equation using the protonation constants and the corresponding enthalpies of formation at 25°C. The protonation constants at elevated temperature increase almost in the same order as that of the medium of 1:1 electrolytes at 25°C, which suggests that the difference in the species distribution of HP₂O₇^3- and H₂P₂O₇^2- plays a primary role in the ionic medium effect. Their intrinsic rate constants of hydrolysis were then extracted on the basis of the protonation constants at each temperature. Interestingly, the intrinsic rate constant of H₂P₂O₇^2- exhibits practically no dependence on the ionic medium, while the corresponding rate constant of HP₂O₇^3- increases in the order Li⁺<Na⁺≈K⁺≈(CH₃)₄N⁺<(n-C₃H₇)₄N⁺. The latter is unexpected if we take into account the acceleration 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 HP₂O₇^3- ion owing to the overlap with that of medium cations plays an essential role in the ionic medium effect.